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Filter

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Task
Filter
You are encouraged to solve this task according to the task description, using any language you may know.
Task

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.

Contents

11l[edit]

V array = Array(1..10)
V even = array.filter(n -> n % 2 == 0)
print(even)
Output:
[2, 4, 6, 8, 10]

ACL2[edit]

(defun filter-evens (xs)
(cond ((endp xs) nil)
((evenp (first xs))
(cons (first xs) (filter-evens (rest xs))))
(t (filter-evens (rest xs)))))

Action![edit]

DEFINE PTR="CARD"
 
INT value ;used in predicate
 
PROC PrintArray(INT ARRAY a BYTE size)
BYTE i
 
Put('[)
FOR i=0 TO size-1
DO
PrintI(a(i))
IF i<size-1 THEN
Put(' )
FI
OD
Put(']) PutE()
RETURN
 
;jump addr is stored in X and A registers
BYTE FUNC Predicate=*(PTR jumpAddr)
DEFINE STX="$8E"
DEFINE STA="$8D"
DEFINE JSR="$20"
DEFINE RTS="$60"
[STX Predicate+8
STA Predicate+7
JSR $00 $00
RTS]
 
PROC DoFilter(PTR predicateFun
INT ARRAY src BYTE srcSize
INT ARRAY dst BYTE POINTER dstSize)
 
INT i
 
dstSize^=0
FOR i=0 TO srcSize-1
DO
value=src(i)
IF Predicate(predicateFun) THEN
dst(dstSize^)=value
dstSize^==+1
FI
OD
RETURN
 
PROC DoFilterInplace(PTR predicateFun
INT ARRAY data BYTE POINTER size)
 
INT i,j
 
i=0
WHILE i<size^
DO
value=data(i)
IF Predicate(predicateFun)=0 THEN
FOR j=i TO size^-2
DO
data(j)=data(j+1)
OD
size^==-1
ELSE
i==+1
FI
OD
RETURN
 
BYTE FUNC Even()
IF (value&1)=0 THEN
RETURN (1)
FI
RETURN (0)
 
BYTE FUNC NonNegative()
IF value>=0 THEN
RETURN (1)
FI
RETURN (0)
 
PROC Main()
INT ARRAY src=[65532 3 5 2 65529 1 0 65300 4123],dst(9)
BYTE srcSize=[9],dstSize
 
PrintE("Non destructive operations:") PutE()
PrintE("Original array:")
PrintArray(src,srcSize)
 
DoFilter(Even,src,srcSize,dst,@dstSize)
PrintE("Select all even numbers:")
PrintArray(dst,dstSize)
 
DoFilter(NonNegative,src,srcSize,dst,@dstSize)
PrintE("Select all non negative numbers:")
PrintArray(dst,dstSize)
 
PutE()
PrintE("Destructive operations:") PutE()
PrintE("Original array:")
PrintArray(src,srcSize)
 
DoFilterInplace(Even,src,@srcSize)
PrintE("Select all even numbers:")
PrintArray(src,srcSize)
 
DoFilterInplace(NonNegative,src,@srcSize)
PrintE("Select all non negative numbers:")
PrintArray(src,srcSize)
RETURN
Output:

Screenshot from Atari 8-bit computer

Non destructive operations:

Original array:
[-4 3 5 2 -7 1 0 -236 4123]
Select all even numbers:
[-4 2 0 -236]
Select all non negative numbers:
[3 5 2 1 0 4123]

Destructive operations:

Original array:
[-4 3 5 2 -7 1 0 -236 4123]
Select all even numbers:
[-4 2 0 -236]
Select all non negative numbers:
[2 0]

ActionScript[edit]

var arr:Array = new Array(1, 2, 3, 4, 5);
var evens:Array = new Array();
for (var i:int = 0; i < arr.length(); i++) {
if (arr[i] % 2 == 0)
evens.push(arr[i]);
}

Actionscript 3

var arr:Array = new Array(1, 2, 3, 4, 5);
arr = arr.filter(function(item:int, index:int, array:Array) {
return item % 2 == 0;
});
 

