Programs & Examples On #Mongodb

MongoDB is a scalable, high-performance, open source, document-oriented NoSQL database. It supports a large number of languages and application development platforms. Questions about server administration can be asked on

How to convert a pymongo.cursor.Cursor into a dict?

The MongoDB find method does not return a single result, but a list of results in the form of a Cursor. This latter is an iterator, so you can go through it with a for loop.

For your case, just use the findOne method instead of find. This will returns you a single document as a dictionary.

Node.js Mongoose.js string to ObjectId function

You can do it like this:

var mongoose = require('mongoose');
var _id = mongoose.mongo.BSONPure.ObjectID.fromHexString("4eb6e7e7e9b7f4194e000001");

EDIT: New standard has fromHexString rather than fromString

Add new field to every document in a MongoDB collection

Since MongoDB version 3.2 you can use updateMany():

> db.yourCollection.updateMany({}, {$set:{"someField": "someValue"}})

What is the correct way to start a mongod service on linux / OS X?

Just installed MongoDB via Homebrew. At the end of the installation console, you can see an output as follows:

To start mongodb:

brew services start mongodb

Or, if you don't want/need a background service you can just run:

mongod --config /usr/local/etc/mongod.conf

So, brew services start mongodb, managed to run MongoDB as a service for me.

Mongoose.js: Find user by username LIKE value

db.users.find( { 'username' : { '$regex' : req.body.keyWord, '$options' : 'i' } } )

Failed to connect to, reason: errno:111 Connection refused

In my case the error was due to missing /data/db folder which the mongodb uses to store your data. Please type this command $sudo mongod in your terminal. If the error message is something like :

missing data/db folder error

Simply create the folder and you are good to go.


If you need multiple columns to group by, follow this model. Here I am conducting a count by status and type:

    "$group": {
        _id: {
            status: "$status",
            type: "$type"
        count: {
            $sum: 1

MongoDB and "joins"

The first example you link to shows how MongoDB references behave much like lazy loading not like a join. There isn't a query there that's happening on both collections, rather you query one and then you lookup items from another collection by reference.

Query for documents where array size is greater than 1

I believe this is the fastest query that answers your question, because it doesn't use an interpreted $where clause:

{$nor: [
    {name: {$exists: false}},
    {name: {$size: 0}},
    {name: {$size: 1}}

It means "all documents except those without a name (either non existant or empty array) or with just one name."


>{name: []})
>{name: ['George']})
>{name: ['George', 'Raymond']})
>{name: ['George', 'Raymond', 'Richard']})
>{name: ['George', 'Raymond', 'Richard', 'Martin']})
> db.test.find({$nor: [{name: {$exists: false}}, {name: {$size: 0}}, {name: {$size: 1}}]})
{ "_id" : ObjectId("511907e3fb13145a3d2e225b"), "name" : [ "George", "Raymond" ] }
{ "_id" : ObjectId("511907e3fb13145a3d2e225c"), "name" : [ "George", "Raymond", "Richard" ] }
{ "_id" : ObjectId("511907e3fb13145a3d2e225d"), "name" : [ "George", "Raymond", "Richard", "Martin" ] }

MongoDB "root" user

I noticed a lot of these answers, use this command:

use admin

which switches to the admin database. At least in Mongo v4.0.6, creating a user in the context of the admin database will create a user with "_id" : "admin.administrator":

> use admin
> db.getUsers()
[ ]
> db.createUser({ user: 'administrator', pwd: 'changeme', roles: [ { role: 'root', db: 'admin' }  ] })
> db.getUsers()
        "_id" : "admin.administrator",
        "user" : "administrator",
        "db" : "admin",
        "roles" : [
                "role" : "root",
                "db" : "admin"
        "mechanisms" : [

I emphasize "admin.administrator", for I have a Mongoid (mongodb ruby adapter) application with a different database than admin and I use the URI to reference the database in my mongoid.yml configuration:

      uri: <%= ENV['MONGODB_URI'] %>
        connect_timeout: 15
        retry_writes: false

This references the following environment variable:

export MONGODB_URI='mongodb://administrator:[email protected]/mysite_development?retryWrites=true&w=majority'

Notice the database is mysite_development, not admin. When I try to run the application, I get an error "User administrator (mechanism: scram256) is not authorized to access mysite_development".

So I return to the Mongo shell delete the user, switch to the specified database and recreate the user:

$ mongo
> db.dropUser('administrator')
> db.getUsers()
> use mysite_development
> db.createUser({ user: 'administrator', pwd: 'changeme', roles: [ { role: 'root', db: 'admin' }  ] })
> db.getUsers()
        "_id" : "mysite_development.administrator",
        "user" : "administrator",
        "db" : "mysite_development",
        "roles" : [
                "role" : "root",
                "db" : "admin"
        "mechanisms" : [

Notice that the _id and db changed to reference the specific database my application depends on:

"_id" : "mysite_development.administrator",
"db" : "mysite_development",

After making this change, the error went away and I was able to connect to MongoDB fine inside my application.

Extra Notes:

In my example above, I deleted the user and recreated the user in the right database context. Had you already created the user in the right database context but given it the wrong roles, you could assign a mongodb built-in role to the user:

db.grantRolesToUser('administrator', [{ role: 'root', db: 'admin' }])

There is also a db.updateUser command, albiet typically used to update the user password.

How to start mongodb shell?

You were in the correct folder if you got the ./mongod working! You now need to open another terminal, go to the same folder and type ./mongo the first terminal window serves as your server, the second is where you enter your commands!

How to export JSON from MongoDB using Robomongo

You can use tojson to convert each record to JSON in a MongoDB shell script.

Run this script in RoboMongo:

var cursor = db.getCollection('foo').find({}, {});
while(cursor.hasNext()) {

This prints all results as a JSON-like array.

The result is not really JSON! Some types, such as dates and object IDs, are printed as JavaScript function calls, e.g., ISODate("2016-03-03T12:15:49.996Z").

Might not be very efficient for large result sets, but you can limit the query. Alternatively, you can use mongoexport.

Mongodb: failed to connect to server on first connect

I was trying to connect mlab with my company wifi connection and it was giving this error. But when I switched to personal network it worked and got connected.

Get names of all keys in the collection

As per the mongoldb documentation, a combination of distinct

Finds the distinct values for a specified field across a single collection or view and returns the results in an array.

and indexes collection operations are what would return all possible values for a given key, or index:

Returns an array that holds a list of documents that identify and describe the existing indexes on the collection

So in a given method one could do use a method like the following one, in order to query a collection for all it's registered indexes, and return, say an object with the indexes for keys (this example uses async/await for NodeJS, but obviously you could use any other asynchronous approach):

async function GetFor(collection, index) {

    let currentIndexes;
    let indexNames = [];
    let final = {};
    let vals = [];

    try {
        currentIndexes = await collection.indexes();
        await ParseIndexes();
        //Check if a specific index was queried, otherwise, iterate for all existing indexes
        if (index && typeof index === "string") return await ParseFor(index, indexNames);
        await ParseDoc(indexNames);
        await Promise.all(vals);
        return final;
    } catch (e) {
        throw e;

    function ParseIndexes() {
        return new Promise(function (result) {
            let err;
            for (let ind in currentIndexes) {
                let index = currentIndexes[ind];
                if (!index) {
                    err = "No Key For Index "+index; break;
                let Name = Object.keys(index.key);
                if (Name.length === 0) {
                    err = "No Name For Index"; break;
            return result(err ? Promise.reject(err) : Promise.resolve());

    async function ParseFor(index, inDoc) {
        if (inDoc.indexOf(index) === -1) throw "No Such Index In Collection";
        try {
            await DistinctFor(index);
            return final;
        } catch (e) {
            throw e
    function ParseDoc(doc) {
        return new Promise(function (result) {
            let err;
            for (let index in doc) {
                let key = doc[index];
                if (!key) {
                    err = "No Key For Index "+index; break;
                vals.push(new Promise(function (pushed) {
                        .catch(function (err) {
                            return pushed(Promise.resolve());
            return result(err ? Promise.reject(err) : Promise.resolve());

    async function DistinctFor(key) {
        if (!key) throw "Key Is Undefined";
        try {
            final[key] = await collection.distinct(key);
        } catch (e) {
            final[key] = 'failed';
            throw e;

So querying a collection with the basic _id index, would return the following (test collection only has one document at the time of the test):

Mongo.MongoClient.connect(url, function (err, client) {
    assert.equal(null, err);

    let collection = client.db('my db').collection('the targeted collection');

    GetFor(collection, '_id')
        .then(function () {
            // { _id: [ 5ae901e77e322342de1fb701 ] }
        .catch(function (err) {
            //manage your error..

Mind you, this uses methods native to the NodeJS Driver. As some other answers have suggested, there are other approaches, such as the aggregate framework. I personally find this approach more flexible, as you can easily create and fine-tune how to return the results. Obviously, this only addresses top-level attributes, not nested ones. Also, to guarantee that all documents are represented should there be secondary indexes (other than the main _id one), those indexes should be set as required.

MySQL vs MongoDB 1000 reads


10 rows

mysql insert: 1702ms
mysql select: 11ms

mongo insert: 47ms
mongo select: 12ms

100 rows

mysql insert: 8171ms
mysql select: 10ms

mongo insert: 167ms
mongo select: 60ms

1000 rows

mysql insert: 94813ms (1.58 minutes)
mysql select: 13ms

mongo insert: 1013ms
mongo select: 677ms

10.000 rows

mysql insert: 924695ms (15.41 minutes)
mysql select: 144ms

mongo insert: 9956ms (9.95 seconds)
mongo select: 4539ms (4.539 seconds)

Find document with array that contains a specific value

For Loopback3 all the examples given did not work for me, or as fast as using REST API anyway. But it helped me to figure out the exact answer I needed.

{"where":{"arrayAttribute":{ "all" :[String]}}}

Referencing another schema in Mongoose

It sounds like the populate method is what your looking for. First make small change to your post schema:

var postSchema = new Schema({
    name: String,
    postedBy: {type: mongoose.Schema.Types.ObjectId, ref: 'User'},
    dateCreated: Date,
    comments: [{body:"string", by: mongoose.Schema.Types.ObjectId}],

Then make your model:

var Post = mongoose.model('Post', postSchema);

Then, when you make your query, you can populate references like this:

Post.findOne({_id: 123})
.exec(function(err, post) {
    // do stuff with post

Get the latest record from mongodb collection

This is a rehash of the previous answer but it's more likely to work on different mongodb versions.


How do I start Mongo DB from Windows?

It is showing admin web console waiting for connections on port 28017.

The above message means that mongodb has started successfully and is listening on port 28017.

You can use the mongo shell(mongo.exe) to connect to the mongoDB and perform db operations.

There are several GUI tools available for MongoDB like MongoVUE, etc.

Couldn't connect to server

Adding the bin to PATH in Environment Variables helped.

GOTO Installation Path and copy the ../bin to the PATH variables in environment variables in Windows

Cannot authenticate into mongo, "auth fails"

This fixed my issue:

Go to terminal shell and type mongo.

Then type use db_name.

Then type:

     user: "mongodb",
     pwd: "dogmeatsubparflavour1337",
     roles: [ { role: "dbOwner", db: "db_name" } ]

Also try: db.getUsers()

Quick sample:

const MongoClient = require('mongodb').MongoClient;

// MongoDB Connection Info
const url = 'mongodb://mongodb:[email protected]:27017/?authMechanism=DEFAULT&authSource=db_name';
// Additional options:

// Use Connect Method to connect to the Server
  .then((db) => {
    console.log('Casually connected correctly to server.');
    // Be careful with db.close() when working asynchronously
  .catch((error) => {

New to MongoDB Can not run command mongo

Specify the database path explicitly like so, and see if that resolves the issue.

mongod --dbpath data/db

add created_at and updated_at fields to mongoose schemas

Since mongo 3.6 you can use 'change stream':

To use it you need to create a change stream object by the 'watch' query, and for each change, you can do whatever you want...

python solution:

def update_at_by(change):
    update_fields = change["updateDescription"]["updatedFields"].keys()
    print("update_fields: {}".format(update_fields))

    collection = change["ns"]["coll"]
    db = change["ns"]["db"]
    key = change["documentKey"]

    if len(update_fields) == 1 and "update_at" in update_fields:
        pass  # to avoid recursion updates...
        client[db][collection].update(key, {"$set": {"update_at":}})

client = MongoClient("")
db = client["Data"]

change_stream =

for change in change_stream:

Note, to use the change_stream object, your mongodb instance should run as 'replica set'. It can be done also as a 1-node replica set (almost no change then the standalone use):

Run mongo as a replica set:

Replica set configuration vs Standalone: Mongo DB - difference between standalone & 1-node replica set

How to drop a database with Mongoose?

Since the remove method is depreciated in the mongoose library we can use the deleteMany function with no parameters passed.


This will delete all content of this particular Model and your collection will be empty.

Understanding MongoDB BSON Document size limit

Many in the community would prefer no limit with warnings about performance, see this comment for a well reasoned argument:

My take, the lead developers are stubborn about this issue because they decided it was an important "feature" early on. They're not going to change it anytime soon because their feelings are hurt that anyone questioned it. Another example of personality and politics detracting from a product in open source communities but this is not really a crippling issue.

how to query child objects in mongodb

Assuming your "states" collection is like:

{"name" : "Spain", "cities" : [ { "name" : "Madrid" }, { "name" : null } ] }
{"name" : "France" }

The query to find states with null cities would be:

db.states.find({"" : {"$eq" : null, "$exists" : true}});

It is a common mistake to query for nulls as:

db.states.find({"" : null});

because this query will return all documents lacking the key (in our example it will return Spain and France). So, unless you are sure the key is always present you must check that the key exists as in the first query.

How can I rename a field for all documents in MongoDB?

This nodejs code just do that , as @Felix Yan mentioned former way seems to work just fine , i had some issues with other snipets hope this helps.

This will rename column "oldColumnName" to be "newColumnName" of table "documents"

var MongoClient = require('mongodb').MongoClient
  , assert = require('assert');

// Connection URL
//var url = 'mongodb://localhost:27017/myproject';
var url = 'mongodb://myuser:[email protected]:portNumber/databasename';

// Use connect method to connect to the server
MongoClient.connect(url, function(err, db) {
  assert.equal(null, err);
  console.log("Connected successfully to server");

  renameDBColumn(db, function() {


// This function should be used for renaming a field for all documents
var renameDBColumn = function(db, callback) {
  // Get the documents collection
  console.log("renaming database column of table documents");
  //use the former way:
  remap = function (x) {
    if (x.oldColumnName){
      db.collection('documents').update({_id:x._id}, {$set:{"newColumnName":x.oldColumnName}, $unset:{"oldColumnName":1}});

  console.log("db table documents remap successfully!");

When to Redis? When to MongoDB?

And you should use neither if you have plenty of RAM. Redis and MongoDB come to the price of a general purpose tool. This introduce a lot of overhead.

There was the saying that Redis is 10 times faster than Mongo. That might not be that true anymore. MongoDB (if i remember correctly) claimed to beat memcache for storing and caching documents as long as the memory configurations are the same.

Anyhow. Redis good, MongoDB is good. If you care about substructures and need aggregation go for MongoDB. If storing keys and values is your main concern its all about Redis. (or any other key value store).

When to use MongoDB or other document oriented database systems?

Like said previously, you can choose between a lot of choices, take a look at all those choices:

What I suggest is to find your best combination: MySQL + Memcache is really great if you need ACID and you want to join some tables MongoDB + Redis is perfect for document store Neo4J is perfect for graph database

What i do: I start with MySQl + Memcache because I'm use to, then I start using others database framework. In a single project, you can combine MySQL and MongoDB for instance !

MongoDB: How to update multiple documents with a single command?

MongoDB will find only one matching document which matches the query criteria when you are issuing an update command, whichever document matches first happens to be get updated, even if there are more documents which matches the criteria will get ignored.

so to overcome this we can specify "MULTI" option in your update statement, meaning update all those documnets which matches the query criteria. scan for all the documnets in collection finding those which matches the criteria and update :

db.test.update({"foo":"bar"},{"$set":{"test":"success!"}}, {multi:true} )

E11000 duplicate key error index in mongodb mongoose

same issue after removing properties from a schema after first building some indexes on saving. removing property from schema leads to an null value for a non existing property, that still had an index. dropping index or starting with a new collection from scratch helps here.

note: the error message will lead you in that case. it has a path, that does not exist anymore. im my case the old path was ...$uuid_1 (this is an index!), but the new one is ....*priv.uuid_1

How to execute mongo commands through shell scripts?

In my setup I have to use:

mongo --host="the.server.ip:port" databaseName theScript.js 

Formatting ISODate from Mongodb

you can use mongo query like this yearMonthDayhms: { $dateToString: { format: "%Y-%m-%d-%H-%M-%S", date: {$subtract:["$cdt",14400000]}}}

HourMinute: { $dateToString: { format: "%H-%M-%S", date: {$subtract:["$cdt",14400000]}}}

enter image description here

MongoDB: How to query for records where field is null or not set?

You can also try this:


Retrieve only the queried element in an object array in MongoDB collection

Likewise you can find for the multiple

    // Get just the docs that contain a shapes element where color is 'red'
  {$match: {'shapes.color': {$in : ['red','yellow'] } }},
  {$project: {
     shapes: {$filter: {
        input: '$shapes',
        as: 'shape',
        cond: {$in: ['$$shape.color', ['red', 'yellow']]}

Combine two OR-queries with AND in Mongoose

It's probably easiest to create your query object directly as:

      $and: [
          { $or: [{a: 1}, {b: 1}] },
          { $or: [{c: 1}, {d: 1}] }
  }, function (err, results) {

But you can also use the Query#and helper that's available in recent 3.x Mongoose releases:

          { $or: [{a: 1}, {b: 1}] },
          { $or: [{c: 1}, {d: 1}] }
      .exec(function (err, results) {

MongoDB: Is it possible to make a case-insensitive query?

You can use Case Insensitive Indexes:

The following example creates a collection with no default collation, then adds an index on the name field with a case insensitive collation. International Components for Unicode

/* strength: CollationStrength.Secondary
* Secondary level of comparison. Collation performs comparisons up to secondary * differences, such as diacritics. That is, collation performs comparisons of 
* base characters (primary differences) and diacritics (secondary differences). * Differences between base characters takes precedence over secondary 
* differences.
db.users.createIndex( { name: 1 }, collation: { locale: 'tr', strength: 2 } } )

To use the index, queries must specify the same collation.

db.users.insert( [ { name: "Oguz" },
                            { name: "oguz" },
                            { name: "OGUZ" } ] )

// does not use index, finds one result
db.users.find( { name: "oguz" } )

// uses the index, finds three results
db.users.find( { name: "oguz" } ).collation( { locale: 'tr', strength: 2 } )

// does not use the index, finds three results (different strength)
db.users.find( { name: "oguz" } ).collation( { locale: 'tr', strength: 1 } )

or you can create a collection with default collation:

db.createCollection("users", { collation: { locale: 'tr', strength: 2 } } )
db.users.createIndex( { name : 1 } ) // inherits the default collation

'Field required a bean of type that could not be found.' error spring restful API using mongodb

Add @Repository in your dao class

    public interface UserDao extends CrudRepository<User, Long> {
         User findByUsername(String username);
         User findByEmail(String email);    

MongoDB Show all contents from all collections

This way:


DynamoDB vs MongoDB NoSQL

For quick overview comparisons, I really like this website, that has many comparison pages, eg AWS DynamoDB vs MongoDB;

Mongoose: Get full list of users

To make function to wait for list to be fetched.

getArrayOfData() {
    return DataModel.find({}).then(function (storedDataArray) {
        return storedDataArray;
        if (err) {
            throw new Error(err.message);

MongoDB: How To Delete All Records Of A Collection in MongoDB Shell?

