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

As an option, give a second solution which filters destructively, by modifying the original Array rather than creating a new Array.

ACL2

<lang Lisp>(defun filter-evens (xs)

  (cond ((endp xs) nil)
        ((evenp (first xs))
         (cons (first xs) (filter-evens (rest xs))))
        (t (filter-evens (rest xs)))))</lang>

ALGOL 68

Works with: ALGOL 68 version Standard - no extensions to language used

Works with: ALGOL 68G version Any - tested with release mk15-0.8b.fc9.i386

Works with: ELLA ALGOL 68 version Any (with appropriate job cards) - tested with release 1.8.8d.fc9.i386

<lang algol68>MODE TYPE = INT;

PROC select = ([]TYPE from, PROC(TYPE)BOOL where)[]TYPE: BEGIN

 FLEX[0]TYPE result;
 FOR key FROM LWB from TO UPB from DO
   IF where(from[key]) THEN
     [UPB result+1]TYPE new result;
     new result[:UPB result] := result;
     new result[UPB new result] := from[key];
     result := new result
   FI
 OD;
 result

END;

[]TYPE from values = (1,2,3,4,5,6,7,8,9,10); PROC where even = (TYPE value)BOOL: NOT ODD value;

print((select(from values, where even), new line));

Or as a simple one line query #

print((select((1,4,9,16,25,36,49,64,81,100), (TYPE x)BOOL: NOT ODD x ), new line))</lang> Output:

         +2         +4         +6         +8        +10
         +4        +16        +36        +64       +100

AppleScript

<lang applescript>set array to {1, 2, 3, 4, 5, 6} set evens to {} repeat with i in array if (i mod 2 = 0) then set end of evens to i's contents end repeat

return evens</lang>Result is (a list):

{2, 4, 6}

Here's how you might implement a more generic filter, passing a script object to represent the test that elements must pass (obviously overkill for this simple example):

<lang AppleScript>to filter(inList, acceptor)

 set outList to {}
 repeat with anItem in inList
   if acceptor's accept(anItem) then
     set end of outList to contents of anItem
   end
 end
 return outList

end

script isEven

 to accept(aNumber)
   aNumber mod 2 = 0
 end accept

end script

filter({1,2,3,4,5,6}, isEven)</lang>

AutoHotkey

<lang autohotkey>array = 1,2,3,4,5,6,7 loop, parse, array, `, {

   if IsEven(A_LoopField)
       evens = %evens%,%A_LoopField%

} stringtrimleft, evens, evens, 1 msgbox % evens return

IsEven(number) {

   return !mod(number, 2)

}

----- Another version: always with csv string ------

array = 1,2,3,4,5,6,7

even(s) { loop, parse, s, `, if !mod(A_LoopField, 2) r .= "," A_LoopField return SubStr(r, 2) }

MsgBox % "Array => " array "`n" "Result => " even(array)

----- Yet another version: with array (requires AutoHotKey_L) ------

array2 := [1,2,3,4,5,6,7]

even2(a) { r := [] For k, v in a if !mod(v, 2) r.Insert(v) return r }

Add "join" method to string object (just like python)

s_join(o, a) { Loop, % a.MaxIndex() r .= o a[A_Index] return SubStr(r, StrLen(o) + 1) } "".base.join := Func("s_join")

MsgBox % "Array => " ",".join(array2) "`n" "Result => " ",".join(even2(array2))

</lang>

In this example, an array is filled with the numbers 1..9. In a loop, even elements are collected into the string r. Note that sequence is not necessarily maintained. <lang awk>$ awk 'BEGIN{split("1 2 3 4 5 6 7 8 9",a);for(i in a)if(!(a[i]%2))r=r" "a[i];print r}' 4 6 8 2</lang>

BBC BASIC

<lang bbcbasic> REM Create the test array:

     items% = 1000
     DIM array%(items%)
     FOR index% = 1 TO items%
       array%(index%) = RND
     NEXT
     
     REM Count the number of filtered items:
     filtered% = 0
     FOR index% = 1 TO items%
       IF FNfilter(array%(index%)) filtered% += 1
     NEXT
     
     REM Create a new array containing the filtered items:
     DIM new%(filtered%)
     filtered% = 0
     FOR index% = 1 TO items%
       IF FNfilter(array%(index%)) THEN
         filtered% += 1
         new%(filtered%) = array%(index%)
       ENDIF
     NEXT
     
     REM Alternatively modify the original array:
     filtered% = 0
     FOR index% = 1 TO items%
       IF FNfilter(array%(index%)) THEN
         filtered% += 1
         array%(filtered%) = array%(index%)
       ENDIF
     NEXT
     END
     
     DEF FNfilter(A%) = ((A% AND 1) = 0)</lang>

Bracmat

<lang bracmat>( :?odds & ( 1 2 3 4 5 6 7 8 9 10 16 25 36 49 64 81 100:? (=.!sjt*1/2:/&!odds !sjt:?odds)$() ()

 | !odds
 )

) </lang>

1 3 5 7 9 25 49 81

Brat

<lang brat>#Prints [2, 4, 6, 8, 10] p 1.to(10).select { x | x % 2 == 0 }</lang>

C

<lang C>#include <stdio.h>

include <stdlib.h>

int even_sel(int x) { return !(x & 1); } int tri_sel(int x) { return x % 3; }

/* using a predicate function sel() to select elements */ int* grep(int *in, int len, int *outlen, int (*sel)(int), int inplace) { int i, j, *out;

if (inplace) out = in; else out = malloc(sizeof(int) * len);

for (i = j = 0; i < len; i++) if (sel(in[i])) out[j++] = in[i];

if (!inplace && j < len) out = realloc(out, sizeof(int) * j);