Ada[edit]

with Ada.Integer_Text_Io; use Ada.Integer_Text_Io;
with Ada.Text_Io; use Ada.Text_Io;
 
procedure Array_Selection is
type Array_Type is array (Positive range <>) of Integer;
Null_Array : Array_Type(1..0);
 
function Evens (Item : Array_Type) return Array_Type is
begin
if Item'Length > 0 then
if Item(Item'First) mod 2 = 0 then
return Item(Item'First) & Evens(Item((Item'First + 1)..Item'Last));
else
return Evens(Item((Item'First + 1)..Item'Last));
end if;
else
return Null_Array;
end if;
end Evens;
 
procedure Print(Item : Array_Type) is
begin
for I in Item'range loop
Put(Item(I));
New_Line;
end loop;
end Print;
 
Foo : Array_Type := (1,2,3,4,5,6,7,8,9,10);
begin
Print(Evens(Foo));
end Array_Selection;

Here is a non-recursive solution:

with Ada.Text_IO;  use Ada.Text_IO;
 
procedure Array_Selection is
type Array_Type is array (Positive range <>) of Integer;
 
function Evens (Item : Array_Type) return Array_Type is
Result : Array_Type (1..Item'Length);
Index  : Positive := 1;
begin
for I in Item'Range loop
if Item (I) mod 2 = 0 then
Result (Index) := Item (I);
Index := Index + 1;
end if;
end loop;
return Result (1..Index - 1);
end Evens;
 
procedure Put (Item : Array_Type) is
begin
for I in Item'range loop
Put (Integer'Image (Item (I)));
end loop;
end Put;
begin
Put (Evens ((1,2,3,4,5,6,7,8,9,10)));
New_Line;
end Array_Selection;

Aime[edit]

integer
even(integer e)
{
return !(e & 1);
}
 
list
filter(list l, integer (*f)(integer))
{
integer i;
list v;
 
i = 0;
while (i < l_length(l)) {
integer e;
 
e = l_q_integer(l, i);
if (f(e)) {
lb_p_integer(v, e);
}
 
i += 1;
}
 
return v;
}
 
integer
main(void)
{
integer i;
list l;
 
i = 0;
while (i < 10) {
lb_p_integer(l, i);
i += 1;
}
 
l = filter(l, even);
 
i = 0;
while (i < l_length(l)) {
o_space(1);
o_integer(l_q_integer(l, i));
i += 1;
}
o_byte('\n');
 
return 0;
}
Output:
 0 2 4 6 8

ALGOL 68[edit]

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
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))
Output:
         +2         +4         +6         +8        +10
         +4        +16        +36        +64       +100

ALGOL W[edit]

In the original Algol W, for procedures passed as parameters to another procedure, the types of the expected parameters could not be specified. (The types of the reuired parameters had to match exactly with those in the call)
In the sample below, the select procedure's parameters do not include the parameters of where - they are commented out.
Recent compilers (such as Awe) require the parameter types be specified and so the parameters to where should be uncommented when compilling with Awe.

begin
 % sets the elements of out to the elements of in that return true from applying the where procedure to them %
 % the bounds of in must be 1 :: inUb - out must be at least as big as in and the number of matching  %
 % elements is returned in outUb - in and out can be the same array  %
procedure select ( integer array in ( * ); integer value inUb
 ; integer array out ( * ); integer result outUb
 ; logical procedure where % ( integer value n ) %
) ;
begin
outUb := 0;
for i := 1 until inUb do begin
if where( in( i ) ) then begin
outUb := outUb + 1;
out( outUb ) := in( i )
end f_where_in_i
end for_i
end select ;
 % test the select procedure %
logical procedure isEven ( integer value n ) ; not odd( n );
integer array t, out ( 1 :: 10 );
integer outUb;
for i := 1 until 10 do t( i ) := i;
select( t, 10, out, outUb, isEven );
for i := 1 until outUb do writeon( i_w := 3, s_w := 0, out( i ) );
write()
end.
 
Output:
  2  4  6  8 10

AmigaE[edit]

PROC main()
DEF l : PTR TO LONG, r : PTR TO LONG, x
l := [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
r := List(ListLen(l))
SelectList({x}, l, r, `Mod(x,2)=0)
ForAll({x}, r, `WriteF('\d\n', x))
ENDPROC

AntLang[edit]

x:range[100]
{1- x mod 2}hfilter x

Apex[edit]