You can delete all the documents from a collection in MongoDB, you can use the following:


Alternatively, you could use the following method as well:


Follow the following MongoDB documentation, for further details.

To remove all documents from a collection, pass an empty filter document {} to either the db.collection.deleteMany() or the db.collection.remove() method.

mongoError: Topology was destroyed

In my case, this error was caused by a db.close(); out of a 'await' section inside of 'async'

MongoClient.connect(url, {poolSize: 10, reconnectTries: Number.MAX_VALUE, reconnectInterval: 1000}, function(err, db) {
    // Validate the connection to Mongo
    assert.equal(null, err);    
    // Query the SQL table 
    .then(function (result) {
        console.log('Print results SQL');
        if(result.length > 0){

            processArray(db, result)
            .then(function (result) {
            .catch(function (err) {
        } else {
            console.log('Nothing to show in MySQL');
    .catch(function (err) {
    db.close(); // <--------------------------------THIS LINE

MongoDB: exception in initAndListen: 20 Attempted to create a lock file on a read-only directory: /data/db, terminating

Fix the permissions of /data/db (or /var/lib/mongodb):

sudo chown -R mongodb: /data/db

then restart MongoDB e.g. using

sudo systemctl restart mongod

In case that does not help, check your error message if you are using a data directory different to /var/lib/mongodb. In that case run

sudo chown -R mongodb: <insert your data directory here>


show dbs gives "Not Authorized to execute command" error

for me it worked by adding

1) "You can run the mongodb instance without username and password first.---OK

2) "Then you can add the user to the system database of the mongodb which is default one using the query below".---OK

  user: "myUserAdmin",
  pwd: "abc123",
  roles: [ { role: "userAdminAnyDatabase", db: "admin" } ],
  mechanisms:[ "SCRAM-SHA-1" ] // I added this line


What are naming conventions for MongoDB?

Even if no convention is specified about this, manual references are consistently named after the referenced collection in the Mongo documentation, for one-to-one relations. The name always follows the structure <document>_id.

For example, in a dogs collection, a document would have manual references to external documents named like this:

  name: 'fido',
  owner_id: '5358e4249611f4a65e3068ab',
  race_id: '5358ee549611f4a65e3068ac',
  colour: 'yellow'

This follows the Mongo convention of naming _id the identifier for every document.

Mongodb service won't start

I solved this by executing C:\mongodb\bin\mongod.exe --repair first. Then when I ran MongoDB again by C:\mongodb\bin\mongod.exe, it successfully started.

Cannot overwrite model once compiled Mongoose

The error is occurring because you already have a schema defined, and then you are defining the schema again. Generally what you should do is instantiate the schema once, and then have a global object call it when it needs it.

For example:


var mongoose = require('mongoose');
var Schema = mongoose.Schema;

var userSchema = new Schema({
module.exports = mongoose.model('users', userSchema);          


var mongoose = require('mongoose');
var User = require('./user_model.js');

var db = mongoose.createConnection('localhost', 'event-db');
db.on('error', console.error.bind(console, 'connection error:'));
var a1= db.once('open',function(){
  User.find({},{},function (err, users) {
    console.log("Username supplied"+username);


var mongoose = require('mongoose');
var User = require('./user_model.js');

var new_user = new User({
  , email:
  , password: req.body.password
  , phone:
  , _enabled:false 
  if(err) console.log(err); 

How to define object in array in Mongoose schema correctly with 2d geo index

Thanks for the replies.

I tried the first approach, but nothing changed. Then, I tried to log the results. I just drilled down level by level, until I finally got to where the data was being displayed.

After a while I found the problem: When I was sending the response, I was converting it to a string via .toString().

I fixed that and now it works brilliantly. Sorry for the false alarm.

How to search in array of object in mongodb

You can do this in two ways:

  1. ElementMatch - $elemMatch (as explained in above answers)

    db.users.find({ awards: { $elemMatch: {award:'Turing Award', year:1977} } })

  2. Use $and with find

    db.getCollection('users').find({"$and":[{"awards.award":"Turing Award"},{"awards.year":1977}]})

"Large data" workflows using pandas

As noted by others, after some years an 'out-of-core' pandas equivalent has emerged: dask. Though dask is not a drop-in replacement of pandas and all of its functionality it stands out for several reasons:

Dask is a flexible parallel computing library for analytic computing that is optimized for dynamic task scheduling for interactive computational workloads of “Big Data” collections like parallel arrays, dataframes, and lists that extend common interfaces like NumPy, Pandas, or Python iterators to larger-than-memory or distributed environments and scales from laptops to clusters.

Dask emphasizes the following virtues:

  • Familiar: Provides parallelized NumPy array and Pandas DataFrame objects
  • Flexible: Provides a task scheduling interface for more custom workloads and integration with other projects.
  • Native: Enables distributed computing in Pure Python with access to the PyData stack.
  • Fast: Operates with low overhead, low latency, and minimal serialization necessary for fast numerical algorithms
  • Scales up: Runs resiliently on clusters with 1000s of cores Scales down: Trivial to set up and run on a laptop in a single process
  • Responsive: Designed with interactive computing in mind it provides rapid feedback and diagnostics to aid humans

and to add a simple code sample:

import dask.dataframe as dd
df = dd.read_csv('2015-*-*.csv')

replaces some pandas code like this:

import pandas as pd
df = pd.read_csv('2015-01-01.csv')

and, especially noteworthy, provides through the concurrent.futures interface a general infrastructure for the submission of custom tasks:

from dask.distributed import Client
client = Client('scheduler:port')

futures = []
for fn in filenames:
    future = client.submit(load, fn)

summary = client.submit(summarize, futures)

MongoDB running but can't connect using shell

On Ubuntu:

Wed Jan 27 10:21:32 Error: couldn't connect to server shell/mongo.js:84 exception: connect failed


look for if mongodb is running by following command:

ps -ef | grep mongo

If mongo is not running you get:

 vimal     1806  1698  0 10:11 pts/0    00:00:00 grep --color=auto mongo

You are seeing that the mongo daemon is not there.

Then start it through configuration file(with root priev):

root@vimal:/data# mongod --config /etc/mongodb.conf &
[1] 2131
root@vimal:/data# all output going to: /var/log/mongodb/mongodb.log

you can see the other details:

root@vimal:~# more /etc/mongodb.conf

Open a new terminal to see the result of mongod --config /etc/mongodb.conf & then type mongo. It should be running or grep

root@vimal:/data# ps -ef | grep mongo

root      3153     1  2 11:39 ?        00:00:23 mongod --config /etc/mongodb.conf
root      3772  3489  0 11:55 pts/1    00:00:00 grep --color=auto mongo 


root@vimal:/data# mongo

MongoDB shell version: 2.0.4
connecting to: test

you get the mongoDB shell

This is not the end of story. I will post the repair method so that it starts automatically every time, most development machine shutdowns every day and the VM must have mongo started automatically at next boot.

Mongoose: CastError: Cast to ObjectId failed for value "[object Object]" at path "_id"

My solution is that i want data from all docs and i dont want _id, so

User.find({}, {_id:0, keyToShow:1, keyToNotShow:0})

Query Mongodb on month, day, year... of a datetime

how about storing the month in its own property since you need to query for it? less elegant than $where, but likely to perform better since it can be indexed.

MongoDB or CouchDB - fit for production?

This question has already accepted answer but now a days one more NoSQL DB is in trend for many of its great features. It is Couchbase; which runs as CouchbaseLite on mobile platform and Couchbase Server on your server side.

Here is some of main features of Couchbase Lite.

Couchbase Lite is a lightweight, document-oriented (NoSQL), syncable database engine suitable for embedding into mobile apps.

Lightweight means:

Embedded—the database engine is a library linked into the app, not a separate server process. Small code size—important for mobile apps, which are often downloaded over cell networks. Quick startup time—important because mobile devices have relatively slow CPUs. Low memory usage—typical mobile data sets are relatively small, but some documents might have large multimedia attachments. Good performance—exact figures depend on your data and application, of course.

Document-oriented means:

Stores records in flexible JSON format instead of requiring predefined schemas or normalization. Documents can have arbitrary-sized binary attachments, such as multimedia content. Application data format can evolve over time without any need for explicit migrations. MapReduce indexing provides fast lookups without needing to use special query languages.

Syncable means:

Any two copies of a database can be brought into sync via an efficient, reliable, proven replication algorithm. Sync can be on-demand or continuous (with a latency of a few seconds). Devices can sync with a subset of a large database on a remote server. The sync engine supports intermittent and unreliable network connections. Conflicts can be detected and resolved, with app logic in full control of merging. Revision trees allow for complex replication topologies, including server-to-server (for multiple data centers) and peer-to-peer, without data loss or false conflicts. Couchbase Lite provides native APIs for seamless iOS (Objective-C) and Android (Java) development. In addition, it includes the Couchbase Lite Plug-in for PhoneGap, which enables you to build iOS and Android apps that you develop by using familiar web-application programming techniques and the PhoneGap mobile development framework.

You can explore more on Couchbase Lite

and Couchbase Server

This is going to the next big thing.

Uninstall mongoDB from ubuntu

In my case mongodb packages are named mongodb-org and mongodb-org-*

So when I type sudo apt purge mongo then tab (for auto-completion) I can see all installed packages that start with mongo.

Another option is to run the following command (which will list all packages that contain mongo in their names or their descriptions):

dpkg -l | grep mongo

In summary, I would do (to purge all packages that start with mongo):

sudo apt purge mongo*

and then (to make sure that no mongo packages are left):

dpkg -l | grep mongo

Of course, as mentioned by @alicanozkara, you will need to manually remove some directories like /var/log/mongodb and /var/lib/mongodb

Running the following find commands:

sudo find /etc/ -name "*mongo*" and sudo find /var/ -name "*mongo*"

may also show some files that you may want to remove, like:




You may also want to remove user and group mongodb, to do so you need to run:

sudo userdel -r mongodb
sudo groupdel mongodb

To check whether mongodb user/group exists or not, try:

cut -d: -f1 /etc/passwd | grep mongo
cut -d: -f1 /etc/group | grep mongo

How do you query for "is not null" in Mongo?

This will return all documents with a key called "IMAGE URL", but they may still have a null value.

db.mycollection.find({"IMAGE URL":{$exists:true}});

This will return all documents with both a key called "IMAGE URL" and a non-null value.

db.mycollection.find({"IMAGE URL":{$ne:null}});

Also, according to the docs, $exists currently can't use an index, but $ne can.

Edit: Adding some examples due to interest in this answer

Given these inserts:

db.test.insert({"num":1, "check":"check value"});
db.test.insert({"num":2, "check":null});

This will return all three documents:


This will return the first and second documents only:


This will return the first document only:


This will return the second and third documents only:


E: Unable to locate package mongodb-org

I'm running Ubuntu 14.04; and apt still couldn't find package; I tried all the answers above and more. The URL that worked for me is this:

echo 'deb dist 10gen' | sudo tee /etc/apt/sources.list.d/mongodb.list


How to get the Mongo database specified in connection string in C#

In this moment with the last version of the C# driver (2.3.0) the only way I found to get the database name specified in connection string is this:

var connectionString = @"mongodb://usr:[email protected],,";
var mongoUrl = new MongoUrl(connectionString);
var dbname = mongoUrl.DatabaseName;
var db = new MongoClient(mongoUrl).GetDatabase(dbname);

Properly close mongoose's connection once you're done

I'm using version 4.4.2 and none of the other answers worked for me. But adding useMongoClient to the options and putting it into a variable that you call close on seemed to work.

var db = mongoose.connect('mongodb://localhost:27017/somedb', { useMongoClient: true })

//do stuff


Changing MongoDB data store directory

Copy the contents of /var/lib/mongodb to /data/db. The files you should be looking for should have names like your_db_name.ns and your_dbname.n where n is a number starting with 0. If you do not see such files under /var/lib/mongodb, search for them on your filesystem.

Once copied over, use --dbpath=/data/db when starting MongoDB via the mongod command.

How do I partially update an object in MongoDB so the new object will overlay / merge with the existing one

You can use $mergeObjects in the aggregation based update. Something like

   { _id:...},
          new info or { param2 : "val2_new", param3 : "val3_new"}

More examples here

How to remove a field completely from a MongoDB document?

Try this: If your collection was 'example'

db.example.update({}, {$unset: {words:1}}, false, true);

Refer this:


The above link no longer covers '$unset'ing. Be sure to add {multi: true} if you want to remove this field from all of the documents in the collection; otherwise, it will only remove it from the first document it finds that matches. See this for updated documentation:


db.example.update({}, {$unset: {words:1}} , {multi: true});

How to secure MongoDB with username and password

Here is a javascript code to add users.

  1. Start mongod with --auth = true

  2. Access admin database from mongo shell and pass the javascript file.

    mongo admin "Filename.js"


    // Adding admin user
    db.addUser("admin_username", " admin_password");
    // Authenticate admin user
    db.auth("admin_username ", " admin_password ");
    // use  database code from java script
    db = db.getSiblingDB("newDatabase");
    // Adding newDatabase database user  
    db.addUser("database_username ", " database_ password ");
  3. Now user addition is complete, we can verify accessing the database from mongo shell

MongoDB what are the default user and password?

In addition with what @Camilo Silva already mentioned, if you want to give free access to create databases, read, write databases, etc, but you don't want to create a root role, you can change the 3rd step with the following:

use admin
    user: "myUserAdmin",
    pwd: "abc123",
    roles: [ { role: "userAdminAnyDatabase", db: "admin" }, 
             { role: "dbAdminAnyDatabase", db: "admin" }, 
             { role: "readWriteAnyDatabase", db: "admin" } ]

How to Update Multiple Array Elements in mongodb

First: your code did not work because you were using the positional operator $ which only identifies an element to update in an array but does not even explicitly specify its position in the array.

What you need is the filtered positional operator $[<identifier>]. It would update all elements that match an array filter condition.


db.collection.update({"events.profile":10}, { $set: { "events.$[elem].handled" : 0 } },
     multi: true,
     arrayFilters: [ { "elem.profile": 10 } ]

Visit mongodb doc here

What the code does:

  1. {"events.profile":10} filters your collection and return the documents matching the filter

  2. The $set update operator: modifies matching fields of documents it acts on.

  3. {multi:true} It makes .update() modifies all documents matching the filter hence behaving like updateMany()

  4. { "events.$[elem].handled" : 0 } and arrayFilters: [ { "elem.profile": 10 } ] This technique involves the use of the filtered positional array with arrayFilters. the filtered positional array here $[elem] acts as a placeholder for all elements in the array fields that match the conditions specified in the array filter.

Array filters

Checking if a field contains a string

This should do the work

db.users.find({ username: { $in: [ /son/i ] } });

The i is just there to prevent restrictions of matching single cases of letters.

You can check the $regex documentation on MongoDB documentation. Here's a link:

MongoDB not equal to

If you want to do multiple $ne then do

db.users.find({name : {$nin : ["mary", "dick", "jane"]}})

How can I run MongoDB as a Windows service?

Working on MongoDB 3.4 [Windows]

  • Create dir C:/mongodb/data
  • Create a file in C:/mongodb/mongodb.config using this configuration:

        engine: wiredTiger
        dbPath: "C:/mongodb/data"
        directoryPerDB: true
            enabled: true
        destination: file
        path: "C:/mongodb/data/mongod.log"
        logAppend: true
        timeStampFormat: iso8601-utc
        port: 27017
        wireObjectCheck : false
  • To install MongoDb as a service, run this command in powershell with admin power

    mongod --config="C:\mongodb\mongodb.config" --install --service

  • Open Services.msc and look for MongoDb, then start it

How to change the type of a field?

To convert int32 to string in mongo without creating an array just add "" to your number :-) { 'mynum' : { $type : 16 } } ).forEach( function (x) {   
  x.mynum = x.mynum + ""; // convert int32 to string;

How to populate a sub-document in mongoose after creating it?

@user1417684 and @chris-foster are right!

excerpt from working code (without error handling):

var SubItemModel = mongoose.model('subitems', SubItemSchema);
var ItemModel    = mongoose.model('items', ItemSchema);

var new_sub_item_model = new SubItemModel(new_sub_item_plain); (error, new_sub_item) {

  var new_item = new ItemModel(new_item);
  new_item.subitem = new_sub_item._id; (error, new_item) {
    // so this is a valid way to populate via the Model
    // as documented in comments above (here @stack overflow):
    ItemModel.populate(new_item, { path: 'subitem', model: 'subitems' }, function(error, new_item) {
    // or populate directly on the result object
    new_item.populate('subitem', function(error, new_item) {


Unable to connect to mongodb Error: couldn't connect to server at src/mongo/shell/mongo.js:L112

If you are on unix-like systems.

Starting in MongoDB 4.4, a startup error is generated if the ulimit value for number of open files is under 64000. View the current val

$ ulimit -n

Change value

$ ulimit -n <val>

How do I drop a MongoDB database from the command line?

Other way:

echo "db.dropDatabase()" | mongo <database name>

MongoDB logging all queries

I ended up solving this by starting mongod like this (hammered and ugly, yeah... but works for development environment):

mongod --profile=1 --slowms=1 &

This enables profiling and sets the threshold for "slow queries" as 1ms, causing all queries to be logged as "slow queries" to the file:


Now I get continuous log outputs using the command:

tail -f /var/log/mongodb/mongodb.log

An example log:

Mon Mar  4 15:02:55 [conn1] query dendro.quads query: { graph: "u:" } ntoreturn:0 ntoskip:0 nscanned:6 keyUpdates:0 locks(micros) r:73163 nreturned:6 reslen:9884 88ms

MongoDB vs. Cassandra

I've used MongoDB extensively (for the past 6 months), building a hierarchical data management system, and I can vouch for both the ease of setup (install it, run it, use it!) and the speed. As long as you think about indexes carefully, it can absolutely scream along, speed-wise.

I gather that Cassandra, due to its use with large-scale projects like Twitter, has better scaling functionality, although the MongoDB team is working on parity there. I should point out that I've not used Cassandra beyond the trial-run stage, so I can't speak for the detail.

The real swinger for me, when we were assessing NoSQL databases, was the querying - Cassandra is basically just a giant key/value store, and querying is a bit fiddly (at least compared to MongoDB), so for performance you'd have to duplicate quite a lot of data as a sort of manual index. MongoDB, on the other hand, uses a "query by example" model.