*outlen = j; return out; }

int main() { int in[] = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 }; int i, len;

int *even = grep(in, 10, &len, even_sel, 0); printf("Filtered even:"); for (i = 0; i < len; i++) printf(" %d", even[i]); printf("\n");

grep(in, 8, &len, tri_sel, 1); printf("In-place filtered not multiple of 3:"); for (i = 0; i < len; i++) printf(" %d", in[i]);

printf("\n");

return 0;

}</lang>output

Filtered even: 2 4 6 8 10
In-place filtered not multiple of 3: 1 2 4 5 7 8 10

C#

Works with: .NET version 1.1

<lang csharp>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() );</lang>

Works with: .NET version 2.0

<lang csharp>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() );</lang>

Works with: .NET version 3.5

<lang csharp>IEnumerable<int> array = new List<int>( new int[] { 1, 2, 3, 4, 5 } ); IEnumerable<int> evens = array.Where( delegate( int i ) { return (i%2)==0; } ); foreach( int i in evens )

      System.Console.WriteLine( i.ToString() );</lang>

Replacing the delegate with the more concise lambda expression syntax. <lang csharp>int[] array = { 1, 2, 3, 4, 5 }; int[] evens = array.Where(i => (i % 2) == 0).ToArray();

foreach (int i in evens)

   Console.WriteLine(i);</lang>

C++

<lang cpp>#include <vector>

include <algorithm>
include <functional>
include <iterator>
include <iostream>

int main() {

 std::vector<int> ary;
 for (int i = 0; i < 10; i++)
   ary.push_back(i);
 std::vector<int> evens;
 std::remove_copy_if(ary.begin(), ary.end(), back_inserter(evens),
                     std::bind2nd(std::modulus<int>(), 2)); // filter copy
 std::copy(evens.begin(), evens.end(),
           std::ostream_iterator<int>(std::cout, "\n"));

 return 0;

}</lang>

Works with: C++11

<lang cpp>#include <vector>

include <algorithm>
include <iterator>
include <iostream>

using namespace std;

int main() {

 vector<int> ary = {1, 2, 3, 4, 5, 6, 7, 8, 9};
 vector<int> evens;

 copy_if(ary.begin(), ary.end(), back_inserter(evens),
     [](int i) { return i % 2 == 0; });

 // print result
 copy(evens.begin(), evens.end(), ostream_iterator<int>(cout, "\n"));

}</lang>

Clean

The standard environment is required for list and array comprehensions. We specify the types of the functions because array comprehensions are overloaded. Clean provides lazy, strict, and unboxed arrays.

<lang clean>module SelectFromArray

import StdEnv</lang>

Create a lazy array where each element comes from the list 1 to 10.

<lang clean>array :: {Int} array = {x \\ x <- [1 .. 10]}</lang>

Create (and print) a strict array where each element (coming from another array) is even.

<lang clean>Start :: {!Int} Start = {x \\ x <-: array | isEven x}</lang>

Clojure

<lang lisp>;; range and filter create lazy seq's (filter even? (range 0 100))

vec will convert any type of seq to an array

(vec (filter even? (vec (range 0 100))))</lang>

Common Lisp

Common Lisp has many ways of accomplishing this task. Most of them involve higher-order sequence functions that take a predicate as the first argument and a list as the second argument. A predicate is a function that returns a boolean. The higher-order functions call the predicate for each element in list, testing the element.

In this example, the goal is to find the even numbers. The most straight-forward function is to use remove-if-not, which removes elements from the list that does not pass the predicate. The predicate, in this case, tests to see if an element is even. Therefore, the remove-if-not acts like a filter:

<lang lisp>(remove-if-not #'evenp '(1 2 3 4 5 6 7 8 9 10)) > (2 4 6 8 10)</lang>

However, this function is non-destructive, meaning the function creates a brand new list. This might be too prohibitive for very large lists.

Destructive

There is a destructive version that modifies the list in-place:

<lang lisp>(delete-if-not #'evenp '(1 2 3 4 5 6 7 8 9 10)) > (2 4 6 8 10)</lang>

D

<lang d>import std.algorithm;

void main() {

   immutable data = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10];

   auto evens = data.filter!(x => x % 2 == 0)(); // lazy

   assert(evens.equal([2, 4, 6, 8, 10]));

}</lang>

Tango Version

Library: Tango

<lang d>import tango.core.Array, tango.io.Stdout;

void main() {

   auto array = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10];

   // removeIf places even elements at the beginnig of the array and returns number of found evens
   auto evens = array.removeIf( ( typeof(array[0]) i ) { return (i % 2) == 1; } );
   Stdout("Evens - ")( array[0 .. evens] ).newline; // The order of even elements is preserved
   Stdout("Odds - ")( array[evens .. $].sort ).newline; // Unlike odd elements

}</lang> Output:

 Evens - [ 2, 4, 6, 8, 10 ]
 Odds - [ 1, 3, 5, 7, 9 ]

Delphi

<lang Delphi>program FilterEven;

{$APPTYPE CONSOLE}

uses SysUtils, Types;

const

 SOURCE_ARRAY: array[0..9] of Integer = (0,1,2,3,4,5,6,7,8,9);

var

 i: Integer;
 lEvenArray: TIntegerDynArray;

begin

 for i in SOURCE_ARRAY do
 begin
   if not Odd(i) then
   begin
     SetLength(lEvenArray, Length(lEvenArray) + 1);
     lEvenArray[Length(lEvenArray) - 1] := i;
   end;
 end;

 for i in lEvenArray do
   Write(i:3);
 Writeln;

end.</lang>

E

There are several ways this could be done.