List<Integer> integers = new List<Integer>{1,2,3,4,5};
Set<Integer> evenIntegers = new Set<Integer>();
for(Integer i : integers)
{
if(math.mod(i,2) == 0)
{
evenIntegers.add(i);
}
}
system.assert(evenIntegers.size() == 2, 'We should only have two even numbers in the set');
system.assert(!evenIntegers.contains(1), '1 should not be a number in the set');
system.assert(evenIntegers.contains(2), '2 should be a number in the set');
system.assert(!evenIntegers.contains(3), '3 should not be a number in the set');
system.assert(evenIntegers.contains(4), '4 should be a number in the set');
system.assert(!evenIntegers.contains(5), '5 should not be a number in the set');

APL[edit]

      (0=2|x)/x←⍳20
2 4 6 8 10 12 14 16 18 20

AppleScript[edit]

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
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):

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)


We can simplify and generalise this further by lifting any ordinary predicate handler into a script on the fly.

In this example, as with JavaScript filter lambdas, the lifted handler can optionally have one or two additional arguments:

  1. The index of the current element
  2. A reference to the whole list.


This allows for context-sensitive filters, which can take account of following or preceding elements in a sequence.

-------------------------- FILTER --------------------------
 
-- filter :: (a -> Bool) -> [a] -> [a]
on filter(f, xs)
tell mReturn(f)
set lst to {}
set lng to length of xs
repeat with i from 1 to lng
set v to item i of xs
if |λ|(v, i, xs) then set end of lst to v
end repeat
return lst
end tell
end filter
 
 
--------------------------- TEST ---------------------------
on run
filter(even, {0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10})
 
--> {0, 2, 4, 6, 8, 10}
end run
 
 
-------------------- GENERIC FUNCTIONS ---------------------
 
-- even :: Int -> Bool
on even(x)
0 = x mod 2
end even
 
 
-- Lift 2nd class handler function into 1st class script wrapper
-- mReturn :: Handler -> Script
on mReturn(f)
if class of f is script then
f
else
script
property |λ| : f
end script
end if
end mReturn
Output:
{0, 2, 4, 6, 8, 10}

Arturo[edit]

arr: [1 2 3 4 5 6 7 8 9 10]
 
print select arr [x][even? x]
Output:
2 4 6 8 10

AutoHotkey[edit]

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))
 
 
 

AWK[edit]

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.

One-liner:

$ 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}'
Output:
4 6 8 2

Regular script:

 
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
}
 

Same output.

Batch File[edit]

 
@echo off
setlocal enabledelayedexpansion
 
set numberarray=1 2 3 4 5 6 7 8 9 10
for %%i in (%numberarray%) do (
set /a tempcount+=1
set numberarray!tempcount!=%%i
)
 
echo Filtering all even numbers from numberarray into newarray...
call:filternew numberarray
echo numberarray - %numberarray%
echo newarray -%newarray%
echo.
echo Filtering numberarray so that only even entries remain...
call:filterdestroy numberarray
echo numberarray -%numberarray%
pause>nul
exit /b
 
:filternew
set arrayname=%1
call:arraylength %arrayname%
set tempcount=0
for /l %%i in (1,1,%length%) do (
set /a cond=!%arrayname%%%i! %% 2
if !cond!==0 (
set /a tempcount+=1
set newarray!tempcount!=!%arrayname%%%i!
set newarray=!newarray! !%arrayname%%%i!
)
)
exit /b
 
:filterdestroy
set arrayname=%1
call:arraylength %arrayname%
set tempcount=0
set "%arrayname%="
for /l %%i in (1,1,%length%) do (
set /a cond=!%arrayname%%%i! %% 2
if !cond!==0 (
set /a tempcount+=1
set %arrayname%!tempcount!=!%arrayname%%%i!
set %arrayname%=!%arrayname%! !%arrayname%%%i!
)
)
exit /b
 
:arraylength
set tempcount=0
set lengthname=%1
set length=0
:lengthloop
set /a tempcount+=1
if "!%lengthname%%tempcount%!"=="" exit /b
set /a length+=1
goto lengthloop
 
Output:
Filtering all even numbers from numberarray into newarray...
numberarray - 1 2 3 4 5 6 7 8 9 10
newarray    - 2 4 6 8 10

Filtering numberarray so that only even entries remain...
numberarray - 2 4 6 8 10

BBC BASIC[edit]

      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)

BCPL[edit]

get "libhdr"
 