For example, say you've got a Collection (MongoDB parlance for the equivalent to a RDMS table) containing Users. MongoDB stores records as Documents, which are basically binary JSON objects. e.g:

   FirstName: "John",
   LastName: "Smith",
   Email: "[email protected]",
   Groups: ["Admin", "User", "SuperUser"]

If you wanted to find all of the users called Smith who have Admin rights, you'd just create a new document (at the admin console using Javascript, or in production using the language of your choice):

   LastName: "Smith",
   Groups: "Admin"

...and then run the query. That's it. There are added operators for comparisons, RegEx filtering etc, but it's all pretty simple, and the Wiki-based documentation is pretty good.

"continue" in cursor.forEach()

Each iteration of the forEach() will call the function that you have supplied. To stop further processing within any given iteration (and continue with the next item) you just have to return from the function at the appropriate point:

  if (!element.shouldBeProcessed)
    return; // stop processing this iteration

  // This part will be avoided if not neccessary

MongoDB vs Firebase

Firebase is designed for real-time updates. It easily integrates with angular. Both are NoSQL databases. MongoDB can also do this with Angular through integration. Meteor.js also makes use of MongoDB with an open socket connection for real-time updates.

MongoDB can be run locally, or hosted on many different cloud based providers. Firebase, in my opinion is great for smaller apps, very quick to get up and running. MongoDB is ideal for more robust larger apps, real-time integration is possible but it takes a bit more work.

How to use mongoose findOne

In my case same error is there , I am using Asyanc / Await functions , for this needs to add AWAIT for findOne

Ex:const foundUser = User.findOne ({ "email" : });

above , foundUser always contains Object value in both cases either user found or not because it's returning values before finishing findOne .

const foundUser = await User.findOne ({ "email" : });

above , foundUser returns null if user is not there in collection with provided condition . If user found returns user document.

How to list all databases in the mongo shell?

For MongoDB shell version 3.0.5 insert the following command in the shell:


or alternatively:


TransactionRequiredException Executing an update/delete query

Nothing seemed to work for me until I realized that my method was declared as public final instead of just public. The error was being caused by the final keyword. Removing it made the error go away.

BadValue Invalid or no user locale set. Please ensure LANG and/or LC_* environment variables are set correctly

Amazon Linux AMI

Permanent solution for ohmyzsh:

$ vim ~/.zshrc

Write there below:

export LC_ALL=en_US.UTF-8
export LANG=en_US.UTF-8
export LANGUAGE=en_US.UTF-8

Update changes in current shell by: $ source ~/.zshrc

How to sort in mongoose?

As of Mongoose 3.8.x:

model.find({ ... }).sort({ field : criteria}).exec(function(err, model){ ... });


criteria can be asc, desc, ascending, descending, 1, or -1

Note: Use quotation marks or double quote

use "asc", "desc", "ascending", "descending", 1, or -1

Mongoose query where value is not null

You should be able to do this like (as you're using the query api):


... which will result in a mongo query resembling:

entrants.find({ pincode: { $ne: null } })

A few links that might help:

MongoDb query condition on comparing 2 fields

You can use $expr ( 3.6 mongo version operator ) to use aggregation functions in regular query.

Compare query operators vs aggregation comparison operators.

Regular Query:

db.T.find({$expr:{$gt:["$Grade1", "$Grade2"]}})

Aggregation Query:

db.T.aggregate({$match:{$expr:{$gt:["$Grade1", "$Grade2"]}}})

When to use CouchDB over MongoDB and vice versa

Ask this questions yourself? And you will decide your DB selection.

  1. Do you need master-master? Then CouchDB. Mainly CouchDB supports master-master replication which anticipates nodes being disconnected for long periods of time. MongoDB would not do well in that environment.
  2. Do you need MAXIMUM R/W throughput? Then MongoDB
  3. Do you need ultimate single-server durability because you are only going to have a single DB server? Then CouchDB.
  4. Are you storing a MASSIVE data set that needs sharding while maintaining insane throughput? Then MongoDB.
  5. Do you need strong consistency of data? Then MongoDB.
  6. Do you need high availability of database? Then CouchDB.
  7. Are you hoping multi databases and multi tables/ collections? Then MongoDB
  8. You have a mobile app offline users and want to sync their activity data to a server? Then you need CouchDB.
  9. Do you need large variety of querying engine? Then MongoDB
  10. Do you need large community to be using DB? Then MongoDB

Delete everything in a MongoDB database

Simplest way to delete a database say blog:

> use blog
switched to db blog
> db.dropDatabase();
{ "dropped" : "blog", "ok" : 1 }

Push items into mongo array via mongoose

Assuming, var friend = { firstName: 'Harry', lastName: 'Potter' };

There are two options you have:

Update the model in-memory, and save (plain javascript array.push):



    { _id: person._id }, 
    { $push: { friends: friend } },

I always try and go for the first option when possible, because it'll respect more of the benefits that mongoose gives you (hooks, validation, etc.).

However, if you are doing lots of concurrent writes, you will hit race conditions where you'll end up with nasty version errors to stop you from replacing the entire model each time and losing the previous friend you added. So only go to the former when it's absolutely necessary.

What's a clean way to stop mongod on Mac OS X?

It's probably because launchctl is managing your mongod instance. If you want to start and shutdown mongod instance, unload that first:

launchctl unload -w ~/Library/LaunchAgents/org.mongodb.mongod.plist

Then start mongod manually:

mongod -f path/to/mongod.conf --fork

You can find your mongod.conf location from ~/Library/LaunchAgents/org.mongodb.mongod.plist.

After that, db.shutdownServer() would work just fine.

Added Feb 22 2014:

If you have mongodb installed via homebrew, homebrew actually has a handy brew services command. To show current running services:

brew services list

To start mongodb:

brew services start mongodb-community

To stop mongodb if it's already running:

brew services stop mongodb-community


As edufinn pointed out in the comment, brew services is now available as user-defined command and can be installed with following command: brew tap gapple/services.

Check that Field Exists with MongoDB

db.<COLLECTION NAME>.find({ "<FIELD NAME>": { $exists: true, $ne: null } })

How to filter array in subdocument with MongoDB

Using aggregate is the right approach, but you need to $unwind the list array before applying the $match so that you can filter individual elements and then use $group to put it back together:

    { $match: {_id: ObjectId("512e28984815cbfcb21646a7")}},
    { $unwind: '$list'},
    { $match: {'list.a': {$gt: 3}}},
    { $group: {_id: '$_id', list: {$push: '$list.a'}}}


  "result": [
      "_id": ObjectId("512e28984815cbfcb21646a7"),
      "list": [
  "ok": 1

MongoDB 3.2 Update

Starting with the 3.2 release, you can use the new $filter aggregation operator to do this more efficiently by only including the list elements you want during a $project:

    { $match: {_id: ObjectId("512e28984815cbfcb21646a7")}},
    { $project: {
        list: {$filter: {
            input: '$list',
            as: 'item',
            cond: {$gt: ['$$item.a', 3]}

$lookup on ObjectId's in an array

2017 update

$lookup can now directly use an array as the local field. $unwind is no longer needed.

Old answer

The $lookup aggregation pipeline stage will not work directly with an array. The main intent of the design is for a "left join" as a "one to many" type of join ( or really a "lookup" ) on the possible related data. But the value is intended to be singular and not an array.

Therefore you must "de-normalise" the content first prior to performing the $lookup operation in order for this to work. And that means using $unwind:

    // Unwind the source
    { "$unwind": "$products" },
    // Do the lookup matching
    { "$lookup": {
       "from": "products",
       "localField": "products",
       "foreignField": "_id",
       "as": "productObjects"
    // Unwind the result arrays ( likely one or none )
    { "$unwind": "$productObjects" },
    // Group back to arrays
    { "$group": {
        "_id": "$_id",
        "products": { "$push": "$products" },
        "productObjects": { "$push": "$productObjects" }

After $lookup matches each array member the result is an array itself, so you $unwind again and $group to $push new arrays for the final result.

Note that any "left join" matches that are not found will create an empty array for the "productObjects" on the given product and thus negate the document for the "product" element when the second $unwind is called.

Though a direct application to an array would be nice, it's just how this currently works by matching a singular value to a possible many.

As $lookup is basically very new, it currently works as would be familiar to those who are familiar with mongoose as a "poor mans version" of the .populate() method offered there. The difference being that $lookup offers "server side" processing of the "join" as opposed to on the client and that some of the "maturity" in $lookup is currently lacking from what .populate() offers ( such as interpolating the lookup directly on an array ).

This is actually an assigned issue for improvement SERVER-22881, so with some luck this would hit the next release or one soon after.

As a design principle, your current structure is neither good or bad, but just subject to overheads when creating any "join". As such, the basic standing principle of MongoDB in inception applies, where if you "can" live with the data "pre-joined" in the one collection, then it is best to do so.

The one other thing that can be said of $lookup as a general principle, is that the intent of the "join" here is to work the other way around than shown here. So rather than keeping the "related ids" of the other documents within the "parent" document, the general principle that works best is where the "related documents" contain a reference to the "parent".

So $lookup can be said to "work best" with a "relation design" that is the reverse of how something like mongoose .populate() performs it's client side joins. By idendifying the "one" within each "many" instead, then you just pull in the related items without needing to $unwind the array first.

How can I save multiple documents concurrently in Mongoose/Node.js?

I know this is an old question, but it worries me that there are no properly correct answers here. Most answers just talk about iterating through all the documents and saving each of them individually, which is a BAD idea if you have more than a few documents, and the process gets repeated for even one in many requests.

MongoDB specifically has a batchInsert() call for inserting multiple documents, and this should be used from the native mongodb driver. Mongoose is built on this driver, and it doesn't have support for batch inserts. It probably makes sense as it is supposed to be a Object document modelling tool for MongoDB.

Solution: Mongoose comes with the native MongoDB driver. You can use that driver by requiring it require('mongoose/node_modules/mongodb') (not too sure about this, but you can always install the mongodb npm again if it doesn't work, but I think it should) and then do a proper batchInsert

How can I tell where mongoDB is storing data? (its not in the default /data/db!)

If you could somehow locate mongod.log and the do a grep over it

grep dbpath mongod.log

The value for dbpath is the data location for mongodb!! All the best :)

Mongodb find() query : return only unique values (no duplicates)

I think you can use db.collection.distinct(fields,query)

You will be able to get the distinct values in your case for NetworkID.

It should be something like this :


ECONNREFUSED error when connecting to mongodb from node.js

I had facing the same issue while writing a simple rest api using node.js eventually found out it was due to wifi blockage and security reason . try once connecting it using your mobile hotspot . if this be the reason it will get resolved immediately.

Mongoose (mongodb) batch insert?

Indeed, you can use the "create" method of Mongoose, it can contain an array of documents, see this example:

Candy.create({ candy: 'jelly bean' }, { candy: 'snickers' }, function (err, jellybean, snickers) {

The callback function contains the inserted documents. You do not always know how many items has to be inserted (fixed argument length like above) so you can loop through them:

var insertedDocs = [];
for (var i=1; i<arguments.length; ++i) {

Update: A better solution

A better solution would to use Candy.collection.insert() instead of Candy.create() - used in the example above - because it's faster (create() is calling on each item so it's slower).

See the Mongo documentation for more information:

(thanks to arcseldon for pointing this out)

How to connect Robomongo to MongoDB

Here's what we do:

  • Create a new connection, set the name, IP address and the appropriate port:

    Connection setup

  • Set up authentication, if required

    Authentication settings

  • Optionally set up other available settings for SSL, SSH, etc.

  • Save and connect

Shell Scripting: Using a variable to define a path

To add to the above correct answer :- For my case in shell, this code worked (working on sqoop)

--options-file  $ROOT_PATH/query.txt

How can I show the table structure in SQL Server query?

On SQL Server 2012, you can use the following stored procedure:

sp_columns '<table name>'

For example, given a database table named users:

sp_columns 'users'

Access Control Origin Header error using Axios in React Web throwing error in Chrome

!!! I had a similar problem and I found that in my case the withCredentials: true in the request was activating the CORS check while issuing the same in the header would avoid the check:

do not use withCredentials: true but set 'Access-Control-Allow-Credentials':true in the headers

How to download python from command-line?

apt-get install python2.7 will work on debian-like linuxes. The python website describes a whole bunch of other ways to get Python.

How can I get query string values in JavaScript?

I used this code (JavaScript) to get the what is passed through the URL:

function getUrlVars() {
            var vars = {};
            var parts = window.location.href.replace(/[?&]+([^=&]+)=([^&]*)/gi, function(m,key,value) {
                vars[key] = value;
            return vars;

Then to assign the value to a variable, you only have to specify which parameter you want to get, ie if the URL is

You can do this to get the values:

var getI = getUrlVars()["I"];
var getP = getUrlVars()["p"];
var getF = getUrlVars()["f"];

then the values would be:

getI = 1, getP = 2 and getF = 3

How to set a default value for an existing column

This will work in SQL Server:


How do I read / convert an InputStream into a String in Java?

Pure Java solution using Streams, works since Java 8.


// ...
public static String inputStreamToString(InputStream is) throws IOException {
    try (BufferedReader br = new BufferedReader(new InputStreamReader(is))) {
        return br.lines().collect(Collectors.joining(System.lineSeparator()));

As mentioned by Christoffer Hammarström below other answer it is safer to explicitly specify the Charset. I.e. The InputStreamReader constructor can be changes as follows:

new InputStreamReader(is, Charset.forName("UTF-8"))

How to multiply individual elements of a list with a number?

You can use built-in map function:

result = map(lambda x: x * P, S)

or list comprehensions that is a bit more pythonic:

result = [x * P for x in S]

Swift: Sort array of objects alphabetically

*import Foundation
import CoreData

extension Messages {

    @nonobjc public class func fetchRequest() -> NSFetchRequest<Messages> {
        return NSFetchRequest<Messages>(entityName: "Messages")

    @NSManaged public var text: String?
    @NSManaged public var date: Date?
    @NSManaged public var friends: Friends?


    //here arrMessage is the array you can sort this array as under bellow 

    var arrMessages = [Messages]()

    arrMessages.sort { (arrMessages1, arrMessages2) -> Bool in
     ! >!

How can I revert a single file to a previous version?

Git doesn't think in terms of file versions. A version in git is a snapshot of the entire tree.

Given this, what you really want is a tree that has the latest content of most files, but with the contents of one file the same as it was 5 commits ago. This will take the form of a new commit on top of the old ones, and the latest version of the tree will have what you want.

I don't know if there's a one-liner that will revert a single file to the contents of 5 commits ago, but the lo-fi solution should work: checkout master~5, copy the file somewhere else, checkout master, copy the file back, then commit.

How to read a configuration file in Java

app.config Sample Code

Source code:

Properties prop = new Properties();
String fileName = "app.config";
InputStream is = null;
try {
    is = new FileInputStream(fileName);
} catch (FileNotFoundException ex) {
try {
} catch (IOException ex) {


Properties Sample Code

How do you comment out code in PowerShell?


# Single line comment in Powershell

Multi-line comment in PowerShell V2+ 

Magento: get a static block as html in a phtml file

<?php echo $this->getLayout()->createBlock('cms/block')->setBlockId('my_static_block_name')->toHtml() ?>

and use this link for more

How can I use grep to show just filenames on Linux?

The standard option grep -l (that is a lowercase L) could do this.

From the Unix standard:

    (The letter ell.) Write only the names of files containing selected
    lines to standard output. Pathnames are written once per file searched.
    If the standard input is searched, a pathname of (standard input) will
    be written, in the POSIX locale. In other locales, standard input may be
    replaced by something more appropriate in those locales.

You also do not need -H in this case.

JPA and Hibernate - Criteria vs. JPQL or HQL

There is a difference in terms of performance between HQL and criteriaQuery, everytime you fire a query using criteriaQuery, it creates a new alias for the table name which does not reflect in the last queried cache for any DB. This leads to an overhead of compiling the generated SQL, taking more time to execute.

Regarding fetching strategies []

  • Criteria respects the laziness settings in your mappings and guarantees that what you want loaded is loaded. This means one Criteria query might result in several SQL immediate SELECT statements to fetch the subgraph with all non-lazy mapped associations and collections. If you want to change the "how" and even the "what", use setFetchMode() to enable or disable outer join fetching for a particular collection or association. Criteria queries also completely respect the fetching strategy (join vs select vs subselect).
  • HQL respects the laziness settings in your mappings and guarantees that what you want loaded is loaded. This means one HQL query might result in several SQL immediate SELECT statements to fetch the subgraph with all non-lazy mapped associations and collections. If you want to change the "how" and even the "what", use LEFT JOIN FETCH to enable outer-join fetching for a particular collection or nullable many-to-one or one-to-one association, or JOIN FETCH to enable inner join fetching for a non-nullable many-to-one or one-to-one association. HQL queries do not respect any fetch="join" defined in the mapping document.

Rails raw SQL example

I know this is old... But I was having the same problem today and found a solution:


If you want to instantiate the results:

  SELECT * FROM clients
  INNER JOIN orders ON = orders.client_id
  ORDER BY clients.created_at desc
# => [<Client id: 1, first_name: "Lucas" >, <Client id: 2, first_name: "Jan">...]


If you just want a hash of values:

Client.connection.select_all("SELECT first_name, created_at FROM clients
   WHERE id = '1'").to_hash
# => [
  {"first_name"=>"Rafael", "created_at"=>"2012-11-10 23:23:45.281189"},
  {"first_name"=>"Eileen", "created_at"=>"2013-12-09 11:22:35.221282"}

Result object:

select_all returns a result object. You can do magic things with it.

result = Post.connection.select_all('SELECT id, title, body FROM posts')
# Get the column names of the result:
# => ["id", "title", "body"]

# Get the record values of the result:
# => [[1, "title_1", "body_1"],
      [2, "title_2", "body_2"],

# Get an array of hashes representing the result (column => value):
# => [{"id" => 1, "title" => "title_1", "body" => "body_1"},
      {"id" => 2, "title" => "title_2", "body" => "body_2"},

# ActiveRecord::Result also includes Enumerable.
result.each do |row|
  puts row['title'] + " " + row['body']


  1. ActiveRecord - Findinig by SQL.
  2. Ruby on Rails - Active Record Result .

How do you get an iPhone's device name

From the UIDevice class:

As an example: [[UIDevice currentDevice] name];

The UIDevice is a class that provides information about the iPhone or iPod Touch device.

Some of the information provided by UIDevice is static, such as device name or system version.


Offical Documentation: Apple Developer Documentation > UIDevice Class Reference

How to use sed to replace only the first occurrence in a file?

Using FreeBSD ed and avoid ed's "no match" error in case there is no include statement in a file to be processed:

#include <stdio.h>
#include <stdlib.h>
#include <inttypes.h>

# using FreeBSD ed
# to avoid ed's "no match" error, see
# *emphasized text* 
cat <<-'EOF' | sed -e 's/^ *//' -e 's/ *$//' | ed -s <(echo "$teststr")
   ,g/# *include/u\
   #include "newfile.h"\

pyplot axes labels for subplots

plt.setp() will do the job:

# plot something
fig, axs = plt.subplots(3,3, figsize=(15, 8), sharex=True, sharey=True)
for i, ax in enumerate(axs.flat):
    ax.set_title(f'Title {i}')

# set labels
plt.setp(axs[-1, :], xlabel='x axis label')
plt.setp(axs[:, 0], ylabel='y axis label')

enter image description here

How to solve 'Redirect has been blocked by CORS policy: No 'Access-Control-Allow-Origin' header'?