<lang e>pragma.enable("accumulator") accum [] for x ? (x %% 2 <=> 0) in [1,2,3,4,5,6,7,8,9,10] { _.with(x) }</lang>

<lang e>var result := [] for x ? (x %% 2 <=> 0) in [1,2,3,4,5,6,7,8,9,10] {

   result with= x

} result</lang>

<lang e>def makeSeries := <elang:control.makeSeries> makeSeries([1,2,3,4,5,6,7,8,9,10]).filter(fn x,_{x %% 2 <=> 0}).asList()</lang>

Ela

Using higher-order function (non-strict version)

<lang ela>open list

evenList = filter' (\x -> x % 2 == 0) [1..]</lang>

Using comprehension (non-strict version)

<lang ela>evenList = [& x \\ x <- [1..] | x % 2 == 0]</lang>

Erlang

<lang erlang>Numbers = lists:seq(1, 5). EvenNumbers = lists:filter(fun (X) -> X rem 2 == 0 end, Numbers).</lang>

Or using a list comprehension:

<lang erlang>EvenNumbers = [X || X <- Numbers, X rem 2 == 0].</lang>

Euphoria

<lang euphoria>sequence s, evens s = {1, 2, 3, 4, 5, 6} evens = {} for i = 1 to length(s) do

   if remainder(s[i], 2) = 0 then
       evens = append(evens, s[i])
   end if

end for ? evens</lang>

Output:

{2,4,6}

F#

<lang fsharp>let lst = [1;2;3;4;5;6] List.filter (fun x -> x % 2 = 0) lst;;

val it : int list = [2; 4; 6]</lang>

Factor

This code uses filter on an array.

<lang factor>10 iota >array [ even? ] filter . ! prints { 0 2 4 6 8 }</lang>

10 iota is already a sequence, so we can skip the conversion to array.

<lang factor>10 iota [ even? ] filter . ! prints V{ 0 2 4 5 8 }</lang>

Destructive

This uses filter! to modify the original vector.

<lang factor>USE: vectors 10 iota >vector [ even? ] filter! . ! prints V{ 0 2 4 5 8 }</lang>

To prove that filter! is destructive but filter is non-destructive, I assign the original vector to v.

<lang factor>USE: locals 10 iota >vector [| v |

   v [ even? ] filter drop
   v pprint " after filter" print
   v [ even? ] filter! drop
   v pprint " after filter!" print

] call ! V{ 0 1 2 3 4 5 6 7 8 9 } after filter ! V{ 0 2 4 6 8 } after filter!</lang>

Fantom

<lang fantom> class Main {

 Void main ()
 {
   items := [1, 2, 3, 4, 5, 6, 7, 8]
   // create a new list with just the even numbers
   evens := items.findAll |i| { i.isEven }
   // display the result
   echo (evens.join(","))
 }

} </lang>

Forth

<lang forth>: sel ( dest 0 test src len -- dest len )

 cells over + swap do   ( dest len test )
   i @ over execute if
     i @ 2over cells + !
     >r 1+ r>
   then
 cell +loop drop ;

create nums 1 , 2 , 3 , 4 , 5 , 6 , create evens 6 cells allot

.array 0 ?do dup i cells + @ . loop drop ;

even? ( n -- ? ) 1 and 0= ;

evens 0 ' even? nums 6 sel .array \ 2 4 6</lang>

Fortran

<lang fortran>module funcs

 implicit none

contains

 pure function iseven(x)
   logical :: iseven
   integer, intent(in) :: x
   iseven = mod(x, 2) == 0
 end function iseven

end module funcs</lang>

<lang fortran>program Filter

 use funcs
 implicit none

 integer, parameter                 :: N = 100
 integer, dimension(N)              :: array
 integer, dimension(:), pointer     :: filtered

 integer :: i

 forall(i=1:N) array(i) = i

 filtered => filterwith(array, iseven)
 print *, filtered

contains

 function filterwith(ar, testfunc)
   integer, dimension(:), pointer        :: filterwith
   integer, dimension(:), intent(in)     :: ar
   interface
      elemental function testfunc(x)
        logical :: testfunc
        integer, intent(in) :: x
      end function testfunc
   end interface

   integer :: i, j, n

   n = count( testfunc(ar) )
   allocate( filterwith(n) )

   j = 1
   do i = lbound(ar, dim=1), ubound(ar, dim=1)
      if ( testfunc(ar(i)) ) then
         filterwith(j) = ar(i)
         j = j + 1
      end if
   end do

 end function filterwith

end program Filter</lang>

GAP

<lang gap># Built-in

Filtered([1 .. 100], IsPrime);

[ 2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71, 73, 79, 83, 89, 97 ]

Filtered([1 .. 10], IsEvenInt);

[ 2, 4, 6, 8, 10 ]

Filtered([1 .. 10], IsOddInt);

[ 1, 3, 5, 7, 9 ]</lang>

Go

<lang go>package main

import (

   "fmt"
   "math/rand"

)

func main() {

   a := rand.Perm(20)
   fmt.Println(a)       // show array to filter
   fmt.Println(even(a)) // show result of non-destructive filter
   fmt.Println(a)       // show that original array is unchanged
   reduceToEven(&a)     // destructive filter
   fmt.Println(a)       // show that a is now changed
   // a is not only changed, it is changed in place.  length and capacity
   // show that it still has its original allocated capacity but has now
   // been reduced in length.
   fmt.Println("a len:", len(a), "cap:", cap(a))

}

func even(a []int) (r []int) {

   for _, e := range a {
       if e%2 == 0 {
           r = append(r, e)
       }
   }
   return

}

func reduceToEven(pa *[]int) {

   a := *pa
   var last int
   for _, e := range a {
       if e%2 == 0 {
           a[last] = e
           last++
       }
   }
   *pa = a[:last]