// Copy every value for which p(x) is true from in to out
// This will also work in place by setting out = in
let filter(p, in, ilen, out, olen) be
$(  !olen := 0
for i = 0 to ilen-1 do
if p(in!i) do
$( out!!olen := in!i
 !olen := !olen + 1
$)
$)
 
// Write N elements from vector
let writevec(v, n) be
for i = 0 to n-1 do writef("%N ", v!i)
 
let start() be
$( // Predicates
let even(n) = (n&1) = 0
let mul3(n) = n rem 3 = 0
 
let nums = table 1,2,3,4,5,6,7,8,9,10,11,12,13,14,15
let arr = vec 20
let len = ?
 
writes("Numbers: ")
writevec(nums, 15)
 
// Filter 'nums' into 'arr'
filter(even, nums, 15, arr, @len)
writes("*NEven numbers: ")
writevec(arr, len)
 
// Filter 'arr' in place for multiples of 3
filter(mul3, arr, len, arr, @len)
writes("*NEven multiples of 3: ")
writevec(arr, len)
wrch('*N')
$)
Output:
Numbers: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Even numbers: 2 4 6 8 10 12 14
Even multiples of 3: 6 12

Bracmat[edit]

( :?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
)
)
 
1 3 5 7 9 25 49 81

Brat[edit]

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

Burlesque[edit]

 
blsq ) 1 [email protected]{2.%n!}f[
{2 4 6 8 10 12}
 

BQN[edit]

General filtering is done using the /(Replicate) function, which can fliter elements given a bitmask. We can make a modifier based on it to accept a function for filtering.

_filter ← {(𝔽𝕩)/𝕩}
Odd ← 2⊸|
 
Odd _filter 1‿2‿3‿4‿5
⟨ 1 3 5 ⟩

C[edit]

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

C#[edit]

Works with: .NET version 1.1
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() );
Works with: .NET version 2.0
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() );
Works with: .NET version 3.5
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() );

Replacing the delegate with the more concise lambda expression syntax.

int[] array = { 1, 2, 3, 4, 5 };
int[] evens = array.Where(i => (i % 2) == 0).ToArray();
 
foreach (int i in evens)
Console.WriteLine(i);

C++[edit]

#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;
}


Works with: C++11
#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"));
}

Clean[edit]

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.

module SelectFromArray
 
import StdEnv

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

array :: {Int}
array = {x \\ x <- [1 .. 10]}

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

Start :: {!Int}
Start = {x \\ x <-: array | isEven x}

Clojure[edit]

;; 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))))

CoffeeScript[edit]

[1..10].filter (x) -> not (x%2)
Output:
[ 2,
  4,
  6,
  8,
  10 ]

Common Lisp[edit]

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:

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

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

Destructive[edit]

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

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

Cowgol[edit]

include "cowgol.coh";
 
# Cowgol has strict typing and there are no templates either.
# Defining the type this way makes it easy to change.
typedef FilterT is uint32;
 
# In order to pass functions around, we need to define an
# interface. The 'FilterPredicate' interface will take an argument
# and return zero if it should be filtered out.
interface FilterPredicate(x: FilterT): (keep: uint8);
 
# Filter an array and store it a new location. Returns the new length.
sub Filter(f: FilterPredicate,
items: [FilterT],
length: intptr,
result: [FilterT]):
(newLength: intptr) is
newLength := 0;
while length > 0 loop
var item := [items];
items := @next items;
if f(item) != 0 then
[result] := item;
result := @next result;
newLength := newLength + 1;
end if;
length := length - 1;
end loop;
end sub;
 
# Filter an array in place. Returns the new length.
sub FilterInPlace(f: FilterPredicate,
items: [FilterT],
length: intptr):
(newLength: intptr) is
newLength := Filter(f, items, length, items);
end sub;
 
# Filter that selects even numbers
sub Even implements FilterPredicate is
keep := (~ x as uint8) & 1;
end sub;
 
# Filter an array
var array: uint32[] := {1, 2, 3, 4, 5, 6, 7, 8, 9, 10};
var filtered: uint32[@sizeof array];
var length := Filter(Even, &array[0], @sizeof array, &filtered[0]);
 
# Print result
var i: uint8 := 0;
while i < length as uint8 loop
print_i32(filtered[i]);
print_char(' ');
i := i + 1;
end loop;
print_nl();
 
# Filter the result again in place for numbers less than 8
sub LessThan8 implements FilterPredicate is
if x < 8 then keep := 1;
else keep := 0;
end if;
end sub;
 
length := FilterInPlace(LessThan8, &filtered[0], length);
i := 0;
while i < length as uint8 loop
print_i32(filtered[i]);
print_char(' ');
i := i + 1;
end loop;
print_nl();
Output:
2 4 6 8 10
2 4 6

D[edit]

void main() {
import std.algorithm: filter, equal;
 
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]));
}

Tango Version[edit]

Library: Tango
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
}
Output:
 Evens - [ 2, 4, 6, 8, 10 ]
 Odds - [ 1, 3, 5, 7, 9 ]

Delphi[edit]

Hand-coded version[edit]

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.