$.get('' + c1v + '/' + c1b, function (data) { 
  // some code...

Just put "https" .

Lock down Microsoft Excel macro

Generate a protected application for Mac or Windows from your Excel spreadsheet using OfficeProtect with either AppProtect or QuickLicense/AddLicense. There is a demonstation video called "Protect Excel Spreedsheet" at

How do I compare two Integers?

I just encountered this in my code and it took me a while to figure it out. I was doing an intersection of two sorted lists and was only getting small numbers in my output. I could get it to work by using (x - y == 0) instead of (x == y) during comparison.

How to enter newline character in Oracle?

   dbms_output.put_line( 'hello' ||chr(13) || chr(10) || 'world' );

One-line list comprehension: if-else variants

You can do that with list comprehension too:

A=[[x*100, x][x % 2 != 0] for x in range(1,11)]
print A

How do I make curl ignore the proxy?

Add your proxy preferences into .curlrc

proxy =
noproxy = .dev,localhost,

This make all dev domains and local machine request ignore the proxy.

How to replace comma (,) with a dot (.) using java

Just use replace instead of replaceAll (which expects regex):

str = str.replace(",", ".");


str = str.replace(',', '.');

(replace takes as input either char or CharSequence, which is an interface implemented by String)

Also note that you should reassign the result

How to define optional methods in Swift protocol?

To illustrate the mechanics of Antoine's answer:

protocol SomeProtocol {
    func aMethod()

extension SomeProtocol {
    func aMethod() {

class protocolImplementingObject: SomeProtocol {


class protocolImplementingMethodOverridingObject: SomeProtocol {
    func aMethod() {

let noOverride = protocolImplementingObject()
let override = protocolImplementingMethodOverridingObject()

noOverride.aMethod() //prints "extensionImplementation"
override.aMethod() //prints "classImplementation"

Android Error Building Signed APK: keystore.jks not found for signing config 'externalOverride'

Click on choose existing and again choose the location where your jks file is located.

enter image description here

I hope this trick works for you.

How to add a tooltip to an svg graphic?

You can use the title element as Phrogz indicated. There are also some good tooltips like jQuery's Tipsy (which can be used to replace all title elements), Bob Monteverde's nvd3 or even the Twitter's tooltip from their Bootstrap

GitHub relative link in Markdown file

Just wanted to add this because none of the above solutions worked if target link is directory with spaces in it's name. If target link is a directory and it has space then even escaping space with \ doesn't render the link on Github. Only solution worked for me is using %20 for each space.

e.g.: if directory structure is this

      |----Dir A
      |----Dir B

To make link to Dir A in present in Top_dir you can do this:

[Dir 1](Cur_dir1/Dir%20A)

How to install JDK 11 under Ubuntu?

First check the default-jdk package, good chance it already provide you an OpenJDK >= 11.

Ubuntu 18.04 LTS +

So starting from Ubuntu 18.04 LTS it should be ok.

sudo apt update -qq
sudo apt install -yq default-jdk

note: don't forget to set JAVA_HOME

export JAVA_HOME=/usr/lib/jvm/default-java
mvn -version

Ubuntu 16.04 LTS

For Ubuntu 16.04 LTS, only openjdk-8-jdk is provided in the official repos so you need to find it in a ppa:

sudo add-apt-repository -y ppa:openjdk-r/ppa
sudo apt update -qq
sudo apt install -yq openjdk-11-jdk

note: don't forget to set JAVA_HOME

export JAVA_HOME=/usr/lib/jvm/java-11-openjdk-amd64
mvn -version

Clear text input on click with AngularJS

Just clear the scope model value on click event and it should do the trick for you.

<input type="text" ng-model="searchAll" />
<a class="clear" ng-click="searchAll = null">
    <span class="glyphicon glyphicon-remove"></span>

Or if you keep your controller's $scope function and clear it from there. Make sure you've set your controller correctly.

$scope.clearSearch = function() {
    $scope.searchAll = null;

sys.argv[1] meaning in script

I would like to note that previous answers made many assumptions about the user's knowledge. This answer attempts to answer the question at a more tutorial level.

For every invocation of Python, sys.argv is automatically a list of strings representing the arguments (as separated by spaces) on the command-line. The name comes from the C programming convention in which argv and argc represent the command line arguments.

You'll want to learn more about lists and strings as you're familiarizing yourself with Python, but in the meantime, here are a few things to know.

You can simply create a script that prints the arguments as they're represented. It also prints the number of arguments, using the len function on the list.

from __future__ import print_function
import sys
print(sys.argv, len(sys.argv))

The script requires Python 2.6 or later. If you call this script, you can invoke it with different arguments to see what happens.

> python
[''] 1

> python foo and bar
['', 'foo', 'and', 'bar'] 4

> python "foo and bar"
['', 'foo and bar'] 2

> python "foo and bar" and baz
['', 'foo and bar', 'and', 'baz'] 4

As you can see, the command-line arguments include the script name but not the interpreter name. In this sense, Python treats the script as the executable. If you need to know the name of the executable (python in this case), you can use sys.executable.

You can see from the examples that it is possible to receive arguments that do contain spaces if the user invoked the script with arguments encapsulated in quotes, so what you get is the list of arguments as supplied by the user.

Now in your Python code, you can use this list of strings as input to your program. Since lists are indexed by zero-based integers, you can get the individual items using the list[0] syntax. For example, to get the script name:

script_name = sys.argv[0] # this will always work.

Although interesting, you rarely need to know your script name. To get the first argument after the script for a filename, you could do the following:

filename = sys.argv[1]

This is a very common usage, but note that it will fail with an IndexError if no argument was supplied.

Also, Python lets you reference a slice of a list, so to get another list of just the user-supplied arguments (but without the script name), you can do

user_args = sys.argv[1:] # get everything after the script name

Additionally, Python allows you to assign a sequence of items (including lists) to variable names. So if you expect the user to always supply two arguments, you can assign those arguments (as strings) to two variables:

user_args = sys.argv[1:]
fun, games = user_args # len(user_args) had better be 2

So, to answer your specific question, sys.argv[1] represents the first command-line argument (as a string) supplied to the script in question. It will not prompt for input, but it will fail with an IndexError if no arguments are supplied on the command-line following the script name.

Formatting a field using ToText in a Crystal Reports formula field

if(isnull({uspRptMonthlyGasRevenueByGas;1.YearTotal})) = true then

The above logic should be what you are looking for.

Python/Json:Expecting property name enclosed in double quotes

I've checked your JSON data

{'': {'': [{'type': 'literal', 'value': "Anna's Homepage"}]}}

in and the results were:

Error: Parse error on line 1:
{   '
Expecting 'STRING', '}', got 'undefined'

modifying it to the following string solve the JSON error:

    "": {
        "": [{
            "type": "literal",
            "value": "Anna's Homepage"

jQuery get the image src

for full url use


for relative image url use


function showImgUrl(){_x000D_
  console.log('for full image url ' + $('#imageId').prop('src') );_x000D_
  console.log('for relative image url ' + $('#imageId').attr('src'));_x000D_
<script src=""></script>_x000D_
<img id='imageId' src='images/image1.jpg' height='50px' width='50px'/>_x000D_
<input type='button' onclick='showImgUrl()' value='click to see the url of the img' />

Javascript: console.log to html

You can override the default implementation of console.log()

(function () {
    var old = console.log;
    var logger = document.getElementById('log');
    console.log = function (message) {
        if (typeof message == 'object') {
            logger.innerHTML += (JSON && JSON.stringify ? JSON.stringify(message) : message) + '<br />';
        } else {
            logger.innerHTML += message + '<br />';

Demo: Fiddle

PHP function to make slug (URL string)

Since I've Seen a lot of methods here but I've found a simplest method for myself.Maybe it will help someone.

$slug = strtolower(preg_replace('/[^a-zA-Z0-9\-]/', '',preg_replace('/\s+/', '-', $string) ));

nodemon command is not recognized in terminal for node js server

Does it need to be installed globally? Do you need to be able to just run nodemon server.js ? If not, you could always just call it from your local project directory. Should be here:


How to increase Neo4j's maximum file open limit (ulimit) in Ubuntu?

What you are doing will not work for root user. Maybe you are running your services as root and hence you don't get to see the change.

To increase the ulimit for root user you should replace the * by root. * does not apply for root user. Rest is the same as you did. I will re-quote it here.

Add the following lines to the file: /etc/security/limits.conf

root soft  nofile 40000

root hard  nofile 40000

And then add following line in the file: /etc/pam.d/common-session

session required

This will update the ulimit for root user. As mentioned in comments, you may don't even have to reboot to see the change.

How to compute precision, recall, accuracy and f1-score for the multiclass case with scikit learn?

I think there is a lot of confusion about which weights are used for what. I am not sure I know precisely what bothers you so I am going to cover different topics, bear with me ;).

Class weights

The weights from the class_weight parameter are used to train the classifier. They are not used in the calculation of any of the metrics you are using: with different class weights, the numbers will be different simply because the classifier is different.

Basically in every scikit-learn classifier, the class weights are used to tell your model how important a class is. That means that during the training, the classifier will make extra efforts to classify properly the classes with high weights.
How they do that is algorithm-specific. If you want details about how it works for SVC and the doc does not make sense to you, feel free to mention it.

The metrics

Once you have a classifier, you want to know how well it is performing. Here you can use the metrics you mentioned: accuracy, recall_score, f1_score...

Usually when the class distribution is unbalanced, accuracy is considered a poor choice as it gives high scores to models which just predict the most frequent class.

I will not detail all these metrics but note that, with the exception of accuracy, they are naturally applied at the class level: as you can see in this print of a classification report they are defined for each class. They rely on concepts such as true positives or false negative that require defining which class is the positive one.

             precision    recall  f1-score   support

          0       0.65      1.00      0.79        17
          1       0.57      0.75      0.65        16
          2       0.33      0.06      0.10        17
avg / total       0.52      0.60      0.51        50

The warning

F1 score:/usr/local/lib/python2.7/site-packages/sklearn/metrics/ DeprecationWarning: The 
default `weighted` averaging is deprecated, and from version 0.18, 
use of precision, recall or F-score with multiclass or multilabel data  
or pos_label=None will result in an exception. Please set an explicit 
value for `average`, one of (None, 'micro', 'macro', 'weighted', 
'samples'). In cross validation use, for instance, 
scoring="f1_weighted" instead of scoring="f1".

You get this warning because you are using the f1-score, recall and precision without defining how they should be computed! The question could be rephrased: from the above classification report, how do you output one global number for the f1-score? You could:

  1. Take the average of the f1-score for each class: that's the avg / total result above. It's also called macro averaging.
  2. Compute the f1-score using the global count of true positives / false negatives, etc. (you sum the number of true positives / false negatives for each class). Aka micro averaging.
  3. Compute a weighted average of the f1-score. Using 'weighted' in scikit-learn will weigh the f1-score by the support of the class: the more elements a class has, the more important the f1-score for this class in the computation.

These are 3 of the options in scikit-learn, the warning is there to say you have to pick one. So you have to specify an average argument for the score method.

Which one you choose is up to how you want to measure the performance of the classifier: for instance macro-averaging does not take class imbalance into account and the f1-score of class 1 will be just as important as the f1-score of class 5. If you use weighted averaging however you'll get more importance for the class 5.

The whole argument specification in these metrics is not super-clear in scikit-learn right now, it will get better in version 0.18 according to the docs. They are removing some non-obvious standard behavior and they are issuing warnings so that developers notice it.

Computing scores

Last thing I want to mention (feel free to skip it if you're aware of it) is that scores are only meaningful if they are computed on data that the classifier has never seen. This is extremely important as any score you get on data that was used in fitting the classifier is completely irrelevant.

Here's a way to do it using StratifiedShuffleSplit, which gives you a random splits of your data (after shuffling) that preserve the label distribution.

from sklearn.datasets import make_classification
from sklearn.cross_validation import StratifiedShuffleSplit
from sklearn.metrics import accuracy_score, f1_score, precision_score, recall_score, classification_report, confusion_matrix

# We use a utility to generate artificial classification data.
X, y = make_classification(n_samples=100, n_informative=10, n_classes=3)
sss = StratifiedShuffleSplit(y, n_iter=1, test_size=0.5, random_state=0)
for train_idx, test_idx in sss:
    X_train, X_test, y_train, y_test = X[train_idx], X[test_idx], y[train_idx], y[test_idx], y_train)
    y_pred = svc.predict(X_test)
    print(f1_score(y_test, y_pred, average="macro"))
    print(precision_score(y_test, y_pred, average="macro"))
    print(recall_score(y_test, y_pred, average="macro"))    

Hope this helps.

Getting selected value of a combobox

The problem you have with the SelectedValue is not converting into integer. This is the main problem so usinge the following code snippet will help you:

int selectedValue;
bool parseOK = Int32.TryParse(cmb.SelectedValue.ToString(), out selectedValue);

Getting values from query string in an url using AngularJS $location

My fix is more simple, create a factory, and implement as one variable. For example

angular.module('myApp', [])_x000D_
// This a searchCustom factory. Copy the factory and implement in the controller_x000D_
.factory("searchCustom", function($http,$log){    _x000D_
    return {_x000D_
        valuesParams : function(params){_x000D_
            paramsResult = [];_x000D_
            params = params.replace('(', '').replace(')','').split("&");_x000D_
            for(x in params){_x000D_
                paramsKeyTmp = params[x].split("=");_x000D_
                // Si el parametro esta disponible anexamos al vector paramResult_x000D_
                if (paramsKeyTmp[1] !== '' && paramsKeyTmp[1] !== ' ' && _x000D_
                    paramsKeyTmp[1] !== null){          _x000D_
            return paramsResult;_x000D_
.controller("SearchController", function($scope, $http,$routeParams,$log,searchCustom){_x000D_
    $ctrl = this;_x000D_
    var valueParams = searchCustom.valuesParams($routeParams.value);_x000D_
    valueParams = valueParams.join('&');_x000D_
        method : "GET",_x000D_
        url: webservice+"q?"+valueParams_x000D_
    }).then( function successCallback(response){_x000D_
        data =;_x000D_
        $scope.cantEncontrados = data.length; _x000D_
        $scope.dataSearch = data;_x000D_
    } , function errorCallback(response){_x000D_
<body ng-app="myApp">_x000D_
<div ng-controller="SearchController">_x000D_
<form action="#" >_x000D_
                            <input ng-model="param1" _x000D_
                                   placeholder="param1" />_x000D_
                            <input  ng-model="param2" _x000D_
<!-- Implement in the html code _x000D_
(param1={{param1}}&param2={{param2}}) -> this is a one variable, the factory searchCustom split and restructure in the array params_x000D_
-->          _x000D_
                        <a href="#seach/(param1={{param1}}&param2={{param2}})">_x000D_
                            <buttom ng-click="searchData()" >Busqueda</buttom>_x000D_
                    </form> _x000D_

How to set javascript variables using MVC4 with Razor

It works if you do something like this:

var proID = @proID + 0;

Which produces code that is something like:

var proID = 4 + 0;

A bit odd for sure, but no more fake syntax errors at least. Sadly the errors are still reported in VS2013, so this hasn't been properly addressed (yet).

Android: why is there no maxHeight for a View?

Have you tried using the layout_weight value? If you set one it to a value greater than 0, it will stretch that view into the remaining space available.

If you had multiple views that needed to be stretched, then the value will become a weight between them.

So if you had two views both set to a layout_weight value of 1, then they would both stretch to fill in the space but they would both stretch to an equal amount of space. If you set one of them to the value of 2, then it would stretch twice as much as the other view.

Some more info here listed under Linear Layout.

An unhandled exception occurred during the execution of the current web request. ASP.NET

Here is the code with line 156, it has try and catch above it

    /// <summary>
    /// Execute a SQL Query statement, using the default SQL connection for the application
    /// </summary>
    /// <param name="query">SQL query to execute</param>
    /// <returns>DataTable of results</returns>
    public static DataTable Query(string query)
        DataTable results = new DataTable();
        string configConnectionString = "ApplicationServices";

        System.Configuration.Configuration WebConfig = System.Web.Configuration.WebConfigurationManager.OpenWebConfiguration("~/Web.config");
        System.Configuration.ConnectionStringSettings connString;

        if (WebConfig.ConnectionStrings.ConnectionStrings.Count > 0)
            connString = WebConfig.ConnectionStrings.ConnectionStrings[configConnectionString];

            if (connString != null)
                    using (SqlConnection conn = new SqlConnection(connString.ToString()))
                    using (SqlCommand cmd = new SqlCommand(query, conn))
                    using (SqlDataAdapter dataAdapter = new SqlDataAdapter(cmd))

                    return results;
                catch (Exception ex)
                    throw new SqlException(string.Format("SqlException occurred during query execution: ", ex));
                throw new SqlException(string.Format("Connection string for " + configConnectionString + "is null."));
            throw new SqlException(string.Format("No connection strings found in Web.config file."));

Logging levels - Logback - rule-of-thumb to assign log levels

I answer this coming from a component-based architecture, where an organisation may be running many components that may rely on each other. During a propagating failure, logging levels should help to identify both which components are affected and which are a root cause.

  • ERROR - This component has had a failure and the cause is believed to be internal (any internal, unhandled exception, failure of encapsulated dependency... e.g. database, REST example would be it has received a 4xx error from a dependency). Get me (maintainer of this component) out of bed.

  • WARN - This component has had a failure believed to be caused by a dependent component (REST example would be a 5xx status from a dependency). Get the maintainers of THAT component out of bed.

  • INFO - Anything else that we want to get to an operator. If you decide to log happy paths then I recommend limiting to 1 log message per significant operation (e.g. per incoming http request).

For all log messages be sure to log useful context (and prioritise on making messages human readable/useful rather than having reams of "error codes")

  • DEBUG (and below) - Shouldn't be used at all (and certainly not in production). In development I would advise using a combination of TDD and Debugging (where necessary) as opposed to polluting code with log statements. In production, the above INFO logging, combined with other metrics should be sufficient.

A nice way to visualise the above logging levels is to imagine a set of monitoring screens for each component. When all running well they are green, if a component logs a WARNING then it will go orange (amber) if anything logs an ERROR then it will go red.

In the event of an incident you should have one (root cause) component go red and all the affected components should go orange/amber.

How to Delete Session Cookie?

There are known issues with IE and Opera not removing session cookies when setting the expire date to the past (which is what the jQuery cookie plugin does)

This works fine in Safari and Mozilla/FireFox.

PHP Fatal error: Cannot access empty property


  1. You declared an array var $my_value = array();
  2. Pushed value into it $a->my_value[] = 'b';
  3. Assigned a string to variable. (so it is no more array) $a->set_value ('c');
  4. Tried to push a value into array, that does not exist anymore. (it's string) $a->my_class('d');

And your foreach wont work anymore.

What is the easiest/best/most correct way to iterate through the characters of a string in Java?

Two options

for(int i = 0, n = s.length() ; i < n ; i++) { 
    char c = s.charAt(i); 


for(char c : s.toCharArray()) {
    // process c

The first is probably faster, then 2nd is probably more readable.