}</lang> Output:

[15 1 7 3 4 8 19 0 17 18 14 5 16 9 13 11 12 10 2 6]
[4 8 0 18 14 16 12 10 2 6]
[15 1 7 3 4 8 19 0 17 18 14 5 16 9 13 11 12 10 2 6]
[4 8 0 18 14 16 12 10 2 6]
a len: 10 cap: 20

Groovy

<lang groovy> def evens = [1, 2, 3, 4, 5].findAll{it % 2 == 0}</lang>

Haskell

In Haskell, a list is often more basic than an array:

<lang haskell>ary = [1..10] evens = [x | x <- ary, even x]</lang> or <lang haskell>evens = filter even ary</lang>

To do the same operation on an array, the simplest way it to convert it lazily into a list:

<lang haskell>import Data.Array

ary = listArray (1,10) [1..10] evens = listArray (1,n) l where

 n = length l
 l = [x | x <- elems ary, even x]</lang>

Note that the bounds must be known before creating the array, so the temporary list will be fully evaluated before the array is created.

Icon and Unicon

<lang Icon>procedure main()

every put(A := [],1 to 10) # make a list of 1..10 every put(B := [],iseven(!A)) # make a second list and filter out odd numbers every writes(!B," ") | write() # show end

procedure iseven(x) #: return x if x is even or fail if x % 2 = 0 then return x end</lang>

IDL

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

<lang idl>result = array[where(NOT array AND 1)]</lang>

J

Solution:
With any verb (function) f that returns a boolean for each element of a vector v, the following is the generic solution: <lang j> (#~ f) v</lang>

Examples: <lang j> ] v=: 20 ?@$ 100 NB. vector of 20 random integers between 0 and 99 63 92 51 92 39 15 43 89 36 69 40 16 23 2 29 91 57 43 55 22

  v #~ -.2| v

92 92 36 40 16 2 22</lang>

Or using the generic form suggested above: <lang j> isEven=: 0 = 2&| NB. verb testing for even numbers

  (#~ isEven) v

92 92 36 40 16 2 22</lang>

We might decide that we use this pattern so often that it is worthwhile creating a new adverb select that filters an array using the verb to its left. <lang j> select=: adverb def '(#~ u)'

  isPrime=: 1&p:

  isEven select v

92 92 36 40 16 2 22

  isPrime select v

43 89 23 2 29 43

  (isEven *. isPrime) select v

2</lang>

Destructive example:

<lang j> v=: isEven select v</lang>

(That said, note that in a highly parallel computing environment the destruction either happens after the filtering or you have to repeatedly stall the filtering to ensure that some sort of partially filtered result has coherency.)

Java

<lang java>int[] array = {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]);</lang>

JavaFX Script

<lang javafx>def array = [1..100]; def evens = array[n | n mod 2 == 0];</lang>

JavaScript

The standard way is to use the Array.prototype.filter function (

Works with: JavaScript version 1.6

):

<lang javascript>var arr = [1,2,3,4,5]; var evens = arr.filter(function(a) {return a % 2 == 0});</lang>

Other ways: <lang 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]);</lang>

Works with: Firefox version 2.0

<lang javascript>var numbers = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10]; var evens = [i for (i in numbers) if (i % 2 == 0)];

function range(limit) {

 for(var i = 0; i < limit; i++) {
   yield i;
 }

}

var evens2 = [i for (i in range(100)) if (i % 2 == 0)];</lang>

Library: Functional

<lang javascript>Functional.select("+1&1", [1,2,3,4]) // [2, 4]</lang>

K

<lang K> / even is a boolean function

  even:{0=x!2}
  even 1 2 3 4 5

0 1 0 1 0

  / filtering the even numbers
  a@&even'a:1+!10

2 4 6 8 10

  / as a function
  evens:{x@&even'x}
  a:10?100

45 5 79 77 44 15 83 88 33 99

  evens a

44 88</lang>

Alternative syntax: <lang K> {x[&0=x!2]}

  {x[&even x]}</lang>

Destructive: <lang K> a:evens a 44 88</lang>

Liberty BASIC

<lang lb>' write random nos between 1 and 100 ' to array1 counting matches as we go dim array1(100) count=100 for i = 1 to 100

   array1(i) = int(rnd(0)*100)+1
   count=count-(array1(i) mod 2)

   if not(array1(i) mod 2) then
       n=n+1
       array2(n)=array1(i)
   end if

   print array2(n)

next</lang>

Lisaac

<lang Lisaac>+ a, b : ARRAY[INTEGER]; a := ARRAY[INTEGER].create_with_capacity 10 lower 0; b := ARRAY[INTEGER].create_with_capacity 10 lower 0; 1.to 10 do { i : INTEGER;

 a.add_last i;

}; a.foreach { item : INTEGER;

 (item % 2 = 0).if {
   b.add_last item;
 };

};</lang>

Logo

<lang logo>to even? :n

 output equal? 0 modulo :n 2

end show filter "even? [1 2 3 4] ; [2 4]

show filter [equal? 0 modulo ? 2] [1 2 3 4]</lang>

Lua

<lang lua>function filter(t, func)

 local ret = {}
 for i, v in ipairs(t) do ret[#ret+1] = func(v) and v or nil end
 return ret

end

function even(a) return a % 2 == 0 end

print(unpack(filter({1,2,3,4,5,6,7,8,9,10},even)))</lang>

Mathematica

Check for even integers: <lang Mathematica>Select[{4, 5, Pi, 2, 1.3, 7, 6, 8.0}, EvenQ]</lang> gives: <lang Mathematica>{4, 2, 6}</lang> To check also for approximate number (like 8.0 in the example above) a possible solution is: <lang Mathematica>Select[{4, 5, Pi, 2, 1.3, 7, 6, 8.0}, Mod[#, 2] == 0 &]</lang> gives: <lang Mathematica>{4, 2, 6, 8.}</lang> notice that the function returns 8. not 8 (the dot indicates that it is a float number, not an integer).

