Swift print vs println vs NSLog

Question

What s the difference between print  NSLog and println and when should I use each   For example  in Python if I wanted to print a dictionary  I d just print myDict  but now I have 2 other options  How and when should I use each

User · Answer

Moreover  Swift 2 has debugPrint    and CustomDebugStringConvertible protocol    Don t forget about debugPrint   which works like print   but most suitable for debugging   Examples    Strings   print  Hello World    becomes Hello World  debugPrint  Hello World    becomes  Hello World   Quotes    Ranges   print 1   lt 6  becomes 1   lt 6  debugPrint 1   lt 6  becomes Range 1   lt 6     Any class can customize their debug string representation via CustomDebugStringConvertible protocol

User · Answer

A few differences   print vs println  The print function prints messages in the Xcode console when debugging apps  The println is a variation of this that was removed in Swift 2 and is not used any more  If you see old code that is using println  you can now safely replace it with print  Back in Swift 1 x  print did not add newline characters at the end of the printed string  whereas println did  But nowadays  print always adds the newline character at the end of the string  and if you don t want it to do that  supply a terminator parameter of  quot  quot    NSLog   NSLog adds a timestamp and identifier to the output  whereas print will not   NSLog statements appear in both the device   s console and debugger   s console whereas print only appears in the debugger console   NSLog in iOS 10-13 macOS 10 12-10 x uses printf-style format strings  e g    NSLog  quot  0 4f quot   CGFloat pi   that will produce   2017-06-09 11 57 55 642328-0700 MyApp 28937 1751492  3 1416   NSLog from iOS 14 macOS 11 can use string interpolation   Then  again  in iOS 14 and macOS 11  we would generally favor Logger over NSLog  See next point     Nowadays  while NSLog still works  we would generally use    unified logging     see below  rather than NSLog   Effective iOS 14 macOS 11  we have Logger interface to the    unified logging    system  For an introduction to Logger  see WWDC 2020 Explore logging in Swift   To use Logger  you must import os  import os   Like NSLog  unified logging will output messages to both the Xcode debugging console and the device console  too  Create a Logger and log a message to it  let logger   Logger subsystem  Bundle main bundleIdentifier   category   quot network quot   logger log  quot url     url  quot    When you observe the app via the external Console app  you can filter on the basis of the subsystem and category  It is very useful to differentiate your debugging messages from  a  those generated by other subsystems on behalf of your app  or  b  messages from other categories or types   You can specify different types of logging messages  either  info   debug   error   fault   critical   notice   trace  etc   logger error  quot web service did not respond   error localizedDescription  quot    So  if using the external Console app  you can choose to only see messages of certain categories  e g  only show debugging messages if you choose    Include Debug Messages    on the Console    Action    menu   These settings also dictate many subtle issues details about whether things are logged to disk or not  See WWDC video for more details   By default  non-numeric data is redacted in the logs  In the example where you logged the URL  if the app were invoked from the device itself and you were watching from your macOS Console app  you would see the following in the macOS Console   url    lt private gt   If you are confident that this message will not include user confidential data and you wanted to see the strings in your macOS console  you would have to do  os log  quot url     url  privacy   public  quot       Prior to iOS 14 macOS 11  iOS 10 macOS 10 12 introduced  os log for    unified logging     For an introduction to unified logging in general  see WWDC 2016 video Unified Logging and Activity Tracing   Import os log  import os log   You should define the subsystem and category  let log   OSLog subsystem  Bundle main bundleIdentifier   category   quot network quot    When using os log  you would use a printf-style pattern rather than string interpolation  os log  quot url      quot   log  log  url absoluteString    You can specify different types of logging messages  either  info   debug   error   fault  or  default   os log  quot web service did not respond quot   type   error    You cannot use string interpolation when using os log  For example with print and Logger you do  logger log  quot url     url  quot    But with os log  you would have to do  os log  quot url      quot   url absoluteString    The os log enforces the same data privacy  but you specify the public visibility in the printf formatter  e g    public   rather than      E g   if you wanted to see it from an external device  you d have to do  os log  quot url     public   quot   url absoluteString    You can also use the    Points of Interest    log if you want to watch ranges of activities from Instruments  let pointsOfInterest   OSLog subsystem  Bundle main bundleIdentifier   category   pointsOfInterest   And start a range with  os signpost  begin  log  pointsOfInterest  name   quot Network request quot    And end it with  os signpost  end  log  pointsOfInterest  name   quot Network request quot    For more information  see https   stackoverflow com a 39416673 1271826      Bottom line  print is sufficient for simple logging with Xcode  but unified logging  whether Logger or os log  achieves the same thing but offers far greater capabilities  The power of unified logging comes into stark relief when debugging iOS apps that have to be tested outside of Xcode  For example  when testing background iOS app processes like background fetch  being connected to the Xcode debugger changes the app lifecycle  So  you frequently will want to test on a physical device  running the app from the device itself  not starting the app from Xcode   s debugger  Unified logging lets you still watch your iOS device log statements from the macOS Console app

User · Answer

There s another method called dump   which can also be used for logging    func dump lt T gt  T  name  String   indent  Int  maxDepth  Int  maxItems  Int        Dumps an object   s contents using its mirror to standard output    From Swift Standard Library Functions

User · Answer

If you re using Swift 2  now you can only use print   to write something to the output      Apple has combined both println   and print   functions into   one    Updated to iOS 9  By default  the function terminates the line it prints by adding a line break   print  Hello Swift     Terminator  To print a value without a line break after it  pass an empty string as the terminator  print  Hello Swift   terminator        Separator  You now can use separator to concatenate multiple items  print  Hello    Swift   2  separator        Both  Or you could combine using in this way  print  Hello    Swift   2  separator      terminator

User · Answer

NSLog - add meta info  like timestamp and identifier  and allows you to output 1023 symbols  Also print message into Console  The slowest method   import Foundation NSLog  quot SomeString quot     print - prints all string to Xcode  Has better performance than previous   import Foundation print  quot SomeString quot     println  only  available Swift v1  and add  n at the end of string os log  from iOS v10  - prints 32768 symbols also prints to console  Has better performance than previous   import os log os log  quot SomeIntro     quot   log   default  type   info   quot someString quot     Logger  from iOS v14  - prints 32768 symbols also prints to console  Has better performance than previous   import os let logger   Logger subsystem  Bundle main bundleIdentifier   category   quot someCategory quot   logger log  quot   s  quot

User · Answer

To add to Rob s answer  since iOS 10 0  Apple has introduced an entirely new  Unified Logging  system that supersedes existing logging systems  including ASL and Syslog  NSLog   and also surpasses existing logging approaches in performance  thanks to its new techniques including log data compression and deferred data collection   From Apple      The unified logging system provides a single  efficient  performant API for capturing messaging across all levels of the system  This unified system centralizes the storage of log data in memory and in a data store on disk    Apple highly recommends using os log going forward to log all kinds of messages  including info  debug  error messages because of its much improved performance compared to previous logging systems  and its centralized data collection allowing convenient log and activity inspection for developers  In fact  the new system is likely so low-footprint that it won t cause the  observer effect  where your bug disappears if you insert a logging command  interfering the timing of the bug to happen     You can learn more about this in details here   To sum it up  use print   for your personal debugging for convenience  but the message won t be logged when deployed on user devices   Then  use Unified Logging  os log  as much as possible for everything else

[swift] Swift: print() vs println() vs NSLog()

Moreover, Swift 2 has `debugPrint()` (and `CustomDebugStringConvertible` protocol)!

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