HttpUtility does not exist in the current context

SLaks has the right answer... but let me be a bit more specific for people, like me, who are annoyed by this and can't find it right away :

Project -> Properties -> Application -> Target Framework -> select ".Net Framework 4"

the project will then save and reload.

Calculating a 2D Vector's Cross Product

Implementation 1 returns the magnitude of the vector that would result from a regular 3D cross product of the input vectors, taking their Z values implicitly as 0 (i.e. treating the 2D space as a plane in the 3D space). The 3D cross product will be perpendicular to that plane, and thus have 0 X & Y components (thus the scalar returned is the Z value of the 3D cross product vector).

Note that the magnitude of the vector resulting from 3D cross product is also equal to the area of the parallelogram between the two vectors, which gives Implementation 1 another purpose. In addition, this area is signed and can be used to determine whether rotating from V1 to V2 moves in an counter clockwise or clockwise direction. It should also be noted that implementation 1 is the determinant of the 2x2 matrix built from these two vectors.

Implementation 2 returns a vector perpendicular to the input vector still in the same 2D plane. Not a cross product in the classical sense but consistent in the "give me a perpendicular vector" sense.

Note that 3D euclidean space is closed under the cross product operation--that is, a cross product of two 3D vectors returns another 3D vector. Both of the above 2D implementations are inconsistent with that in one way or another.

Hope this helps...

How do I get logs from all pods of a Kubernetes replication controller?

I use this command.

kubectl -n <namespace> logs -f deployment/<app-name> --all-containers=true --since=10m

Easiest way to compare arrays in C#

For .NET 4.0 and higher you can compare elements in array or tuples via using StructuralComparisons type:

object[] a1 = { "string", 123, true };
object[] a2 = { "string", 123, true };

Console.WriteLine (a1 == a2);        // False (because arrays is reference types)
Console.WriteLine (a1.Equals (a2));  // False (because arrays is reference types)

IStructuralEquatable se1 = a1;
//Next returns True
Console.WriteLine (se1.Equals (a2, StructuralComparisons.StructuralEqualityComparer)); 

How to link to apps on the app store

Apple just announced the urls.

There are three types of App Store Short Links, in two forms, one for iOS apps, another for Mac Apps:

Company Name

iOS: for example,

Mac: for example,

App Name

iOS: for example,

Mac: for example,

App by Company

iOS: for example,

Mac: for example,

Most companies and apps have a canonical App Store Short Link. This canonical URL is created by changing or removing certain characters (many of which are illegal or have special meaning in a URL (for example, "&")).

To create an App Store Short Link, apply the following rules to your company or app name:

Remove all whitespace

Convert all characters to lower-case

Remove all copyright (©), trademark (™) and registered mark (®) symbols

Replace ampersands ("&") with "and"

Remove most punctuation (See Listing 2 for the set)

Replace accented and other "decorated" characters (ü, å, etc.) with their elemental character (u, a, etc.)

Leave all other characters as-is.

Listing 2 Punctuation characters that must be removed.


Below are some examples to demonstrate the conversion that takes place.

App Store

Company Name examples

Gameloft =>

Activision Publishing, Inc. =>

Chen's Photography & Software =>

App Name examples

Ocarina =>

Where’s My Perry? =>

Brain Challenge™ =>

Select current date by default in ASP.Net Calendar control

I was trying to make the calendar selects a date by default and highlights it for the user. However, i tried using all the options above but i only managed to set the calendar's selected date.

protected void Page_Load(object sender, EventArgs e)
    Calendar1.SelectedDate = DateTime.Today;

the previous code did NOT highlight the selection, although it set the SelectedDate to today.

However, to select and highlight the following code will work properly.

protected void Page_Load(object sender, EventArgs e)
    DateTime today = DateTime.Today;
    Calendar1.TodaysDate = today;
    Calendar1.SelectedDate = Calendar1.TodaysDate;

check this link:

Add colorbar to existing axis

Couldn't add this as a comment, but in case anyone is interested in using the accepted answer with subplots, the divider should be formed on specific axes object (rather than on the numpy.ndarray returned from plt.subplots)

import numpy as np
import matplotlib.pyplot as plt
from mpl_toolkits.axes_grid1 import make_axes_locatable
data = np.arange(100, 0, -1).reshape(10, 10)
fig, ax = plt.subplots(ncols=2, nrows=2)
for row in ax:
    for col in row:
        im = col.imshow(data, cmap='bone')
        divider = make_axes_locatable(col)
        cax = divider.append_axes('right', size='5%', pad=0.05)
        fig.colorbar(im, cax=cax, orientation='vertical')

Python socket receive - incoming packets always have a different size

You could try always sending the first 4 bytes of your data as data size and then read complete data in one shot. Use the below functions on both client and server-side to send and receive data.

def send_data(conn, data):
    serialized_data = pickle.dumps(data)
    conn.sendall(struct.pack('>I', len(serialized_data)))

def receive_data(conn):
    data_size = struct.unpack('>I', conn.recv(4))[0]
    received_payload = b""
    reamining_payload_size = data_size
    while reamining_payload_size != 0:
        received_payload += conn.recv(reamining_payload_size)
        reamining_payload_size = data_size - len(received_payload)
    data = pickle.loads(received_payload)

    return data

you could find sample program at

Maximum length for MD5 input/output

MD5 processes an arbitrary-length message into a fixed-length output of 128 bits, typically represented as a sequence of 32 hexadecimal digits.

Simple java program of pyramid

enter image description here

 import java.util.Scanner;
    public class Print {
        public static void main(String[] args) {
            int row,temp,c,n;
            Scanner s=new Scanner(;
            temp = n;
            for ( row = 1 ; row <= n ; row++ )
                  for ( c = 1 ; c < temp ; c++ )
                    System.out.print(" ");


                  for ( c = 1 ; c <= 2*row - 1 ; c++ )



How to change the font size on a matplotlib plot

The changes to the rcParams are very granular, most of the time all you want is just scaling all of the font sizes so they can be seen better in your figure. The figure size is a good trick but then you have to carry it for all of your figures. Another way (not purely matplotlib, or maybe overkill if you don't use seaborn) is to just set the font scale with seaborn:

sns.set_context('paper', font_scale=1.4)

DISCLAIMER: I know, if you only use matplotlib then probably you don't want to install a whole module for just scaling your plots (I mean why not) or if you use seaborn, then you have more control over the options. But there's the case where you have the seaborn in your data science virtual env but not using it in this notebook. Anyway, yet another solution.

Check for false

If you want it to check explicit for it to not be false (boolean value) you have to use

if(borrar() !== false)

But in JavaScript we usually use falsy and truthy and you could use


but then values 0, '', null, undefined, null and NaN would not generate the alert.

The following values are always falsy:

,0 (zero)
,'' or "" (empty string)

Everything else is truthy. That includes:

'0' (a string containing a single zero)
,'false' (a string containing the text “false”)
,[] (an empty array)
,{} (an empty object)
,function(){} (an “empty” function)


As an extra perk to convert any value to true or false (boolean type), use double exclamation mark:

!![] === true
!!'false' === true
!!false === false
!!undefined === false

How to "test" NoneType in python?

if variable is None:

if variable is not None:

Is there a git-merge --dry-run option?

This might be interesting: From the documentation:

If you tried a merge which resulted in complex conflicts and want to start over, you can recover with git merge --abort.

But you could also do it the naive (but slow) way:

rm -Rf /tmp/repository
cp -r repository /tmp/
cd /tmp/repository
git merge ...
...if successful, do the real merge. :)

(Note: It won't work just cloning to /tmp, you'd need a copy, in order to be sure that uncommitted changes will not conflict).

Angular 1 - get current URL parameters

Better would have been generate url like

and then use

var type=$;
var id=$;

and inject $location in controller.

How do I write outputs to the Log in Android?

Use android.util.Log and the static methods defined there (e.g., e(), w()).

What does "restore purchases" in In-App purchases mean?

You typically restore purchases with this code:

[[SKPaymentQueue defaultQueue] restoreCompletedTransactions];

It will reinvoke -paymentQueue:updatedTransactions on the observer(s) for the purchased items. This is useful for users who reinstall the app after deletion or install it on a different device.

Not all types of In-App purchases can be restored.

How to get just the parent directory name of a specific file

For Kotlin :

 fun getFolderName() {
            val uri: Uri
            val cursor: Cursor?
            uri = MediaStore.Audio.Media.EXTERNAL_CONTENT_URI
            val projection = arrayOf(MediaStore.Audio.AudioColumns.DATA)
            cursor = requireActivity().contentResolver.query(uri, projection, null, null, null)
            if (cursor != null) {
                column_index_data = cursor.getColumnIndexOrThrow(MediaStore.Audio.AudioColumns.DATA)
            while (cursor!!.moveToNext()) {
                absolutePathOfImage = cursor.getString(column_index_data)
                val fileName: String = File(absolutePathOfImage)

jQuery move to anchor location on page load

Did you tried JQuery's scrollTo method?

Or you can extend JQuery and add your custom mentod:

 scrollToMe: function () {
   var x = jQuery(this).offset().top - 100;
   jQuery('html,body').animate({scrollTop: x}, 400);

Then you can call this method like:


Visual Studio Code - Target of URI doesn't exist 'package:flutter/material.dart'

Open the project after deleting .idea folder and .dart_tool

ERROR in ./node_modules/css-loader?

try using

npm rebuild node-sass

How to set value of input text using jQuery

Using jQuery, we can use the following code:

Select by input name:

$('input[name="textboxname"]').val('some value')

Select by input class:

$('input[type=text].textboxclass').val('some value')

Select by input id:

$('#textboxid').val('some value')

CSS Progress Circle

Another pure css based solution that is based on two clipped rounded elements that i rotate to get to the right angle:

That's the basic css that enables it:

.clip1 {
.slice1 {



and the js rotates it as required.

quite easy to understand..

Hope it helps, Maayan

How to run .NET Core console app from the command line

You can also run your app like any other console applications but only after the publish.

Let's suppose you have the simple console app named MyTestConsoleApp. Open the package manager console and run the following command:

dotnet publish -c Debug -r win10-x64 

-c flag mean that you want to use the debug configuration (in other case you should use Release value) - r flag mean that your application will be runned on Windows platform with x64 architecture.

When the publish procedure will be finished your will see the *.exe file located in your bin/Debug/publish directory.

Now you can call it via command line tools. So open the CMD window (or terminal) move to the directory where your *.exe file is located and write the next command:

>> MyTestConsoleApp.exe argument-list

For example:

>> MyTestConsoleApp.exe --input some_text -r true

IE11 Document mode defaults to IE7. How to reset?

By default, IE displays webpages in the Intranet zone in compatibility view. To change this:

  • Press Alt to display the IE menu.
  • Choose Tools | Compatibility View settings
  • Remove the checkmark next to Display intranet sites in Compatibility View.
  • Choose Close.

At this point, IE should rely on the webpage itself (or any relevant group policies) to determine the compatibility settings for your Intranet webpages.

Note that certain sites may no longer function correctly after making this change. You can use the same dialog box to add specific sites to enable compatibility view when needed.

How open PowerShell as administrator from the run window

The easiest way to open an admin Powershell window in Windows 10 (and Windows 8) is to add a "Windows Powershell (Admin)" option to the "Power User Menu". Once this is done, you can open an admin powershell window via Win+X,A or by right-clicking on the start button and selecting "Windows Powershell (Admin)":

[Windows 10/Windows 8 Power User menu with "Windows Powershell (Admin)

Here's where you replace the "Command Prompt" option with a "Windows Powershell" option:

[Taskbar and Start Menu Properties: "Replace Command Prompt With Windows Powershell"

What is SaaS, PaaS and IaaS? With examples

Difference between IaaS PaaS & SaaS

In the following tabular format we will be explaining the difference in context of

  pizza as a service 

Using gdb to single-step assembly code outside specified executable causes error "cannot find bounds of current function"

You can use stepi or nexti (which can be abbreviated to si or ni) to step through your machine code.

mySQL :: insert into table, data from another table?


Using NSLog for debugging

NSLog(@"%@", digit);

what is shown in console?

Override console.log(); for production

After read a lot of posts, I made my own solution as follow:


function extendConsole() {
    "use strict";
    try {
        var disabledConsoles = {};

        console.enable = function (level, enabled) {
            // Prevent errors in browsers without console[level]
            if (window.console === 'undefined' || !window.console || window.console === null) {
                window.console = {};
            if (window.console[level] === 'undefined' || !window.console[level] || window.console[level] == null) {
                window.console[level] = function() {};

            if (enabled) {
                if (disabledConsoles[level]) {
                    window.console[level] = disabledConsoles[level];
      "console." + level + "() was enabled.");
            } else {
                disabledConsoles[level] = window.console[level];
                window.console[level] = function () { };
      "console." + level + "() was disabled.");
    } catch (exception) {
        console.error("extendConsole() threw an exception.");


console.enable("debug", window.debugMode);


Rename a column in MySQL

Rename column name in mysql

alter table categories change  type  category_type varchar(255);

Remove characters from a String in Java

Java strings are immutable. But you has many options:

You can use:

The StringBuilder class instead, so you can remove everything you want and control your string.

The replace method.

And you can actually use a loop £:

Python Requests library redirect new url

This is answering a slightly different question, but since I got stuck on this myself, I hope it might be useful for someone else.

If you want to use allow_redirects=False and get directly to the first redirect object, rather than following a chain of them, and you just want to get the redirect location directly out of the 302 response object, then r.url won't work. Instead, it's the "Location" header:

r = requests.get('', allow_redirects=False)
r.status_code  # 302
r.url  #, not https.
r.headers['Location']  # -- the redirect destination

Which characters are valid/invalid in a JSON key name?

Unicode codepoints U+D800 to U+DFFF must be avoided: they are invalid in Unicode because they are reserved for UTF-16 surrogate pairs. Some JSON encoders/decoders will replace them with U+FFFD. See for example how the Go language and its JSON library deals with them.

So avoid "\uD800" to "\uDFFF" alone (not in surrogate pairs).

How do I get the list of keys in a Dictionary?

To get list of all keys

using System.Linq;
List<String> myKeys = myDict.Keys.ToList();

System.Linq is supported in .Net framework 3.5 or above. See the below links if you face any issue in using System.Linq

Visual Studio Does not recognize System.Linq

System.Linq Namespace

converting json to string in python

There are other differences. For instance, {'time':} cannot be serialized to JSON, but can be converted to string. You should use one of these tools depending on the purpose (i.e. will the result later be decoded).

C# loop - break vs. continue

A really easy way to understand this is to place the word "loop" after each of the keywords. The terms now make sense if they are just read like everyday phrases.

break loop - looping is broken and stops.

continue loop - loop continues to execute with the next iteration.

View JSON file in Browser

For Firefox's Bookmarks JSON files, use this excellent Bookmarklet:

javascript:(function(){var E=document.getElementsByTagName('PRE')[0],T=E.innerHTML,i=0,r1,r2;t=new Array();while(/("uri":"([^"]*)")/g.exec(T)){r1=RegExp.$1;r2=RegExp.$2;if(/^https?:/.exec(r2)){t[i++]='['+(i)+']:<a href='+r2+'>'+r2+'<\/a>';}}with({for(i=0;t[i];i++)write(t[i]+'<br>');close();}})();

Source: "alterna" from

jQuery click event not working after adding class

You should use the following:

$('#gentab').on('click', 'a.tabclick', function(event) {
    var liId = $(this).closest("li").attr("id");

This will attach your event to any anchors within the #gentab element, reducing the scope of having to check the whole document element tree and increasing efficiency.

Reading Email using Pop3 in C#

downloading the email via the POP3 protocol is the easy part of the task. The protocol is quite simple and the only hard part could be advanced authentication methods if you don't want to send a clear text password over the network (and cannot use the SSL encrypted communication channel). See RFC 1939: Post Office Protocol - Version 3 and RFC 1734: POP3 AUTHentication command for details.

The hard part comes when you have to parse the received email, which means parsing MIME format in most cases. You can write quick&dirty MIME parser in a few hours or days and it will handle 95+% of all incoming messages. Improving the parser so it can parse almost any email means:

  • getting email samples sent from the most popular mail clients and improve the parser in order to fix errors and RFC misinterpretations generated by them.
  • Making sure that messages violating RFC for message headers and content will not crash your parser and that you will be able to read every readable or guessable value from the mangled email
  • correct handling of internationalization issues (e.g. languages written from righ to left, correct encoding for specific language etc)
  • Attachments and hierarchical message item tree as seen in "Mime torture email sample"
  • S/MIME (signed and encrypted emails).
  • and so on

Debugging a robust MIME parser takes months of work. I know, because I was watching my friend writing one such parser for the component mentioned below and was writing a few unit tests for it too ;-)

Back to the original question.

Following code taken from our POP3 Tutorial page and links would help you:

// create client, connect and log in 
Pop3 client = new Pop3();
client.Login("username", "password");

// get message list 
Pop3MessageCollection list = client.GetMessageList();

if (list.Count == 0)
    Console.WriteLine("There are no messages in the mailbox.");
    // download the first message 
    MailMessage message = client.GetMailMessage(list[0].SequenceNumber);


Byte Array to Hex String

If you have a numpy array, you can do the following:

>>> import numpy as np
>>> a = np.array([133, 53, 234, 241])
>>> a.astype(np.uint8).data.hex()

Why doesn't Python have a sign function?

"copysign" is defined by IEEE 754, and part of the C99 specification. That's why it's in Python. The function cannot be implemented in full by abs(x) * sign(y) because of how it's supposed to handle NaN values.

>>> import math
>>> math.copysign(1, float("nan"))
>>> math.copysign(1, float("-nan"))
>>> math.copysign(float("nan"), 1)
>>> math.copysign(float("nan"), -1)
>>> float("nan") * -1
>>> float("nan") * 1

That makes copysign() a more useful function than sign().

As to specific reasons why IEEE's signbit(x) is not available in standard Python, I don't know. I can make assumptions, but it would be guessing.

The math module itself uses copysign(1, x) as a way to check if x is negative or non-negative. For most cases dealing with mathematical functions that seems more useful than having a sign(x) which returns 1, 0, or -1 because there's one less case to consider. For example, the following is from Python's math module:

static double
m_atan2(double y, double x)
        if (Py_IS_NAN(x) || Py_IS_NAN(y))
                return Py_NAN;
        if (Py_IS_INFINITY(y)) {
                if (Py_IS_INFINITY(x)) {
                        if (copysign(1., x) == 1.)
                                /* atan2(+-inf, +inf) == +-pi/4 */
                                return copysign(0.25*Py_MATH_PI, y);
                                /* atan2(+-inf, -inf) == +-pi*3/4 */
                                return copysign(0.75*Py_MATH_PI, y);
                /* atan2(+-inf, x) == +-pi/2 for finite x */
                return copysign(0.5*Py_MATH_PI, y);

There you can clearly see that copysign() is a more effective function than a three-valued sign() function.

You wrote:

If I were a python designer, I would been the other way around: no cmp() builtin, but a sign()

That means you don't know that cmp() is used for things besides numbers. cmp("This", "That") cannot be implemented with a sign() function.

Edit to collate my additional answers elsewhere:

You base your justifications on how abs() and sign() are often seen together. As the C standard library does not contain a 'sign(x)' function of any sort, I don't know how you justify your views. There's an abs(int) and fabs(double) and fabsf(float) and fabsl(long) but no mention of sign. There is "copysign()" and "signbit()" but those only apply to IEEE 754 numbers.