MATLAB

<lang MATLAB>function evens = selectEvenNumbers(list)

   evens = list( mod(list,2) == 0 );

end</lang>

Sample Output: <lang MATLAB>>> selectEvenNumbers([0 1 2 3 4 5 6 7 8 9 10])

ans =

    0     2     4     6     8    10</lang>

Maxima

<lang maxima>a: makelist(i, i, 1, 20); [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20]

sublist(a, evenp); [2, 4, 6, 8, 10, 12, 14, 16, 18, 20]

sublist(a, lambda([n], mod(n, 3) = 0)); [3, 6, 9, 12, 15, 18]</lang>

MAXScript

<lang maxscript>arr = #(1, 2, 3, 4, 5, 6, 7, 8, 9) newArr = for i in arr where (mod i 2 == 0) collect i</lang>

MUMPS

<lang MUMPS>FILTERARRAY

;NEW I,J,A,B - Not making new, so we can show the values
;Populate array A
FOR I=1:1:10 SET A(I)=I
;Move even numbers into B
SET J=0 FOR I=1:1:10 SET:A(I)#2=0 B($INCREMENT(J))=A(I)
QUIT</lang>

Testing:

WRITE

A(1)=1
A(2)=2
A(3)=3
A(4)=4
A(5)=5
A(6)=6
A(7)=7
A(8)=8
A(9)=9
A(10)=10
B(1)=2
B(2)=4
B(3)=6
B(4)=8
B(5)=10
I=10
J=5

Nemerle

Lists have a built-in method for filtering: <lang Nemerle>def original = $[1 .. 100]; def filtered = original.Filter(fun(n) {n % 2 == 0}); WriteLine($"$filtered");</lang> The following would work for arrays: <lang Nemerle>Filter[T] (a : array[T], f : T -> bool) : array[T] {

   def b = $[x | x in a, (f(x))];
   b.ToArray()

}</lang>

NewLISP

<lang NewLISP>> (filter (fn (x) (= (% x 2) 0)) '(1 2 3 4 5 6 7 8 9 10)) (2 4 6 8 10) </lang>

Nial

<lang nial>filter (= [0 first, mod [first, 2 first] ] ) 0 1 2 3 4 5 6 7 8 9 10 =0 2 4 6 8 10</lang>

Objeck

<lang objeck> use Structure;

bundle Default {

 class Evens {
   function : Main(args : String[]) ~ Nil {
     values := IntVector->New([1, 2, 3, 4, 5]);
     f := Filter(Int) ~ Bool;
     evens := values->Filter(f);

     each(i : evens) {
       evens->Get(i)->PrintLine();
     };
   }

   function : Filter(v : Int) ~ Bool {
     return v % 2 = 0;
   }
 }

} </lang>

Objective-C

Works with: Cocoa version Mac OS X 10.6+

<lang objc>NSArray *numbers = [NSArray arrayWithObjects:[NSNumber numberWithInt:1],

                                            [NSNumber numberWithInt:2],
                                            [NSNumber numberWithInt:3],
                                            [NSNumber numberWithInt:4],
                                            [NSNumber numberWithInt:5], nil];

NSArray *evens = [numbers objectsAtIndexes:[numbers indexesOfObjectsPassingTest:

 ^BOOL(id obj, NSUInteger idx, BOOL *stop) { return [obj intValue] % 2 == 0; } ]];</lang>

Works with: Cocoa version Mac OS X 10.5+

<lang objc>NSArray *numbers = [NSArray arrayWithObjects:[NSNumber numberWithInt:1],

                                            [NSNumber numberWithInt:2],
                                            [NSNumber numberWithInt:3],
                                            [NSNumber numberWithInt:4],
                                            [NSNumber numberWithInt:5], nil];

NSPredicate *isEven = [NSPredicate predicateWithFormat:@"modulus:by:(SELF, 2) == 0"]; NSArray *evens = [numbers filteredArrayUsingPredicate:isEven];</lang>

Works with: GNUstep

<lang objc>#import <Foundation/Foundation.h>

@interface NSNumber ( ExtFunc ) -(int) modulo2; @end

@implementation NSNumber ( ExtFunc ) -(int) modulo2 {

 return [self intValue] % 2;

} @end

int main() {

 NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];

 NSArray *numbers = [NSArray arrayWithObjects:[NSNumber numberWithInt:1],
                                              [NSNumber numberWithInt:2],
                                              [NSNumber numberWithInt:3],
                                              [NSNumber numberWithInt:4],
                                              [NSNumber numberWithInt:5], nil];

 NSPredicate *isEven = [NSPredicate predicateWithFormat:@"modulo2 == 0"];
 NSArray *evens = [numbers filteredArrayUsingPredicate:isEven];
 
 NSLog(@"%@", evens);

 [pool release];
 return 0;

}</lang>

OCaml

It is easier to do it with a list: <lang ocaml>let lst = [1;2;3;4;5;6] let even_lst = List.filter (fun x -> x mod 2 = 0) lst</lang>

Octave

<lang octave>arr = [1:100]; evennums = arr( mod(arr, 2) == 0 ); disp(evennums);</lang>

Oz

It is easier to do it with a list: <lang oz>declare Lst = [1 2 3 4 5] LstEven = {Filter Lst IsEven}</lang>

PARI/GP

<lang parigp>selecteven(v)=vecextract(v,1<<(#v\2*2+1)\3);</lang>

Pascal

Arrays are supported in all versions of pascal so this simple example will cover the entire gamut.