Using Boost.Int library[edit]

Alternative using Boost.Int[1]:

Library: Types
Library: Boost.Int
program FilterEven;
 
{$APPTYPE CONSOLE}
 
uses
System.SysUtils,
Types,
Boost.Int;
 
var
Source, Destiny: TIntegerDynArray;
 
begin
Source.Assign([0, 1, 2, 3, 4, 5, 6, 7, 8, 9]);
 
// Non-destructively
Destiny := Source.Filter(
function(Item: Integer): Boolean
begin
Result := not odd(Item) and (Item <> 0);
end);
 
Writeln('[' + Destiny.Comma + ']');
Readln;
end.
 
// Destructively
Source.Remove(
function(Item: Integer): Boolean
begin
Result := odd(Item) or (Item = 0);
end);
 
Writeln('[' + Source.Comma + ']');
End.
 
Output:
[2,4,6,8]
[2,4,6,8]

Dyalect[edit]

Non-destructively[edit]

func Array.Filter(pred) {
var arr = []
for x in this when pred(x) {
arr.Add(x)
}
arr
}
 
var arr = [1..20].Filter(x => x % 2 == 0)
print(arr)
Output:
[2, 4, 6, 8, 10, 12, 14, 16, 18, 20]

Destructively[edit]

func Array.Filter(pred) {
var i = 0
while i < this.Length() {
if !pred(this[i]) {
this.RemoveAt(i)
}
i += 1
}
}
 
var arr = [1..20]
arr.Filter(x => x % 2 == 0)
print(arr)
Output:
[2, 4, 6, 8, 10, 12, 14, 16, 18, 20]

Idiomatic approach[edit]

Idiomatic approach in Dy is to use non-strict iterators (which can be combined without intermedate data structures) and translate the result to an array if needed:

var xs = [1..20]
var arr = xs.Iterate().Filter(x => x % 2 == 0).Map(x => x.ToString())
print(arr.ToArray())
Output:
["2", "4", "6", "8", "10", "12", "14", "16", "18", "20"]

Déjà Vu[edit]

Non-destructively[edit]

filter pred lst:
]
for value in copy lst:
if pred @value:
@value
[
 
even x:
= 0 % x 2
 
!. filter @even [ 0 1 2 3 4 5 6 7 8 9 ]
Output:
[ 0 2 4 6 8 ]

Destructively[edit]

local :lst [ 0 1 2 3 4 5 6 7 8 9 ]
 
filter-destructively pred lst:
local :tmp []
while lst:
pop-from lst
if pred dup:
push-to tmp
else:
drop
while tmp:
push-to lst pop-from tmp
 
filter-destructively @even lst
 
!. lst
Output:
[ 0 2 4 6 8 ]

E[edit]

There are several ways this could be done.

pragma.enable("accumulator")
accum [] for x ? (x %% 2 <=> 0) in [1,2,3,4,5,6,7,8,9,10] { _.with(x) }
var result := []
for x ? (x %% 2 <=> 0) in [1,2,3,4,5,6,7,8,9,10] {
result with= x
}
result
def makeSeries := <elang:control.makeSeries>
makeSeries([1,2,3,4,5,6,7,8,9,10]).filter(fn x,_{x %% 2 <=> 0}).asList()

EasyLang[edit]

a[] = [ 1 2 3 4 5 6 7 8 9 ]
for i range len a[]
if a[i] mod 2 = 0
b[] &= a[i]
.
.
print b[]

EchoLisp[edit]

 
(iota 12){ 0 1 2 3 4 5 6 7 8 9 10 11 }
 
;; lists
(filter even? (iota 12))
(0 2 4 6 8 10)
 
;; array (non destructive)
(vector-filter even? #(1 2 3 4 5 6 7 8 9 10 11 12 13))
→ #( 2 4 6 8 10 12)
 
;; sequence, infinite, lazy
(lib 'sequences)
(define evens (filter even? [0 ..]))
 