With complex numbers, what would sign(-3+4j) return in Python, were it to be implemented? abs(-3+4j) return 5.0. That's a clear example of how abs() can be used in places where sign() makes no sense.

Suppose sign(x) were added to Python, as a complement to abs(x). If 'x' is an instance of a user-defined class which implements the __abs__(self) method then abs(x) will call x.__abs__(). In order to work correctly, to handle abs(x) in the same way then Python will have to gain a sign(x) slot.

This is excessive for a relatively unneeded function. Besides, why should sign(x) exist and nonnegative(x) and nonpositive(x) not exist? My snippet from Python's math module implementation shows how copybit(x, y) can be used to implement nonnegative(), which a simple sign(x) cannot do.

Python should support have better support for IEEE 754/C99 math function. That would add a signbit(x) function, which would do what you want in the case of floats. It would not work for integers or complex numbers, much less strings, and it wouldn't have the name you are looking for.

You ask "why", and the answer is "sign(x) isn't useful." You assert that it is useful. Yet your comments show that you do not know enough to be able to make that assertion, which means you would have to show convincing evidence of its need. Saying that NumPy implements it is not convincing enough. You would need to show cases of how existing code would be improved with a sign function.

And that it outside the scope of StackOverflow. Take it instead to one of the Python lists.

413 Request Entity Too Large - File Upload Issue

First edit the Nginx configuration file (nginx.conf)

Location: sudo nano /etc/nginx/nginx.conf

Add following codes:

http {
        client_max_body_size 100M;

Then Add the following lines in PHP configuration file(php.ini)

Location: sudo gedit /etc/php5/fpm/php.ini

Add following codes:

memory_limit = 128M 
post_max_size = 20M  
upload_max_filesize = 10M

.htaccess not working apache

  1. Enable Apache mod_rewrite module

    a2enmod rewrite

  2. add the following code to /etc/apache2/sites-available/default

    AllowOverride All

  3. Restart apache

    /etc/init.d/apache2 restart

What are the differences between char literals '\n' and '\r' in Java?

\n is a line feed (LF) character, character code 10. \r is a carriage return (CR) character, character code 13. What they do differs from system to system. On Windows, for instance, lines in text files are terminated using CR followed immediately by LF (e.g., CRLF). On Unix systems and their derivatives, only LF is used. (Macs prior to Mac OS X used CR, but Mac OS X is a *nix derivative and so uses LF.)

In the old days, LF literally did just a line feed on printers (moving down one line without moving where you are horizonally on the page), and CR similarly moved back to the beginning of the line without moving the paper up, hence some systems (like Windows) sending CR (return to the left-hand side) and LF (and feed the paper up).

Because of all this confusion, some output targets will accept multiple line break sequences, so you could see the same effect from either character depending on what you're outputting to.

Count number of files within a directory in Linux?

this is one:

ls -l . | egrep -c '^-'


ls -1 | wc -l

Which means: ls: list files in dir

-1: (that's a ONE) only one entry per line. Change it to -1a if you want hidden files too

|: pipe output onto...

wc: "wordcount"

-l: count lines.

How to bring a window to the front?

The rules governing what happens when you .toFront() a JFrame are the same in windows and in linux :

-> if a window of the existing application is currently the focused window, then focus swaps to the requested window -> if not, the window merely flashes in the taskbar


-> new windows automatically get focus

So let's exploit this ! You want to bring a window to the front, how to do it ? Well :

  1. Create an empty non-purpose window
  2. Show it
  3. Wait for it to show up on screen (setVisible does that)
  4. When shown, request focus for the window you actually want to bring the focus to
  5. hide the empty window, destroy it

Or, in java code :

// unminimize if necessary
this.setExtendedState(this.getExtendedState() & ~JFrame.ICONIFIED);

// don't blame me, blame my upbringing
// or better yet, blame java !
final JFrame newFrame = new JFrame();
newFrame.add(new JLabel("boembabies, is this in front ?"));



// I'm not 100% positive invokeLater is necessary, but it seems to be on
// WinXP. I'd be lying if I said I understand why
SwingUtilities.invokeLater(new Runnable() {
  @Override public void run() {

Update records using LINQ

You have two options as far as I know:

  1. Perform your query, iterate over it to modify the entities, then call SaveChanges().
  2. Execute a SQL command like you mentioned at the top of your question. To see how to do this, take a look at this page.

If you use option 2, you're losing some of the abstraction that the Entity Framework gives you, but if you need to perform a very large update, this might be the best choice for performance reasons.

JavaScript and getElementById for multiple elements with the same ID

I know this is an old question and that an HTML page with multiple IDs is invalid. However, I ran into this issues while needing to scrape and reformat someone else's API's HTML documentation that contained duplicate IDs (invalid HTML).

So for anyone else, here is the code I used to work around the issue using querySelectorAll:

var elms = document.querySelectorAll("[id='duplicateID']");

for(var i = 0; i < elms.length; i++) 
  elms[i].style.display='none'; // <-- whatever you need to do here.

Add a pipe separator after items in an unordered list unless that item is the last on a line

Slightly modified SCSS version which gives you control of the pipe | size and will eliminate padding from first and last list items while respects borders.

$pipe-list-height: 20px;
$pipe-list-padding: 15px;

.pipe-list {
    position: relative;
    overflow: hidden;
    height: $pipe-list-height;

    > ul {
        display: flex;
        flex-direction: row;

        > li {
            position: relative;
            padding: 0 $pipe-list-padding;

            &:after {
                content: " ";
                position: absolute;
                border-right: 1px solid gray;
                top: 10%;
                right: 0;
                height: 75%;
                margin-top: auto;
                margin-bottom: auto;

            &:first-child {
                padding-left: 0;

            &:last-child {
                padding-right: 0;

                &:after {
                    border-right: none;
<div class="pipe-list">

TypeError: 'dict' object is not callable

The syntax for accessing a dict given a key is number_map[int(x)]. number_map(int(x)) would actually be a function call but since number_map is not a callable, an exception is raised.

Check if a string has white space

A few others have posted answers. There are some obvious problems, like it returns false when the Regex passes, and the ^ and $ operators indicate start/end, whereas the question is looking for has (any) whitespace, and not: only contains whitespace (which the regex is checking).

Besides that, the issue is just a typo.

Change this...

var reWhiteSpace = new RegExp("/^\s+$/");

To this...

var reWhiteSpace = new RegExp("\\s+");

When using a regex within RegExp(), you must do the two following things...

  • Omit starting and ending / brackets.
  • Double-escape all sequences code, i.e., \\s in place of \s, etc.

Full working demo from source code....

$(document).ready(function(e) { function hasWhiteSpace(s) {
        var reWhiteSpace = new RegExp("\\s+");
        // Check for white space
        if (reWhiteSpace.test(s)) {
            //alert("Please Check Your Fields For Spaces");
            return 'true';
        return 'false';
  $('#whitespace1').html(hasWhiteSpace(' '));
<script src=""></script>
" ": <span id="whitespace1"></span><br>
"123": <span id="whitespace2"></span>

Oracle "Partition By" Keyword

the over partition keyword is as if we are partitioning the data by client_id creation a subset of each client id

select client_id, operation_date,
       row_number() count(*) over (partition by client_id order by client_id ) as operationctrbyclient
from client_operations e
order by e.client_id;

this query will return the number of operations done by the client_id

Update row with data from another row in the same table

If you just need to insert a new row with a data from another row,

    insert into ORDER_ITEM select * from ORDER_ITEM where ITEM_NUMBER =123;

HighCharts Hide Series Name from the Legend

Looks like HighChart 2.2.0 has resolved this issue. I tried it here with the same code you have, and the first series is hidden now. Could you try it with HighChart 2.2.0?

How to create a directory using Ansible

You can directly run the command and create directly using ansible

ansible -v targethostname -m shell -a "mkdir /srv/www" -u targetuser


ansible -v targethostname -m file -a "path=/srv/www state=directory" -u targetuser

Headers and client library minor version mismatch

My hosting company told me to fix this by deactivating "mysqli" and activating "nd_mysqli" in the php extensions.

The error is gone, but I don't have the knowledge to understand if this is the right method to fix this.

How can I map "insert='false' update='false'" on a composite-id key-property which is also used in a one-to-many FK?

"Dino TW" has provided the link to the comment Hibernate Mapping Exception : Repeated column in mapping for entity which has the vital information.

The link hints to provide "inverse=true" in the set mapping, I tried it and it actually works. It is such a rare situation wherein a Set and Composite key come together. Make inverse=true, we leave the insert & update of the table with Composite key to be taken care by itself.

Below can be the required mapping,

<class name="com.example.CompanyEntity" table="COMPANY">
    <id name="id" column="COMPANY_ID"/>
    <set name="names" inverse="true" table="COMPANY_NAME" cascade="all-delete-orphan" fetch="join" batch-size="1" lazy="false">
        <key column="COMPANY_ID" not-null="true"/>
        <one-to-many entity-name="vendorName"/>

A terminal command for a rooted Android to remount /System as read/write

This is what works on my first generation Droid X with Android version 2.3.4. I suspect that this will be universal. Steps:

  1. root system and install su.

  2. Install busybox

  3. Install a terminal program.

  4. to mount system rw first su then

    busybox mount -o rw,remount system
  5. To remount ro

    busybox mount -o ro,remount system

Note that there are no slashes on "system".

How to add elements of a Java8 stream into an existing List

Lets say we have existing list, and gonna use java 8 for this activity `

import java.util.*;

public class AddingArray {

    public void addArrayInList(){
        List<Integer> list = Arrays.asList(3, 7, 9);

   // And we have an array of Integer type 

        int nums[] = {4, 6, 7};

   //Now lets add them all in list
   // converting array to a list through stream and adding that list to previous list
        list.addAll( -> 


Remove specific commit

i would see a very simple way

git reset --hard HEAD <YOUR COMMIT ID>

and then reset remote branch

git push origin -f

How to delete SQLite database from Android programmatically

I used Android database delete method and database removed successfully

public bool DeleteDatabase()
            var dbName = "TenderDb.db";
            var documentDirectoryPath = System.Environment.GetFolderPath(System.Environment.SpecialFolder.Personal);
            var path = Path.Combine(documentDirectoryPath, dbName);
            return Android.Database.Sqlite.SQLiteDatabase.DeleteDatabase(new Java.IO.File(path));

How to show progress bar while loading, using ajax

Here is an example that's working for me with MVC and Javascript in the Razor. The first function calls an action via ajax on my controller and passes two parameters.

        function redirectToAction(var1, var2)

                var url = '../actionnameinsamecontroller/' + routeId;

                    type: "GET",
                    url: url,
                    data: { param1: var1, param2: var2 },
                    dataType: 'html',
                    success: function(){
                    error: function(xhr, ajaxOptions, thrownError){


Use the ajaxStart to start your progress bar code.

                // showing a modal

                var i = 0;
                var timeout = 750;

                (function progressbar()
                    if(i < 1000)
                        // some code to make the progress bar move in a loop with a timeout to 
                        // control the speed of the bar
                        setTimeout(progressbar, timeout);

When the process completes close the progress bar

            // hide the progress bar

Replace HTML page with contents retrieved via AJAX

The simplest way is to set the new HTML content using:;

Strings as Primary Keys in SQL Database

Indices imply lots of comparisons.

Typically, strings are longer than integers and collation rules may be applied for comparison, so comparing strings is usually more computationally intensive task than comparing integers.

Sometimes, though, it's faster to use a string as a primary key than to make an extra join with a string to numerical id table.

Rails ActiveRecord date between

There should be a default active record behavior on this I reckon. Querying dates is hard, especially when timezones are involved.

Anyway, I use:

  scope :between, ->(start_date=nil, end_date=nil) {
    if start_date && end_date
      where("#{self.table_name}.created_at BETWEEN :start AND :end", start: start_date.beginning_of_day, end: end_date.end_of_day)
    elsif start_date
      where("#{self.table_name}.created_at >= ?", start_date.beginning_of_day)
    elsif end_date
      where("#{self.table_name}.created_at <= ?", end_date.end_of_day)

How do I remove an item from a stl vector with a certain value?


C++ community has heard your request :)


C++ 20 provides an easy way of doing it now. It gets as simple as :

#include <vector>
vector<int> cnt{5, 0, 2, 8, 0, 7};
std::erase(cnt, 0);

You should check out std::erase and std::erase_if.

Not only will it remove all elements of the value (here '0'), it will do it in O(n) time complexity. Which is the very best you can get.

If your compiler does not support C++ 20, you should use erase-remove idiom:

#include <algorithm>
vec.erase(std::remove(vec.begin(), vec.end(), 0), vec.end());

How do I store the select column in a variable?

This is how to assign a value to a variable:

  FROM dbo.Employee

However, the above query is returning more than one value. You'll need to add a WHERE clause in order to return a single Id value.

JAVA Unsupported major.minor version 51.0

This is because of a higher JDK during compile time and lower JDK during runtime. So you just need to update your JDK version, possible to JDK 7

You may also check Unsupported major.minor version 51.0

ADB server version (36) doesn't match this client (39) {Not using Genymotion}

In my case this error occured when I set up my environment adb path as ~/.android-sdk/platform-tools (which happens when e.g. android-platform-tools is installed via homebrew), which version was 36, but Android Studio project has Android SDK next path ~/Library/Android/sdk which adb version was 39.

I have changed my PATH to platform-tools to ~/Library/Android/sdk/platform-tools and error was solved

What does "Failure [INSTALL_FAILED_OLDER_SDK]" mean in Android Studio?

I fixed this problem.The device system version is older then the sdk minSdkVersion? I just modified the minSdkVersion from android_L to 19 to target my nexus 4.4.4.

buildscript {
    repositories {
    dependencies {
        classpath ''
apply plugin: ''

repositories {

android {
    **compileSdkVersion 'android-L'** modified to 19
    buildToolsVersion "20.0.0"

    defaultConfig {
        applicationId "com.antwei.uiframework.ui"
        minSdkVersion 14
        targetSdkVersion 'L'
        versionCode 1
        versionName "1.0"
    buildTypes {
        release {
            runProguard false
            proguardFiles getDefaultProguardFile('proguard-android.txt'), ''

dependencies {
    compile fileTree(dir: 'libs', include: ['*.jar'])
    **compile ''** modified to compile ''

how to modified the value by ide. select file->Project Structure -> Facets -> android-gradle and then modified the compile Sdk Version from android_L to 19

sorry I don't have enough reputation to add pictures

Unable to negotiate with XX.XXX.XX.XX: no matching host key type found. Their offer: ssh-dss

How would one specify multiple algorithms? I ask because git just updated on my work laptop, (Windows 10, using the official Git for Windows build,) and I got this error when I tried to push a project branch to my Azure DevOps remote. I tried to push --set-upstream and got this:

Unable to negotiate with port 22: no matching key exchange method found. Their offer: diffie-hellman-group1-sha1,diffie-hellman-group14-sha1
fatal: Could not read from remote repository.

Please make sure you have the correct access rights
and the repository exists.

So how would one implement the suggestions above allowing for both of those? (As a quick get-it-done, I used @golvok's solution with group14 and it worked, but I really don't know if 1 or 14 is better, etc.)

C non-blocking keyboard input

As already stated, you can use sigaction to trap ctrl-c, or select to trap any standard input.

Note however that with the latter method you also need to set the TTY so that it's in character-at-a-time rather than line-at-a-time mode. The latter is the default - if you type in a line of text it doesn't get sent to the running program's stdin until you press enter.

You'd need to use the tcsetattr() function to turn off ICANON mode, and probably also disable ECHO too. From memory, you also have to set the terminal back into ICANON mode when the program exits!

Just for completeness, here's some code I've just knocked up (nb: no error checking!) which sets up a Unix TTY and emulates the DOS <conio.h> functions kbhit() and getch():

#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <sys/select.h>
#include <termios.h>

struct termios orig_termios;

void reset_terminal_mode()
    tcsetattr(0, TCSANOW, &orig_termios);

void set_conio_terminal_mode()
    struct termios new_termios;

    /* take two copies - one for now, one for later */
    tcgetattr(0, &orig_termios);
    memcpy(&new_termios, &orig_termios, sizeof(new_termios));

    /* register cleanup handler, and set the new terminal mode */
    tcsetattr(0, TCSANOW, &new_termios);

int kbhit()
    struct timeval tv = { 0L, 0L };
    fd_set fds;
    FD_SET(0, &fds);
    return select(1, &fds, NULL, NULL, &tv);

int getch()
    int r;
    unsigned char c;
    if ((r = read(0, &c, sizeof(c))) < 0) {
        return r;
    } else {
        return c;

int main(int argc, char *argv[])

    while (!kbhit()) {
        /* do some work */
    (void)getch(); /* consume the character */

Directly assigning values to C Pointers

In the first example, ptr has not been initialized, so it points to an unspecified memory location. When you assign something to this unspecified location, your program blows up.

In the second example, the address is set when you say ptr = &q, so you're OK.

How can I get the baseurl of site?

I'm using following code from Application_Start

String baseUrl = Path.GetDirectoryName(HttpContext.Current.Request.Url.OriginalString);

How to log cron jobs?

cron already sends the standard output and standard error of every job it runs by mail to the owner of the cron job.

You can use MAILTO=recipient in the crontab file to have the emails sent to a different account.

For this to work, you need to have mail working properly. Delivering to a local mailbox is usually not a problem (in fact, chances are ls -l "$MAIL" will reveal that you have already been receiving some) but getting it off the box and out onto the internet requires the MTA (Postfix, Sendmail, what have you) to be properly configured to connect to the world.

If there is no output, no email will be generated.

A common arrangement is to redirect output to a file, in which case of course the cron daemon won't see the job return any output. A variant is to redirect standard output to a file (or write the script so it never prints anything - perhaps it stores results in a database instead, or performs maintenance tasks which simply don't output anything?) and only receive an email if there is an error message.

To redirect both output streams, the syntax is

42 17 * * * script >>stdout.log 2>>stderr.log

Notice how we append (double >>) instead of overwrite, so that any previous job's output is not replaced by the next one's.

As suggested in many answers here, you can have both output streams be sent to a single file; replace the second redirection with 2>&1 to say "standard error should go wherever standard output is going". (But I don't particularly endorse this practice. It mainly makes sense if you don't really expect anything on standard output, but may have overlooked something, perhaps coming from an external tool which is called from your script.)

cron jobs run in your home directory, so any relative file names should be relative to that. If you want to write outside of your home directory, you obviously need to separately make sure you have write access to that destination file.

A common antipattern is to redirect everything to /dev/null (and then ask Stack Overflow to help you figure out what went wrong when something is not working; but we can't see the lost output, either!)

From within your script, make sure to keep regular output (actual results, ideally in machine-readable form) and diagnostics (usually formatted for a human reader) separate. In a shell script,

echo "$results"  # regular results go to stdout
echo "$0: something went wrong" >&2

Some platforms (and e.g. GNU Awk) allow you to use the file name /dev/stderr for error messages, but this is not properly portable; in Perl, warn and die print to standard error; in Python, write to sys.stderr, or use logging; in Ruby, try $stderr.puts. Notice also how error messages should include the name of the script which produced the diagnostic message.