Works with: Delphi

Works with: Turbo Pascal

<lang pascal>const

numbers:array[0..9] of integer = (0,1,2,3,4,5,6,7,8,9);

for x = 1 to 10 do

    if odd(numbers[x]) then
        writeln( 'The number ',numbers[x],' is odd.');
    else
        writeln( 'The number ',numbers[x],' is even.');</lang>

The odd() function is a standard library function of pascal as is the function even().

Perl

Perl 6

Works with: Rakudo Star version 2010-08

Alternatively:

Destructive:

PHP

Using a standard loop <lang php>$arr = range(1,5); $evens = array(); foreach ($arr as $val){

     if ($val % 2 == 0) $evens[] = $val);

} print_r($evens);</lang>

Using a filter function <lang php>function is_even($var) { return(!($var & 1)); } $arr = range(1,5); $evens = array_filter($arr, "is_even"); print_r($evens);</lang>

PicoLisp

<lang PicoLisp>(filter '((N) (not (bit? 1 N)))

  (1 2 3 4 5 6 7 8 9) )</lang>

Output:

-> (2 4 6 8)

Pop11

Most natural solution in Pop11 would probably use list. Below we accumulate filtered elements on the stack and then allocate array for the result:

<lang pop11>;;; Generic filtering procedure which selects from ar elements

satisfying pred

define filter_array(ar, pred); lvars i, k;

   stacklength() -> k;
   for i from 1 to length(ar) do
      ;;; if element satisfies pred we leave it on the stack
      if pred(ar(i)) then ar(i) endif;
   endfor;
   ;;; Collect elements from the stack into a vector
   return (consvector(stacklength() - k));

enddefine;

Use it

filter_array({1, 2, 3, 4, 5},

            procedure(x); not(testbit(x, 0)); endprocedure) =></lang>

PostScript

Library: initlib

PowerShell

<lang powershell>$array = -15..37 $array | Where-Object { $_ % 2 -eq 0 }</lang>

Prolog

findall

<lang prolog>evens(D, Es) :- findall(E, (member(E, D), E mod 2 =:= 0), Es).</lang>

Usage:

<lang prolog>?- evens([1,2,3,4,5,6,7,8,9,10],E). E = [2, 4, 6, 8, 10]</lang>

Anonymous functions

Works with SWI-Prolog and module(lambda) written by Ulrich Neumerkel, "lambda.pl" can be found there : http://www.complang.tuwien.ac.at/ulrich/Prolog-inedit/lambda.pl <lang Prolog>?- use_module(library(lambda)). true.

?- include((\X^(X mod 2 =:= 0)), [1,2,3,4,5,6,7,8,9], L). L = [2,4,6,8].</lang>

filter and anonymous functions

Works with SWI-Prolog and module(lambda) written by Ulrich Neumerkel, "lambda.pl" can be found there : http://www.complang.tuwien.ac.at/ulrich/Prolog-inedit/lambda.pl <lang prolog>:- use_module(lambda).

%% filter(Pred, LstIn, LstOut) %% filter(_Pre, [], []).

filter(Pred, [H|T], L) :- filter(Pred, T, L1), ( call(Pred,H) -> L = [H|L1]; L = L1). </lang> Usage : <lang prolog> ?- filter(\X^(X mod 2 =:= 0), [1,2,3,4,5,6,7,8,9], L). L = [2,4,6,8] . </lang>

Protium

Fixed length English dialect <lang html><@ LETCNWLSTLIT>numbers|1 2 3 4 5 6 7 8 9 10 11 12</@> <@ DEFLST>evens</@> <@ ENULSTLIT>numbers| <@ TSTEVEELTLST>...</@> <@ IFF> <@ LETLSTELTLST>evens|...</@> </@> </@></lang>

PureBasic

<lang PureBasic>Dim Tal.i(9) Dim Evens.i(0)

- Set up an array with random numbers

For i=0 To ArraySize(Tal())

 Tal(i)=Random(100)

- Pick out all Even and save them

j=0 For i=0 To ArraySize(Tal())

 If Tal(i)%2=0
   ReDim Evens(j)    ; extend the Array as we find new Even's
   Evens(j)=tal(i)
   j+1
 EndIf

- Display the result

PrintN("List of Randoms") For i=0 To ArraySize(Tal())

 Print(Str(Tal(i))+" ")

Next PrintN(#CRLF$+#CRLF$+"List of Even(s)") For i=0 To ArraySize(Evens())

 Print(Str(Evens(i))+" ")

Next</lang>

Output can look like

List of Randoms
32 35 89 91 11 33 12 22 42 43

List of Even(s)
32 12 22 42

Python

Works with: Python version 2.4

<lang 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

alternately but less idiomatic:

evens = filter(lambda x: not x & 1, values)</lang>

Alternative using the slice syntax with its optional "stride" expression:

<lang python>values = range(10) evens = values[::2]</lang>

This works for all versions of Python (at least as far back as 1.5). Lists (arrays) can be "sliced" by indexing them with a range (lower and upper bounds). Thus mylist[1:9] evaluates into a list from the second item (excluding the first item which is mylist[0], of course) up to but not including the ninth item. In Python the expression mylist[:] is synonymous with mylist[0:len(mylist)] ... returning a copy of the complete list. also mylist[:x] returns the first x items from the list and negative numbers can be used such that mylist[-x:] returns the last x items from the list. The relatively obscure and optional stride expression can skip items and/or force the evaluation from the end of the list downward towards it's lower elements. Thus mylist[::-1] returns a reversed copy of the list, mylist[::2] returns all even elements, mylist[1::2] returns all odd elements, and so on.