(take evens 12)
(0 2 4 6 8 10 12 14 16 18 20 22)
 
 

Ela[edit]

Using higher-order function (non-strict version)[edit]

open list
 
evenList = filter' (\x -> x % 2 == 0) [1..]

Using comprehension (non-strict version)[edit]

evenList = [& x \\ x <- [1..] | x % 2 == 0]

Elena[edit]

ELENA 5.0 :

import system'routines;
import system'math;
import extensions;
import extensions'routines;
 
public program()
{
auto array := new int[]{1,2,3,4,5};
 
var evens := array.filterBy:(n => n.mod:2 == 0).toArray();
 
evens.forEach:printingLn
}

Using strong typed collections and extensions:

import system'collections;
import system'routines'stex;
import system'math;
import extensions;
 
public program()
{
int[] array := new int[]{1,2,3,4,5};
 
array
.filterBy:(int n => n.mod:2 == 0)
.forEach:(int i){ console.printLine(i) }
}
Output:
2
4

Elixir[edit]

iex(10)> numbers = Enum.to_list(1..9)
[1, 2, 3, 4, 5, 6, 7, 8, 9]
iex(11)> Enum.filter(numbers, fn x -> rem(x,2)==0 end)
[2, 4, 6, 8]
iex(12)> for x <- numbers, rem(x,2)==0, do: x # comprehension
[2, 4, 6, 8]

Erlang[edit]

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

Or using a list comprehension:

EvenNumbers = [X || X <- Numbers, X rem 2 == 0].

Euphoria[edit]

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
Output:
{2,4,6}

F#[edit]

let lst = [1;2;3;4;5;6]
List.filter (fun x -> x % 2 = 0) lst;;
 
val it : int list = [2; 4; 6]

Factor[edit]

Works with: Factor version 0.98

This code uses filter on an array.

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

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

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

Destructive[edit]

This uses filter! to modify the original vector.

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

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

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!

Fantom[edit]

 
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(","))
}
}
 

Forth[edit]

: 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

Fortran[edit]

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
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

FreeBASIC[edit]

' FB 1.05.0 Win64
 
Type FilterType As Function(As Integer) As Boolean
 
Function isEven(n As Integer) As Boolean
Return n Mod 2 = 0
End Function
 
Sub filterArray(a() As Integer, b() As Integer, filter As FilterType)
If UBound(a) = -1 Then Return '' empty array
Dim count As Integer = 0
Redim b(0 To UBound(a) - LBound(a))
For i As Integer = LBound(a) To UBound(a)
If filter(a(i)) Then
b(count) = a(i)
count += 1
End If
Next
 
If count > 0 Then Redim Preserve b(0 To count - 1) '' trim excess elements
End Sub
 
' Note that da() must be a dynamic array as static arrays can't be redimensioned
Sub filterDestructArray(da() As Integer, filter As FilterType)
If UBound(da) = -1 Then Return '' empty array
Dim count As Integer = 0
For i As Integer = LBound(da) To UBound(da)
If i > UBound(da) - count Then Exit For
If Not filter(da(i)) Then '' remove this element by moving those still to be examined down one
For j As Integer = i + 1 To UBound(da) - count
da(j - 1) = da(j)
Next j
count += 1
i -= 1
End If
Next i
 
If count > 0 Then
Redim Preserve da(LBound(da) To UBound(da) - count) '' trim excess elements
End If
End Sub
 
Dim n As Integer = 12
Dim a(1 To n) As Integer '' creates dynamic array as upper bound is a variable
For i As Integer = 1 To n : Read a(i) : Next
Dim b() As Integer '' array to store results
filterArray a(), b(), @isEven
Print "The even numbers are (in new array)  : ";
For i As Integer = LBound(b) To UBound(b)
Print b(i); " ";
Next
Print : Print
filterDestructArray a(), @isEven
Print "The even numbers are (in original array) : ";
For i As Integer = LBound(a) To UBound(a)
Print a(i); " ";
Next
Print : Print
Print "Press any key to quit"
Sleep
End
 
Data 1, 2, 3, 7, 8, 10, 11, 16, 19, 21, 22, 27
Output:
The even numbers are (in new array)      :  2  8  10  16  22

