Filter

Select certain elements from an Array into a new Array in a generic way. To demonstrate, select all even numbers from an Array.

AppleScript

set array to {1, 2, 3, 4, 5}
set evens to {}
repeat with i in array
    -- very important -- list index starts at 1 not 0
    if (i mod 2 = 0) then set evens to evens & i
end repeat

C

       int arr[5] = {1,2,3,4,5};
       int *result;
       int memoryReqd = 0,length = sizeof(arr)/sizeof(arr[0]), i,j;
       for (i=0; i<length; i++)
       {
               if(0 == arr[i]%2)
               {
                       memoryReqd++;
               }
       }
       result = (int*)malloc(memoryReqd*sizeof(int));
       for(i=0,j=0; i<length; i++)
       {
               if(0 == arr[i]%2)
               {
                       result[j++]=arr[i];
               }
       }

Test our resultant array:

       for(i=0; i<memoryReqd; i++)
       {
               printf("%d\n",result[i]);
       }

C++

Compiler: Visual C++ .NET 2005

Libraries: STL, Boost.Lambda

 using namespace std;
 using namespace boost::lambda;
 vector<int> ary(10);
 int i = 0;
 for_each(ary.begin(), ary.end(), _1 = ++var(i)); // init array
 vector<int> evens;
 remove_copy_if(ary.begin(), ary.end(), back_inserter(evens), _1 % 2); // filter copy

C#

       // .NET 1.x solution
       ArrayList array = new ArrayList( new int[] { 1, 2, 3, 4, 5 } );
       ArrayList evens = new ArrayList();
       foreach( int i in array )
       {
               if( (i%2) == 0 )
                       evens.Add( i );
       }
       foreach( int i in evens )
              System.Console.WriteLine( i.ToString() );

       // .NET 2.0 solution
       List<int> array = new List<int>( new int[] { 1, 2, 3, 4, 5 } );
       List<int> evens = array.FindAll( delegate( int i ) { return (i%2)==0; } );
       foreach( int i in evens )
              System.Console.WriteLine( i.ToString() );

Haskell

 ary = [1..10]
 evens = [ x | x <- ary, even x ]

IDL

The where() function can select elements on any logical expression. For example

 result = array[where(NOT array AND 1)]

Java

   int[] array = new int[] {1, 2, 3, 4, 5 };
   List<Integer> evensList = new ArrayList<Integer>();
   for (int  i: array) {
       if (i % 2 == 0) evensList.add(i);
   }
   int[] evens = evensList.toArray(new int[0]);

JavaScript

 var arr = [1,2,3,4,5];
 var evens = [];
 for (var i=0, ilen=arr.length; i<ilen; i++)
 	if (arr[i] % 2 == 0)
 		evens.push(arr[i]);

Perl

 my @list = (1, 2, 3, 4, 5, 6);
 my @even = grep { 0 == $_%2; } @list;

PHP

Using a standard loop

 $arr = range(1,5);
 $evens = array();
 foreach ($arr as $val)
 	if ($val % 2 == 0)
 		array_push($evens,$val);
 print_r($evens);

Using a filter function

 function is_even($var) { return(!($var & 1)); }
 $arr = range(1,5);
 $evens = array_filter($arr, "is_even");
 print_r($evens);

Python

values = range(10)
evens = [x for x in values if not(x & 1)]
ievens = (x for x in values if not(x & 1)) # lazy

Ruby

 ary = [1,2,3,4,5,6] #or ary = (1..6).to_a
 even_ary = ary.select{|el|el%2==0}
 # => [2, 4, 6]

Standard ML

 val ary = [1,2,3,4,5,6];
 List.filter (fn x => x mod 2 = 0) ary

TCL doesn't really have a concept of a "number" per se - strictly speaking its only data type is the string (but a string can be interpreted as a number, of course). The generic way of getting certain elements from an array looks roughly like this:

 foreach key [array names arr] {if { <condition> } then {puts $tst($key)}}