Filtering collections in C

Question

I am looking for a very fast way to filter down a collection in C    I am currently using generic List lt object gt  collections  but am open to using other structures if they perform better   Currently  I am just creating a new List lt object gt  and looping thru the original list   If the filtering criteria matches  I put a copy into the new list   Is there a better way to do this   Is there a way to filter in place so there is no temporary list required

User · Accepted Answer

If you re using C  3 0 you can use linq  way better and way more elegant   List lt int gt  myList   GetListOfIntsFromSomewhere        This will filter out the list of ints that are  gt  than 7  Where returns an    IEnumerable lt T gt  so a call to ToList is required to convert back to a List lt T gt   List lt int gt  filteredList   myList Where  x   gt  x  gt  7  ToList      If you can t find the  Where  that means you need to import using System Linq  at the top of your file

User · Answer

Using LINQ is relatively much slower than using a predicate supplied to the Lists FindAll method  Also be careful with LINQ as the enumeration of the list is not actually executed until you access the result  This can mean that  when you think you have created a filtered list  the content may differ to what you expected when you actually read it

User · Answer

If your list is very big and you are filtering repeatedly - you can sort the original list on the filter attribute  binary search to find the start and end points   Initial time O n log n   then O log n     Standard filtering will take O n  each time

User · Answer

Here is a code block   example of some list filtering using three different methods that I put together to show Lambdas and LINQ based list filtering    region List Filtering  static void Main string   args        ListFiltering        Console ReadLine       private static void ListFiltering         var PersonList   new List lt Person gt          PersonList Add new Person     Age   23  Name    Jon   Gender    M        Non-Constructor Object Property Initialization     PersonList Add new Person     Age   24  Name    Jack   Gender    M          PersonList Add new Person     Age   29  Name    Billy   Gender    M           PersonList Add new Person     Age   33  Name    Bob   Gender    M          PersonList Add new Person     Age   45  Name    Frank   Gender    M           PersonList Add new Person     Age   24  Name    Anna   Gender    F          PersonList Add new Person     Age   29  Name    Sue   Gender    F          PersonList Add new Person     Age   35  Name    Sally   Gender    F          PersonList Add new Person     Age   36  Name    Jane   Gender    F          PersonList Add new Person     Age   42  Name    Jill   Gender    F             Logic  Show me all males that are less than 30 years old       Console WriteLine            Iterative Method     Console WriteLine  List Filter Normal Way         foreach  var p in PersonList          if  p Gender     M   amp  amp  p Age  lt  30              Console WriteLine p Name     is     p Age        Console WriteLine            Lambda Filter Method     Console WriteLine  List Filter Lambda Way        foreach  var p in PersonList Where p   gt   p Gender     M   amp  amp  p Age  lt  30       Where is an extension method         Console WriteLine p Name     is     p Age        Console WriteLine            LINQ Query Method     Console WriteLine  List Filter LINQ Way         foreach  var v in from p in PersonList                       where p Gender     M   amp  amp  p Age  lt  30                       select new   p Name  p Age            Console WriteLine v Name     is     v Age      private class Person       public Person           public int Age   get  set        public string Name   get  set        public string Gender   get  set        endregion

User · Answer

You can use IEnumerable to eliminate the need of a temp list   public IEnumerable lt T gt  GetFilteredItems IEnumerable lt T gt  collection        foreach  T item in collection      if  Matches lt T gt  item                 yield return item            where Matches is the name of your filter method  And you can use this like   IEnumerable lt MyType gt  filteredItems   GetFilteredItems myList   foreach  MyType item in filteredItems           do sth with your filtered items     This will call GetFilteredItems function when needed and in some cases that you do not use all items in the filtered collection  it may provide some good performance gain

User · Answer

List lt T gt  has a FindAll method that will do the filtering for you and return a subset of the list    MSDN has a great code example here  http   msdn microsoft com en-us library aa701359 VS 80  aspx  EDIT  I wrote this before I had a good understanding of LINQ and the Where   method  If I were to write this today i would probably use the method Jorge mentions above  The FindAll method still works if you re stuck in a  NET 2 0 environment though

User · Answer

To do it in place  you can use the RemoveAll method of the  List lt    class along with a custom  Predicate  class   but all that does is clean up the code    under the hood it s doing the same thing you are   but yes  it does it in place  so you do same the temp list

User · Answer

If you re using C  3 0 you can use linq      Or  if you prefer  use the special query syntax provided by the C  3 compiler   var filteredList   from x in myList                    where x  gt  7                    select x

User · Answer

You can use the FindAll method of the List  providing a delegate to filter on   Though  I agree with  IainMH that it s not worth worrying yourself too much unless it s a huge list

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