How do you beta test an iphone app?

In 2014 along with iOS 8 and XCode 6 apple introduced Beta Testing of iOS App using iTunes Connect.

You can upload your build to iTunes connect and invite testers using their mail id's. You can invite up to 2000 external testers using just their email address. And they can install the beta app through TestFlight

Could not find default endpoint element

"This error can arise if you are calling the service in a class library and calling the class library from another project."

"In this case you will need to include the WS configuration settings into the main projects app.config if its a winapp or web.config if its a web app. This is the way to go even with PRISM and WPF/Silverlight."

Yes, but if you can't change main project (Orchard CMS for example), you can keep WCF service config in your project.

You need to create a service helper with client generation method:

public static class ServiceClientHelper
    public static T GetClient<T>(string moduleName) where T : IClientChannel
        var channelType = typeof(T);
        var contractType = channelType.GetInterfaces().First(i => i.Namespace == channelType.Namespace);
        var contractAttribute = contractType.GetCustomAttributes(typeof(ServiceContractAttribute), false).First() as ServiceContractAttribute;

        if (contractAttribute == null)
            throw new Exception("contractAttribute not configured");

        //path to your lib app.config (mark as "Copy Always" in properties)
        var configPath = HostingEnvironment.MapPath(String.Format("~/Modules/{0}/bin/{0}.dll.config", moduleName)); 

        var configuration = ConfigurationManager.OpenMappedExeConfiguration(new ExeConfigurationFileMap { ExeConfigFilename = configPath }, ConfigurationUserLevel.None);
        var serviceModelSectionGroup = ServiceModelSectionGroup.GetSectionGroup(configuration);

        if (serviceModelSectionGroup == null)
            throw new Exception("serviceModelSectionGroup not configured");

        var endpoint = serviceModelSectionGroup.Client.Endpoints.OfType<ChannelEndpointElement>().First(e => e.Contract == contractAttribute.ConfigurationName);
        var channelFactory = new ConfigurationChannelFactory<T>(endpoint.Name, configuration, null);
        var client = channelFactory.CreateChannel();
        return client;

and use it:

using (var client = ServiceClientHelper.GetClient<IDefaultNameServiceChannel>(yourLibName)) {
                ... get data from service ...

See details in this article.

MySQL: Error dropping database (errno 13; errno 17; errno 39)

Simply go to /opt/lampp/var/mysql

There You can find your database name. Open that folder. Remove if any files in it

Now come to phpmyadmin and drop that database

delete image from folder PHP

First Check that is image exists? if yes then simply Call unlink(your file path) function to remove you file otherwise show message to the user.

              if (file_exists($filePath)) 
                  echo "File Successfully Delete."; 
               echo "File does not exists"; 

Using Regular Expressions to Extract a Value in Java

Full example:

private static final Pattern p = Pattern.compile("^([a-zA-Z]+)([0-9]+)(.*)");
public static void main(String[] args) {
    // create matcher for pattern p and given string
    Matcher m = p.matcher("Testing123Testing");

    // if an occurrence if a pattern was found in a given string...
    if (m.find()) {
        // ...then you can use group() methods.
        System.out.println(; // whole matched expression
        System.out.println(; // first expression from round brackets (Testing)
        System.out.println(; // second one (123)
        System.out.println(; // third one (Testing)

Since you're looking for the first number, you can use such regexp:


and will return you the first number. Note that signed numbers can contain a minus sign:


ggplot legends - change labels, order and title

You need to do two things:

  1. Rename and re-order the factor levels before the plot
  2. Rename the title of each legend to the same title

The code:

dtt$model <- factor(dtt$model, levels=c("mb", "ma", "mc"), labels=c("MBB", "MAA", "MCC"))

ggplot(dtt, aes(x=year, y=V, group = model, colour = model, ymin = lower, ymax = upper)) +
  geom_ribbon(alpha = 0.35, linetype=0)+ 
  geom_line(aes(linetype=model), size = 1) +       
  geom_point(aes(shape=model), size=4)  +      
  theme(legend.position=c(.6,0.8)) +
  theme(legend.background = element_rect(colour = 'black', fill = 'grey90', size = 1, linetype='solid')) +
  scale_linetype_discrete("Model 1") +
  scale_shape_discrete("Model 1") +
  scale_colour_discrete("Model 1")

enter image description here

However, I think this is really ugly as well as difficult to interpret. It's far better to use facets:

ggplot(dtt, aes(x=year, y=V, group = model, colour = model, ymin = lower, ymax = upper)) +
  geom_ribbon(alpha=0.2, colour=NA)+ 
  geom_line() +       
  geom_point()  +      

enter image description here

Remove spaces from std::string in C++

From gamedev

string.erase(std::remove_if(string.begin(), string.end(), std::isspace), string.end());

Swap x and y axis without manually swapping values

-Right click on either axis

-Click "Select Data..."

-Then Press the "Edit" button

-Copy the "Series X values" to the "Series Y values" and vise versa finally hit ok

I found this answer on this youtube video

How to change the timeout on a .NET WebClient object

According to kisp solution this is my edited version working async:

Class WebConnection.cs

internal class WebConnection : WebClient
    internal int Timeout { get; set; }

    protected override WebRequest GetWebRequest(Uri Address)
        WebRequest WebReq = base.GetWebRequest(Address);
        WebReq.Timeout = Timeout * 1000 // Seconds
        return WebReq;

The async Task

private async Task GetDataAsyncWithTimeout()
    await Task.Run(() =>
        using (WebConnection webClient = new WebConnection())
            webClient.Timeout = 5; // Five seconds
} // await GetDataAsyncWithTimeout()

Else, if you don't want to use async:

private void GetDataSyncWithTimeout()
    using (WebConnection webClient = new WebConnection())
        webClient.Timeout = 5; // Five seconds
} // GetDataSyncWithTimeout()

Display a float with two decimal places in Python

You could use the string formatting operator for that:

>>> '%.2f' % 1.234
>>> '%.2f' % 5.0

The result of the operator is a string, so you can store it in a variable, print etc.

Escape quote in web.config connection string

connectionString="Server=dbsrv;User ID=myDbUser;Password=somepass&quot;word"

Since the web.config is XML, you need to escape the five special characters:

&amp; -> & ampersand, U+0026
&lt; -> < left angle bracket, less-than sign, U+003C
&gt; -> > right angle bracket, greater-than sign, U+003E
&quot; -> " quotation mark, U+0022
&apos; -> ' apostrophe, U+0027

+ is not a problem, I suppose.

Duc Filan adds: You should also wrap your password with single quote ':

connectionString="Server=dbsrv;User ID=myDbUser;Password='somepass&quot;word'"

MySQL 'create schema' and 'create database' - Is there any difference

So, there is no difference between MySQL "database" and MySQL "schema": these are two names for the same thing - a namespace for tables and other DB objects.

For people with Oracle background: MySQL "database" a.k.a. MySQL "schema" corresponds to Oracle schema. The difference between MySQL and Oracle CREATE SCHEMA commands is that in Oracle the CREATE SCHEMA command does not actually create a schema but rather populates it with tables and views. And Oracle's CREATE DATABASE command does a very different thing than its MySQL counterpart.

Opening Android Settings programmatically

You can try to call:

startActivityForResult(new Intent(android.provider.Settings.ACTION_WIFI_SETTINGS));

for other screen in setting screen, you can go to

Hope help you in this case.

Overriding !important style

Rather than injecting style, if you inject a class(for eg: 'show') through java script, it will work. But here you need css like below. the added class css rule should be below your original rule.

td.EvenRow a{
  display: none !important;

  display: block !important;

PHP remove commas from numeric strings

Not tested, but probably something like if(preg_match("/^[0-9,]+$/", $a)) $a = str_replace(...)

Do it the other way around:

$a = "1,435";
$b = str_replace( ',', '', $a );

if( is_numeric( $b ) ) {
    $a = $b;

The easiest would be:

$var = intval(preg_replace('/[^\d.]/', '', $var));

or if you need float:

$var = floatval(preg_replace('/[^\d.]/', '', $var));

Loop through JSON in EJS

JSON.stringify(data).length return string length not Object length, you can use Object.keys.

<% for(var i=0; i < Object.keys(data).length ; i++) {%>

Defining arrays in Google Scripts

Try this

function readRows() {
  var sheet = SpreadsheetApp.getActiveSheet();
  var rows = sheet.getDataRange();
  var numRows = rows.getNumRows();
  //var values = rows.getValues();

  var Names = sheet.getRange("A2:A7");
  var Name = [
    Names.getCell(1, 1).getValue(),
    Names.getCell(2, 1).getValue(),
    Names.getCell(5, 1).getValue()]

You can define arrays simply as follows, instead of allocating and then assigning.

var arr = [1,2,3,5]

Your initial error was because of the following line, and ones like it

var Name[0] = Name_cell.getValue(); 

Since Name is already defined and you are assigning the values to its elements, you should skip the var, so just

Name[0] = Name_cell.getValue();

Pro tip: For most issues that, like this one, don't directly involve Google services, you are better off Googling for the way to do it in javascript in general.

Error: "Adb connection Error:An existing connection was forcibly closed by the remote host"

In my case, which none of the answers above stated. If your device is using the miniUsb connector, make sure you are using a cable that is not charge-only. I became accustom to using developing with a newer Usb-C device and could not fathom a charge-only cable got mixed with my pack especially since there is no visible way to tell the difference.

Before you uninstall and go through a nightmare of driver reinstall and android menu options. Try a different cable first.

nginx 502 bad gateway


fastcgi_pass    unix:/var/run/php-fpm.sock;


fastcgi_pass    unix:/var/run/php5-fpm.sock;

Adding new column to existing DataFrame in Python pandas

Let me just add that, just like for hum3, .loc didn't solve the SettingWithCopyWarning and I had to resort to df.insert(). In my case false positive was generated by "fake" chain indexing dict['a']['e'], where 'e' is the new column, and dict['a'] is a DataFrame coming from dictionary.

Also note that if you know what you are doing, you can switch of the warning using pd.options.mode.chained_assignment = None and than use one of the other solutions given here.

Java Error: "Your security settings have blocked a local application from running"

  • Starting with Java 8, there is no "medium" risk setting in the Security tab under Java

  • You will keep getting this error till you revert to older Java (suggested Java 7, it has hit the end of life though).

  • Install both 32-bit and 64-bit version because browsers are still 32-bit, even on a 64-bit machine, 64-bit OS

How to rotate x-axis tick labels in Pandas barplot

The follows might be helpful:

# Valid font size are xx-small, x-small, small, medium, large, x-large, xx-large, larger, smaller, None


Here is the function xticks[reference] with example and API

def xticks(ticks=None, labels=None, **kwargs):
    Get or set the current tick locations and labels of the x-axis.

    Call signatures::

        locs, labels = xticks()            # Get locations and labels
        xticks(ticks, [labels], **kwargs)  # Set locations and labels

    ticks : array_like
        A list of positions at which ticks should be placed. You can pass an
        empty list to disable xticks.

    labels : array_like, optional
        A list of explicit labels to place at the given *locs*.

        :class:`.Text` properties can be used to control the appearance of
        the labels.

        An array of label locations.
        A list of `.Text` objects.

    Calling this function with no arguments (e.g. ``xticks()``) is the pyplot
    equivalent of calling `~.Axes.get_xticks` and `~.Axes.get_xticklabels` on
    the current axes.
    Calling this function with arguments is the pyplot equivalent of calling
    `~.Axes.set_xticks` and `~.Axes.set_xticklabels` on the current axes.

    Get the current locations and labels:

        >>> locs, labels = xticks()

    Set label locations:

        >>> xticks(np.arange(0, 1, step=0.2))

    Set text labels:

        >>> xticks(np.arange(5), ('Tom', 'Dick', 'Harry', 'Sally', 'Sue'))

    Set text labels and properties:

        >>> xticks(np.arange(12), calendar.month_name[1:13], rotation=20)

    Disable xticks:

        >>> xticks([])

Centering image and text in R Markdown for a PDF report

There is now a much better solution, a lot more elegant, based on fenced div, which have been implemented in pandoc, as explained here:

::: {.center data-latex=""}
Some text here...