Since strings in Python can be treated as a sort of immutable list of characters then the slicing and extended slicing can also be used with them as well. Thus mystring[::-2] will return every other character from the reverse order of the string.

One can also assign to a slice (of a list or other mutable indexed object. Thus the following:

<lang python>values = range(10) values[::2] = [11,13,15,17,19] print values 11, 1, 13, 3, 15, 5, 17, 7, 19, 9</lang>

R

Raven

<lang raven>[ 0 1 2 3 4 5 6 7 8 9 ] as nums group nums each

   dup 1 & if drop

list as evens</lang>

REBOL

<lang REBOL>a: [] repeat i 100 [append a i] ; Build and load array.

evens: [] repeat element a [if even? element [append evens element]]

print mold evens</lang>

Output:

[2 4 6 8 10 12 14 16 18 20 22 24 
26 28 30 32 34 36 38 40 42 44 46 48 50 
52 54 56 58 60 62 64 66 68 70 72 74 76 
78 80 82 84 86 88 90 92 94 96 98 100]

REXX

This example shows that array element indexes may be negative and the array bounds need not be declared. <lang rexx>/*REXX program selects all even numbers from an array ──► a new array. */ numeric digits 210000 /*handle past 1m for Fibonacci.*/ old.= /*the OLD array, all null so far.*/ new.= /*the NEW array, all null so far.*/

 do j=-40 to 40;  old.j=fib(j);  end  /*put 81 Fibonacci numbs ==> OLD */

news=0 /*numb. of elements in NEW so far*/

/*══════════════════════════════════════════════════════════════════════*/

     do k=-40  while old.k\==       /*process the OLD array elements.*/
     if old.k//2 \== 0 then iterate   /*if element isn't even, skip it.*/
     news=news+1                      /*bump the number of NEW elements*/
     new.news=old.k                   /*assign it to the NEW array.    */
     end

/*══════════════════════════════════════════════════════════════════════*/

     do j=1  for news
     say 'new.'j "=" new.j
     end                              /*show all NEW numbers.*/

exit /*stick a fork in it, we're done.*/ /*─────────────────────────────────────FIB subroutine (non-recursive)───*/ fib: procedure; parse arg n; na=abs(n); if na<2 then return na /*special*/ a=0; b=1

               do j=2 to na; s=a+b; a=b; b=s; end

if n>0 | na//2==1 then return s /*if positive or odd negative... */

                  else return -s      /*return a negative Fib number.  */</lang>

output

new.1 = 63245986
new.2 = -14930352
new.3 = 3524578
new.4 = -832040
new.5 = 196418
new.6 = -46368
new.7 = 10946
new.8 = -2584
new.9 = 610
new.10 = -144
new.11 = 34
new.12 = -8
new.13 = 2
new.14 = 0
new.15 = 2
new.16 = 8
new.17 = 34
new.18 = 144
new.19 = 610
new.20 = 2584
new.21 = 10946
new.22 = 46368
new.23 = 196418
new.24 = 832040
new.25 = 3524578
new.26 = 14930352
new.27 = 63245986

Ruby

Enumerable#select is the filter that returns a new Array. This example calls Integer#even?, which is new to Ruby 1.8.7.

Works with: Ruby version 1.8.7

<lang ruby># Enumerable#select returns a new array. ary = [1, 2, 3, 4, 5, 6] even_ary = ary.select {|elem| elem.even?} p even_ary # => [2, 4, 6]

Enumerable#select also works with Range.

range = 1..6 even_ary = range.select {|elem| elem.even?} p even_ary # => [2, 4, 6]</lang>

If you want this code to work with Ruby older than 1.8.7, then you may define Integer#even?.
<lang ruby># Integer#even? is new to Ruby 1.8.7.

Define it for older Ruby.

unless Integer.method_defined? :even? class Integer def even? self % 2 == 0 end end end</lang>

Destructive

Array#select! is the destructive version which modifies the original Array. Array#select! is new to Ruby 1.9.2.

Works with: Ruby version 1.9.2

<lang ruby>ary = [1, 2, 3, 4, 5, 6] ary.select! {|elem| elem.even?} p ary # => [2, 4, 6]</lang>

For Ruby older than 1.9.2, you can easily define Array#select! around the older Array#delete_if method.
<lang ruby># Array#select! is new to Ruby 1.9.2.

Define it for older Ruby.

unless Array.method_defined? :select! class Array def select! enum_for(:select!) unless block_given? delete_if { |elem| not yield elem } self end end end</lang>

Salmon

In this example, [1...10] is a list of the integers from 1 to 10. The comprehend expression walks over this list and selects only the even elements. The result of the comprehend expression is a new list with only the even elements. Then an iterate statement is used to walk over the list of even elements and print them out.

<lang Salmon>iterate(x; comprehend(y; [1...10]; y % 2 == 0) (y))

   x!;</lang>

Here's a version that walks an array destructively removing the non-even elements:

<lang Salmon>variable my_array := [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]; variable write_position := 0; iterate (read_position; [0...9])

 {
   immutable elem := my_array[read_position];
   if (elem % 2 == 0)
     {
       my_array[write_position] := elem;
       ++write_position;
     };
 };

// Chop off the end of the array. my_array := my_array[0...write_position - 1]; iterate(x; my_array)

   x!;</lang>

Sather

<lang sather>class MARRAY{T} < $ARR{T} is

 include ARRAY{T};

 filter_by(r:ROUT{T}:BOOL):SAME is
   o:MARRAY{T} := #;
   loop e ::= elt!;
     if r.call(e) then
       o := o.append(#MARRAY{T}(|e|));
     end;
   end;
   return o;
 end;

end;

class MAIN is

 main is
   a ::= #MARRAY{INT}(|5, 6, 7, 8, 9, 10, 11|);
   sel ::= a.filter_by( bind(_.is_even) );
   loop #OUT + sel.elt! + " "; end;
   #OUT + "\n";
 end;

end;</lang>

Scala

<lang scala>(1 to 100).filter(_ % 2 == 0)</lang>

Scheme

In the interactive prompt: <lang scheme>> (filter even? '(1 2 3 4 5 6 7 8 9 10)) (2 4 6 8 10)</lang> Or as a function: <lang scheme>(define (select-even lst)