All you need to do is to change your css file accordingly. The following chunk for instance does the job:

```{cat, engine.opts = list(file = "style.css")}
.center {
  text-align: center;

(Obviously, you can also directly type the content of the chunk into your .css file...).
The tex file includes the proper centering commands.
The crucial advantage of this method is that it allows writing markdown code inside the block.
In my previous answer, r ctrFmt("Centered **text** in html and pdf!") does not bold for the word "text", but it would if inside a fenced div.

For images, etc... the lua filter is available here

How to make function decorators and chain them together?

This answer has long been answered, but I thought I would share my Decorator class which makes writing new decorators easy and compact.

from abc import ABCMeta, abstractclassmethod

class Decorator(metaclass=ABCMeta):
    """ Acts as a base class for all decorators """

    def __init__(self):
        self.method = None

    def __call__(self, method):
        self.method = method

    def call(self, *args, **kwargs):
        return self.method(*args, **kwargs)

For one I think this makes the behavior of decorators very clear, but it also makes it easy to define new decorators very concisely. For the example listed above, you could then solve it as:

class MakeBold(Decorator):
    def call():
        return "<b>" + self.method() + "</b>"

class MakeItalic(Decorator):
    def call():
        return "<i>" + self.method() + "</i>"

def say():
   return "Hello"

You could also use it to do more complex tasks, like for instance a decorator which automatically makes the function get applied recursively to all arguments in an iterator:

class ApplyRecursive(Decorator):
    def __init__(self, *types):
        if not len(types):
            types = (dict, list, tuple, set)
        self._types = types

    def call(self, arg):
        if dict in self._types and isinstance(arg, dict):
            return {key: for key, value in arg.items()}

        if set in self._types and isinstance(arg, set):
            return set( for value in arg)

        if tuple in self._types and isinstance(arg, tuple):
            return tuple( for value in arg)

        if list in self._types and isinstance(arg, list):
            return list( for value in arg)

        return self.method(arg)

@ApplyRecursive(tuple, set, dict)
def double(arg):
    return 2*arg

print(double({'a': 1, 'b': 2}))
print(double({1, 2, 3}))
print(double((1, 2, 3, 4)))
print(double([1, 2, 3, 4, 5]))

Which prints:

{'a': 2, 'b': 4}
{2, 4, 6}
(2, 4, 6, 8)
[1, 2, 3, 4, 5, 1, 2, 3, 4, 5]

Notice that this example didn't include the list type in the instantiation of the decorator, so in the final print statement the method gets applied to the list itself, not the elements of the list.

How to make circular background using css?

If you want to do it with only 1 element, you can use the ::before and ::after pseudo elements for the same div instead of a wrapper.

Simple PHP calculator

Check string using single quotes

Ex. $_POST['group1'] == 'add'

Execute SQLite script

For those using PowerShell

PS C:\> Get-Content create.sql -Raw | sqlite3 auction.db

Python : Trying to POST form using requests

You can use the Session object

import requests
headers = {'User-Agent': 'Mozilla/5.0'}
payload = {'username':'niceusername','password':'123456'}

session = requests.Session()'',headers=headers,data=payload)
# the session instance holds the cookie. So use it to get/post later.
# e.g. session.get('')

Generating Random Passwords

Added some supplemental code to the accepted answer. It improves upon answers just using Random and allows for some password options. I also liked some of the options from the KeePass answer but did not want to include the executable in my solution.

private string RandomPassword(int length, bool includeCharacters, bool includeNumbers, bool includeUppercase, bool includeNonAlphaNumericCharacters, bool includeLookAlikes)
    if (length < 8 || length > 128) throw new ArgumentOutOfRangeException("length");
    if (!includeCharacters && !includeNumbers && !includeNonAlphaNumericCharacters) throw new ArgumentException("RandomPassword-Key arguments all false, no values would be returned");

    string pw = "";
        pw += System.Web.Security.Membership.GeneratePassword(128, 25);
        pw = RemoveCharacters(pw, includeCharacters, includeNumbers, includeUppercase, includeNonAlphaNumericCharacters, includeLookAlikes);
    } while (pw.Length < length);

    return pw.Substring(0, length);

private string RemoveCharacters(string passwordString, bool includeCharacters, bool includeNumbers, bool includeUppercase, bool includeNonAlphaNumericCharacters, bool includeLookAlikes)
    if (!includeCharacters)
        var remove = new string[] { "a", "b", "c", "d", "e", "f", "g", "h", "i", "j", "k", "l", "m", "n", "o", "p", "q", "r", "s", "t", "u", "v", "w", "x", "y", "z" };
        foreach (string r in remove)
            passwordString = passwordString.Replace(r, string.Empty);
            passwordString = passwordString.Replace(r.ToUpper(), string.Empty);

    if (!includeNumbers)
        var remove = new string[] { "0", "1", "2", "3", "4", "5", "6", "7", "8", "9" };
        foreach (string r in remove)
            passwordString = passwordString.Replace(r, string.Empty);

    if (!includeUppercase)
        passwordString = passwordString.ToLower();

    if (!includeNonAlphaNumericCharacters)
        var remove = new string[] { "!", "@", "#", "$", "%", "^", "&", "*", "(", ")", "-", "_", "+", "=", "{", "}", "[", "]", "|", "\\", ":", ";", "<", ">", "/", "?", "." };
        foreach (string r in remove)
            passwordString = passwordString.Replace(r, string.Empty);

    if (!includeLookAlikes)
        var remove = new string[] { "(", ")", "0", "O", "o", "1", "i", "I", "l", "|", "!", ":", ";" };
        foreach (string r in remove)
            passwordString = passwordString.Replace(r, string.Empty);

    return passwordString;

This was the first link when I searched for generating random passwords and the following is out of scope for the current question but might be important to consider.

  • Based upon the assumption that System.Web.Security.Membership.GeneratePassword is cryptographically secure with a minimum of 20% of the characters being Non-Alphanumeric.
  • Not sure if removing characters and appending strings is considered good practice in this case and provides enough entropy.
  • Might want to consider implementing in some way with SecureString for secure password storage in memory.

How to install gem from GitHub source?

Also you can do gem install username-projectname -s

Jquery Open in new Tab (_blank)

Replace this line:

$(this).target = "_blank";


$( this ).attr( 'target', '_blank' );

That will set its HREF to _blank.

How to create string with multiple spaces in JavaScript

Use &nbsp;

It is the entity used to represent a non-breaking space. It is essentially a standard space, the primary difference being that a browser should not break (or wrap) a line of text at the point that this   occupies.

var a = 'something' + '&nbsp &nbsp &nbsp &nbsp &nbsp' + 'something'

Non-breaking Space

A common character entity used in HTML is the non-breaking space (&nbsp;).

Remember that browsers will always truncate spaces in HTML pages. If you write 10 spaces in your text, the browser will remove 9 of them. To add real spaces to your text, you can use the &nbsp; character entity.


var a = 'something' + '&nbsp &nbsp &nbsp &nbsp &nbsp' + 'something';_x000D_
document.body.innerHTML = a;

Convert decimal to binary in python


Execution failed for task ':app:compileDebugJavaWithJavac' Android Studio 3.1 Update

This worked for me!


//Add this....Keep both version same

compileOptions {                                                          
    sourceCompatibility JavaVersion.VERSION_1_8 
    targetCompatibility JavaVersion.VERSION_1_8

Get user location by IP address

    public static string GetLocationIPAPI(string ipaddress)
            IPDataIPAPI ipInfo = new IPDataIPAPI();
            string strResponse = new WebClient().DownloadString("" + ipaddress);
            if (strResponse == null || strResponse == "") return "";
            ipInfo = JsonConvert.DeserializeObject<IPDataIPAPI>(strResponse);
            if (ipInfo == null || ipInfo.status.ToLower().Trim() == "fail") return "";
            else return + "; " + ipInfo.regionName + "; " + + "; " + ipInfo.countryCode;
        catch (Exception)
            return "";

public class IPDataIPINFO
    public string ip { get; set; }
    public string city { get; set; }
    public string region { get; set; }
    public string country { get; set; }
    public string loc { get; set; }
    public string postal { get; set; }
    public int org { get; set; }



    public static string GetLocationIPINFO(string ipaddress)
            IPDataIPINFO ipInfo = new IPDataIPINFO();
            string strResponse = new WebClient().DownloadString("" + ipaddress);
            if (strResponse == null || strResponse == "") return "";
            ipInfo = JsonConvert.DeserializeObject<IPDataIPINFO>(strResponse);
            if (ipInfo == null || ipInfo.ip == null || ipInfo.ip == "") return "";
            else return + "; " + ipInfo.region + "; " + + "; " + ipInfo.postal;
        catch (Exception)
            return "";

public class IPDataIPAPI
    public string status { get; set; }
    public string country { get; set; }
    public string countryCode { get; set; }
    public string region { get; set; }
    public string regionName { get; set; }
    public string city { get; set; }
    public string zip { get; set; }
    public string lat { get; set; }
    public string lon { get; set; }
    public string timezone { get; set; }
    public string isp { get; set; }
    public string org { get; set; }
    public string @as { get; set; }
    public string query { get; set; }


    private static string GetLocationIPSTACK(string ipaddress)
            IPDataIPSTACK ipInfo = new IPDataIPSTACK();
            string strResponse = new WebClient().DownloadString("" + ipaddress + "?access_key=XX384X1XX028XX1X66XXX4X04XXXX98X");
            if (strResponse == null || strResponse == "") return "";
            ipInfo = JsonConvert.DeserializeObject<IPDataIPSTACK>(strResponse);
            if (ipInfo == null || ipInfo.ip == null || ipInfo.ip == "") return "";
            else return + "; " + ipInfo.region_name + "; " + ipInfo.country_name + "; " +;
        catch (Exception)
            return "";

public class IPDataIPSTACK
    public string ip { get; set; }
    public int city { get; set; }
    public string region_code { get; set; }
    public string region_name { get; set; }
    public string country_code { get; set; }
    public string country_name { get; set; }
    public string zip { get; set; }


Binary numbers in Python

I think you're confused about what binary is. Binary and decimal are just different representations of a number - e.g. 101 base 2 and 5 base 10 are the same number. The operations add, subtract, and compare operate on numbers - 101 base 2 == 5 base 10 and addition is the same logical operation no matter what base you're working in.

Java word count program

To count specified words only like John, John99, John_John and John's only. Change regex according to yourself and count the specified words only.

    public static int wordCount(String content) {
        int count = 0;
        String regex = "([a-zA-Z_’][0-9]*)+[\\s]*";     
        Pattern pattern = Pattern.compile(regex);
        Matcher matcher = pattern.matcher(content);
        while(matcher.find()) {
            System.out.println(; //If want to display the matched words
        return count;

How to disable or enable viewpager swiping in android

Best solution for me. -First, you create a class like this:

public class CustomViewPager extends ViewPager {
  private Boolean disable = false;
  public CustomViewPager(Context context) {
  public CustomViewPager(Context context, AttributeSet attrs){
  public boolean onInterceptTouchEvent(MotionEvent event) {
     return !disable && super.onInterceptTouchEvent(event);

  public boolean onTouchEvent(MotionEvent event) {
     return !disable && super.onTouchEvent(event);

  public void disableScroll(Boolean disable){
      //When disable = true not work the scroll and when disble = false work the scroll
      this.disable = disable;

-Then change this in your layout:< for this<com.mypackage.CustomViewPager

-Finally, you can disable it:view_pager.disableScroll(true); or enable it: view_pager.disableScroll(false);

I hope that this help you :)

How do I change the background color of the ActionBar of an ActionBarActivity using XML?

try this:

ActionBar bar = getActionBar();
bar.setBackgroundDrawable(new ColorDrawable(Color.parseColor("#0000ff")));

onChange and onSelect in DropDownList

I bet the onchange is getting fired after the onselect, essentially re-enabling the select.

I'd recommend you implement only the onchange, inspect which option has been selected, and enable or disabled based on that.

To get the value of the selected option use:


Which will yield .. nothing since you haven't specified a value for each option .. :(

<select id="mySelect" onChange="enable();">
<option onSelect="disable();" value="no">No</option>
<option onSelect="enable();" value="yes">Yes</option>

Now it will yield "yes" or "no"

How to run shell script on host from docker container?

My laziness led me to find the easiest solution that wasn't published as an answer here.

It is based on the great article by luc juggery.

All you need to do in order to gain a full shell to your linux host from within your docker container is:

docker run --privileged --pid=host -it alpine:3.8 \
nsenter -t 1 -m -u -n -i sh


--privileged : grants additional permissions to the container, it allows the container to gain access to the devices of the host (/dev)

--pid=host : allows the containers to use the processes tree of the Docker host (the VM in which the Docker daemon is running) nsenter utility: allows to run a process in existing namespaces (the building blocks that provide isolation to containers)

nsenter (-t 1 -m -u -n -i sh) allows to run the process sh in the same isolation context as the process with PID 1. The whole command will then provide an interactive sh shell in the VM

This setup has major security implications and should be used with cautions (if any).

Returning JSON response from Servlet to Javascript/JSP page

Got it working! I should have been building a JSONArray of JSONObjects and then add the array to a final "Addresses" JSONObject. Observe the following:

JSONObject json      = new JSONObject();
JSONArray  addresses = new JSONArray();
JSONObject address;
   int count = 15;

   for (int i=0 ; i<count ; i++)
       address = new JSONObject();
       address.put("CustomerName"     , "Decepticons" + i);
       address.put("AccountId"        , "1999" + i);
       address.put("SiteId"           , "1888" + i);
       address.put("Number"            , "7" + i);
       address.put("Building"          , "StarScream Skyscraper" + i);
       address.put("Street"            , "Devestator Avenue" + i);
       address.put("City"              , "Megatron City" + i);
       address.put("ZipCode"          , "ZZ00 XX1" + i);
       address.put("Country"           , "CyberTron" + i);
   json.put("Addresses", addresses);
catch (JSONException jse)


This worked and returned valid and parse-able JSON. Hopefully this helps someone else in the future. Thanks for your help Marcel

How can I import a large (14 GB) MySQL dump file into a new MySQL database?

I found below SSH commands are robust for export/import huge MySql databases, at least I'm using them for years. Never rely on backups generated via control panels like cPanel WHM, CWP, OVIPanel, etc they may trouble you especially when you're switching between control panels, trust SSH always.


$ mysqldump -u root -p example_database| gzip > example_database.sql.gz 


$ gunzip < example_database.sql.gz | mysql -u root -p example_database 

How to plot an array in python?

if you give a 2D array to the plot function of matplotlib it will assume the columns to be lines:

If x and/or y is 2-dimensional, then the corresponding columns will be plotted.

In your case your shape is not accepted (100, 1, 1, 8000). As so you can using numpy squeeze to solve the problem quickly:

np.squeez doc: Remove single-dimensional entries from the shape of an array.

import numpy as np
import matplotlib.pyplot as plt

data = np.random.randint(3, 7, (10, 1, 1, 80))
newdata = np.squeeze(data) # Shape is now: (10, 80)
plt.plot(newdata) # plotting by columns

But notice that 100 sets of 80 000 points is a lot of data for matplotlib. I would recommend that you look for an alternative. The result of the code example (run in Jupyter) is:

Jupyter matplotlib plot

How does setTimeout work in Node.JS?

setTimeout is a kind of Thread, it holds a operation for a given time and execute.


in here the first argument should be a function type; as an example if you want to print your name after 3 seconds, your code should be something like below.

setTimeout(function(){console.log('your name')},3000);

Key point to remember is, what ever you want to do by using the setTimeout method, do it inside a function. If you want to call some other method by parsing some parameters, your code should look like below:


How to map atan2() to degrees 0-360

I have 2 solutions that seem to work for all combinations of positive and negative x and y.

1) Abuse atan2()

According to the docs atan2 takes parameters y and x in that order. However if you reverse them you can do the following:

double radians = std::atan2(x, y);
double degrees = radians * 180 / M_PI;
if (radians < 0)
    degrees += 360; 

2) Use atan2() correctly and convert afterwards

double degrees = std::atan2(y, x) * 180 / M_PI;
if (degrees > 90)
    degrees = 450 - degrees;
    degrees = 90 - degrees;

Convert an array into an ArrayList

As an ArrayList that line would be

import java.util.ArrayList;
ArrayList<Card> hand = new ArrayList<Card>();

To use the ArrayList you have do

hand.get(i); //gets the element at position i 
hand.add(obj); //adds the obj to the end of the list
hand.remove(i); //removes the element at position i
hand.add(i, obj); //adds the obj at the specified index
hand.set(i, obj); //overwrites the object at i with the new obj

Also read this

Django DB Settings 'Improperly Configured' Error

In 2017 with django 1.11.5 and python 3.6 (from the comment this also works with Python 2.7):

import django
import os
os.environ.setdefault("DJANGO_SETTINGS_MODULE", "mysite.settings")

The .py in which you put this code should be in mysite (the parent one)

Getting the IP Address of a Remote Socket Endpoint

I've made this code in VB.NET but you can translate. Well pretend you have the variable Client as a TcpClient

Dim ClientRemoteIP As String = Client.Client.RemoteEndPoint.ToString.Remove(Client.Client.RemoteEndPoint.ToString.IndexOf(":"))

Hope it helps! Cheers.

T-SQL Cast versus Convert

To expand on the above answercopied by Shakti, I have actually been able to measure a performance difference between the two functions.

I was testing performance of variations of the solution to this question and found that the standard deviation and maximum runtimes were larger when using CAST.

Runtimes in milliseconds *Times in milliseconds, rounded to nearest 1/300th of a second as per the precision of the DateTime type

If statement in select (ORACLE)

    --  <add any columns to outer select from inner query> 
 (  -- your query here --
   select 'CARAT Issue Open' issue_comment, ...., ..., 
          substr(gcrs.stream_name,1,case when instr(gcrs.stream_name,' (')=0 then 100 else  instr(gcrs.stream_name,' (')-1 end) ISSUE_DIVISION,
          case when gcrs.STREAM_NAME like 'NON-GT%' THEN 'NON-GT' ELSE gcrs.STREAM_NAME END as ISSUE_DIVISION_2
     from ....
    where UPPER(ISSUE_STATUS) like '%OPEN%'
 WHERE... -- optional --

formGroup expects a FormGroup instance

There are a few issues in your code

  • <div [formGroup]="form"> outside of a <form> tag
  • <form [formGroup]="form"> but the name of the property containing the FormGroup is loginForm therefore it should be <form [formGroup]="loginForm">
  • [formControlName]="dob" which passes the value of the property dob which doesn't exist. What you need is to pass the string dob like [formControlName]="'dob'" or simpler formControlName="dob"

Plunker example

Easy way to build Android UI?

Creating an App Inventor app begins in your browser, where you design how the app will look. Then, like fitting together puzzle pieces, you set your app's behavior. All the while, through a live connection between your computer and your phone, your app appears on your phone.

Accuracy Score ValueError: Can't Handle mix of binary and continuous target

The sklearn.metrics.accuracy_score(y_true, y_pred) method defines y_pred as:

y_pred : 1d array-like, or label indicator array / sparse matrix. Predicted labels, as returned by a classifier.

Which means y_pred has to be an array of 1's or 0's (predicated labels). They should not be probabilities.

The predicated labels (1's and 0's) and/or predicted probabilites can be generated using the LinearRegression() model's methods predict() and predict_proba() respectively.

1. Generate predicted labels:

LR = linear_model.LinearRegression()


[1 1 0 1]

y_preds can now be used for the accuracy_score() method: accuracy_score(y_true, y_pred)

2. Generate probabilities for labels:

Some metrics such as 'precision_recall_curve(y_true, probas_pred)' require probabilities, which can be generated as follows:

LR = linear_model.LinearRegression()


[0.87812372 0.77490434 0.30319547 0.84999743]

C# Inserting Data from a form into an access Database

 private void Add_Click(object sender, EventArgs e) {
 OleDbConnection con = new OleDbConnection(@ "Provider=Microsoft.Jet.OLEDB.4.0;Data Source=C:\Users\HP\Desktop\DS Project.mdb");
 OleDbCommand cmd = con.CreateCommand();
 cmd.CommandText = "Insert into DSPro (Playlist) values('" + textBox1.Text + "')";
 MessageBox.Show("Record Submitted", "Congrats");

How to put a UserControl into Visual Studio toolBox

I had many users controls but one refused to show in the Toolbox, even though I rebuilt the solution and it was checked in the Choose Items... dialog.


  1. From Solution Explorer I Right-Clicked the offending user control file and selected Exclude From Project
  2. Rebuild the solution
  3. Right-Click the user control and select Include in Project (assuming you have the Show All Files enabled in the Solution Explorer)

Note this also requires you have the AutoToolboxPopulate option enabled. As @DaveF answer suggests.

Alternate Solution: I'm not sure if this works, and I couldn't try it since I already resolved my issue, but if you unchecked the user control from the Choose Items... dialog, hit OK, then opened it back up and checked the user control. That might also work.

How do I set default values for functions parameters in Matlab?

if you would use octave you could do it like this - but sadly matlab does not support this possibility

function hello (who = "World")
  printf ("Hello, %s!\n", who);

(taken from the doc)

Remove a specific character using awk or sed

tr can be more concise for removing characters than sed or awk, especially when you want to remove different characters from a string.

Removing double quotes:

echo '"Hi"' | tr -d \"
# Produces Hi without quotes

Removing different kinds of brackets:

echo '[{Hi}]' | tr -d {}[]
# Produces Hi without brackets

-d stands for "delete".

Input Type image submit form value?

You could use a radio button/checkbox and set it to hide the button in css and then give it a label with an image.

input[type="radio"] {display: none}
input[type="radio"] + label span {display: block}

Then on the page:

<input type="radio" name="emotion" id="mysubmitradio" />
        <label for="mysubmitradio"><img src="images/f.jpg" />
    <span>if you need it</span></label>

And then set it to submit using javascript:


MySQL 8.0 - Client does not support authentication protocol requested by server; consider upgrading MySQL client

I have MYSQL on server and nodejs application on another server

Execute the following query in MYSQL Workbench

ALTER USER 'root'@'%' IDENTIFIED WITH mysql_native_password BY 'password'

How to get first object out from List<Object> using Linq

Try this to get all the list at first, then your desired element (say the First in your case):

var desiredElementCompoundValueList = new List<YourType>();
dic.Values.ToList().ForEach( elem => 
var x = desiredElementCompoundValueList.FirstOrDefault();

To get directly the first element value without a lot of foreach iteration and variable assignment:

var desiredCompoundValue = dic.Values.ToList().Select( elem => elem.CompoundValue("Dep")).FirstOrDefault();

See the difference between the two approaches: in the first one you get the list through a ForEach, then your element. In the second you can get your value in a straight way.

Same result, different computation ;)

Failed to install android-sdk: "java.lang.NoClassDefFoundError: javax/xml/bind/annotation/XmlSchema"

I had a similar problem this morning (trying to build for Android using Unity3D). I ended up uninstalling JDK9 and installing Java SE Development Kit 8u144. Hope this helps.

  1. brew cask uninstall java # uninstall java9
  2. brew tap homebrew/cask-versions
  3. brew cask install java8 # install java8
  4. touch ~/.android/repositories.cfg # without this file, error will occur on next step
  5. brew cask install android-sdk

break out of if and foreach

A safer way to approach breaking a foreach or while loop in PHP is to nest an incrementing counter variable and if conditional inside of the original loop. This gives you tighter control than break; which can cause havoc elsewhere on a complicated page.


// Setup a counter
$ImageCounter = 0;

// Increment through repeater fields
while ( condition ):

   // Only print the first while instance
   if ($ImageCounter == 1) {
    echo 'It worked just once';

// Close while statement

DataTables: Cannot read property style of undefined

Make sure that in your input data, response[i] and response[i][j], are not undefined/null.

If so, replace them with "".

Iterating each character in a string using Python

Well you can also do something interesting like this and do your job by using for loop

#suppose you have variable name
name = "Mr.Suryaa"
for index in range ( len ( name ) ):
    print ( name[index] ) #just like c and c++ 

Answer is

M r . S u r y a a

However since range() create a list of the values which is sequence thus you can directly use the name

for e in name:

This also produces the same result and also looks better and works with any sequence like list, tuple, and dictionary.

We have used tow Built in Functions ( BIFs in Python Community )

1) range() - range() BIF is used to create indexes Example

for i in range ( 5 ) :
can produce 0 , 1 , 2 , 3 , 4

2) len() - len() BIF is used to find out the length of given string

Spark Kill Running Application

First use:

yarn application -list

Note down the application id Then to kill use:

yarn application -kill application_id

Converting list to numpy array

If you have a list of lists, you only needed to use ...

import numpy as np
npa = np.asarray(someListOfLists, dtype=np.float32)

per this LINK in the scipy / numpy documentation. You just needed to define dtype inside the call to asarray.

Is there a way to view two blocks of code from the same file simultaneously in Sublime Text?

In the nav go View => Layout => Columns:2 (alt+shift+2) and open your file again in the other pane (i.e. click the other pane and use ctrl+p

It appears you can also reopen the file using the command File -> New View into File which will open the current file in a new tab

Index of element in NumPy array

I'm torn between these two ways of implementing an index of a NumPy array:

idx = list(classes).index(var)
idx = np.where(classes == var)

Both take the same number of characters, but the first method returns an int instead of a numpy.ndarray.

The import javax.persistence cannot be resolved

My solution was to select the maven profiles I had defined in my pom.xml in which I had declared the hibernate dependencies.

CTRL + ALT + P in eclipse.

In my project I was experiencing this problem and many others because in my pom I have different profiles for supporting Glassfish 3, Glassfish 4 and also WildFly so I have differet versions of Hibernate per container as well as different Java compilation targets and so on. Selecting the active maven profiles resolved my issue.

PostgreSQL: export resulting data from SQL query to Excel/CSV

If you have error like "ERROR: could not open server file "/file": Permission denied" you can fix it that:

Ran through the same problem, and this is the solution I found: Create a new folder (for instance, tmp) under /home $ cd /home make postgres the owner of that folder $ chown -R postgres:postgres tmp copy in tmp the files you want to write into the database, and make sure they also are owned by postgres. That's it. You should be in business after that.