 (filter even? lst))

(select-even '(1 2 3 4 5 6 7 8 9 10))</lang>

Seed7

<lang seed7>var array integer: arr is [] (1, 2, 3, 4, 5); var array integer: evens is 0 times 0; var integer: number is 0;

for number range arr do

 if not odd(number) then
   evens &:= [] (number);
 end if;

end for;</lang>

Slate

<lang slate>#(1 2 3 4 5) select: [| :number | number isEven].</lang>

Smalltalk

<lang smalltalk>#(1 2 3 4 5) select: [:number | number even]</lang>

SQL

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

Works with: MS SQL

<lang sql>--Create the original array (table #nos) with numbers from 1 to 10 create table #nos (v int) declare @n int set @n=1 while @n<=10 begin insert into #nos values (@n) set @n=@n+1 end

--Select the subset that are even into the new array (table #evens) select v into #evens from #nos where v % 2 = 0

-- Show #evens select * from #evens

-- Clean up so you can edit and repeat: drop table #nos drop table #evens</lang>

'

Works with: MySQL

<lang sql>create temporary table nos (v int); insert into nos values (1),(2),(3),(4),(5),(6),(7),(8),(9),(10); create temporary table evens (v int); insert into evens select v from nos where v%2=0; select * from evens order by v; /*2,4,6,8,10*/ drop table nos; drop table evens;</lang>

Or to be shorter, you could create the table evens directly from the query result : <lang sql>create temporary table evens select * from nos where v%2=0;</lang>

Standard ML

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

Tcl

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:

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

In this case, we can do this particular challenge with: <lang tcl>foreach {key val} [array get srcArray] {

   if {[string is integer -strict $key] && !($key%2)} {
       set dstArray($key) $val
   }

}</lang>

If we were using Tcl's lists and interpreting the challenge to mean getting just the elements at index 0, 2, 4, ... <lang tcl>foreach {even odd} $srcList {

   lappend dstList $even

}</lang>

Toka

<lang toka>10 cells is-array table 10 cells is-array even {

 variable source
 [ swap source ! >r reset r> 0
   [ i source @ array.get
     dup 2 mod 0 <> [ drop ] ifTrue 
   ] countedLoop
   depth 0 swap [ i even array.put ] countedLoop
 ]

} is copy-even 10 0 [ i i table array.put ] countedLoop table 10 copy-even</lang>

TUSCRIPT

<lang tuscript> $$ MODE TUSCRIPT arr="1'4'9'16'25'36'49'64'81'100",even="" LOOP nr=arr rest=MOD (nr,2) IF (rest==0) even=APPEND (even,nr) ENDLOOP PRINT even </lang> Output:

4'16'36'64'100

UNIX Shell

Works with: Bash

<lang bash>a=(1 2 3 4 5) unset e[@] for ((i=0;i<${#a[@]};i++)); do

 [ $((a[$i]%2)) -eq 0 ] && e[$i]="${a[$i]}"

done

echo "${a[@]}" echo "${e[@]}"</lang> Output:

1 2 3 4 5
2 4

UnixPipes

<lang bash>yes \ | cat -n | while read a; do ; expr $a % 2 >/dev/null && echo $a ; done</lang>

Ursala

Ursala doesn't have arrays, except when the run time system transparently converts a list to an array as needed for an external math library function call. However, selection can be done on lists.

Unary predicates

The most common way to select items from a list according to a unary predicate p is to write p*~, as shown below.

<lang Ursala>#import std

import nat

x = <89,36,13,15,41,39,21,3,15,92,16,59,52,88,33,65,54,88,93,43>

cast %nL

y = (not remainder\2)*~ x</lang>

The output will be

<36,92,16,52,88,54,88>

Binary predicates

Selection is so frequently useful that the language has a couple of other ways to do it. Selecting according to a binary predicate can be done like this. <lang Ursala>z = (not remainder)~| (36,<1,2,3,4,5,6,7,8,9,10,11,12>)</lang> The value of z will be the divisors of 36 appearing in the list.

<1,2,3,4,6,9,12>

This usage has the advantage over writing (not remainder/36)*~ with the operator above that it allows the 36 to be part of the argument rather than being hard coded into the function.

Operator suffixes

Many operators in Ursala allow suffixes that modify their semantics. For example, the suffix ihB on the identity function ~& makes it ~&ihB, a predicate to detect odd numbers by inspecting the binary representation. If an operator with this kind of suffix is further modified by appending an F, it becomes a selection filter. For example <lang Ursala>shortcut = ~&ihBF x</lang> using the x defined above will evaluate to

<89,13,15,41,39,21,3,15,59,33,65,93,43>

There are also suffixes corresponding to the ~| operator.

(: Long version with a FLWOR expression :) let $long := for $value in $array

            where $value mod 2 = 0
            return $value

(: Show the results :) return

 <result>
   <short>{$short}</short>
   <long>{$short}</long>
 </result>

</lang>

The output of this query is: <lang xml> <?xml version="1.0" encoding="UTF-8"?> <result>

  <short>2 4 6 8 10</short>
  <long>2 4 6 8 10</long>

</result> </lang>

XSLT

 <xsl:value-of select="@value" />

</xsl:for-each></lang>