The even numbers are (in original array) :  2  8  10  16  22

Frink[edit]

 
b = array[1 to 100]
c = select[b, {|x| x mod 2 == 0}]
 

Futhark[edit]

This example is incorrect. Please fix the code and remove this message.
Details: Futhark's syntax has changed, so this example will not compile
 
fun main(as: []int): []int =
filter (fn x => x%2 == 0) as
 

Gambas[edit]

Click this link to run this code

sRandom As New String[]
'______________________________________________________________________________________________________
Public Sub Main()
Dim siCount As Short
 
For siCount = 0 To 19
sRandom.Add(Rand(1, 100))
Next
 
Print sRandom.join(",")
 
NewArray
Destructive
 
End
'______________________________________________________________________________________________________
Public Sub NewArray() 'Select certain elements from an array into a new array in a generic way
Dim sEven As New String[]
Dim siCount As Short
 
For siCount = 0 To sRandom.Max
If Even(sRandom[siCount]) Then sEven.Add(sRandom[siCount])
Next
 
Print sEven.join(",")
 
End
'______________________________________________________________________________________________________
Public Sub Destructive() 'Give a second solution which filters destructively
Dim siIndex As New Short[]
Dim siCount As Short
 
For siCount = 0 To sRandom.Max
If Odd(sRandom[siCount]) Then siIndex.Add(siCount)
Next
 
For siCount = siIndex.max DownTo 0
sRandom.Extract(siIndex[siCount], 1)
Next
 
Print sRandom.join(",")
 
End

Output:

36,13,21,37,68,6,47,4,53,80,90,95,60,29,76,39,6,93,83,91
36,68,6,4,80,90,60,76,6
36,68,6,4,80,90,60,76,6

GAP[edit]

# 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 ]

Go[edit]

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]
}
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[edit]

 def evens = [1, 2, 3, 4, 5].findAll{it % 2 == 0}

Haskell[edit]

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

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

or

evens = filter even ary

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

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]

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[edit]

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

IDL[edit]

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

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

J[edit]

Solution:
With any verb (function) f that returns a boolean for each element of a vector v, the following is the generic solution:

   (#~ f) v

Examples:

   ] v=: 20 [email protected]$ 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

Or using the generic form suggested above:

   isEven=: 0 = 2&|    NB. verb testing for even numbers
(#~ isEven) v
92 92 36 40 16 2 22

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.

   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

Destructive example:

   v=: isEven select v

(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[edit]

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]);

A Java 8 solution with stream and generic types:

public static <T> T[] filter(T[] input, Predicate<T> filterMethod) {
return Arrays.stream(input)
.filter(filterMethod)
.toArray(size -> (T[]) Array.newInstance(input.getClass().getComponentType(), size));
}

Methodcall:

Integer[] array = {1, 2, 3, 4, 5};
Integer[] result = filter(array, i -> (i % 2) == 0);

Warning: This solution works not with primitive types!
For arrays with a primitive type use the wrapper class.

JavaFX Script[edit]

def array = [1..100];
def evens = array[n | n mod 2 == 0];

JavaScript[edit]

ES5[edit]

The standard way is to use the Array.prototype.filter function (
Works with: JavaScript version 1.6
):
var arr = [1,2,3,4,5];
var evens = arr.filter(function(a) {return a % 2 == 0});

Other ways:

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]);
Works with: Firefox version 2.0
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)];
Library: Functional
Functional.select("+1&1", [1,2,3,4])   // [2, 4]

ES6[edit]

(() => {
'use strict';
 
// isEven :: Int -> Bool
const isEven = n => n % 2 === 0;
 
 
// TEST
 
return [1,2,3,4,5,6,7,8,9]
.filter(isEven);
 
// [2, 4, 6, 8]
})();
Output:
[2, 4, 6, 8]

jq[edit]

jq's "select" filter is designed to make it easy to filter both arrays and streams:

(1,2,3,4,5,6,7,8,9) | select(. % 2 == 0)
Output:
2
4
6
8
 
[range(1;10)] | map( select(. % 2 == 0) )
 
Output:
[2,4,6,8]

Julia[edit]

Works with: Julia version 0.6
@show filter(iseven, 1:10)
Output:
filter(iseven, 1:10) = [2, 4, 6, 8, 10]

K[edit]

   / even is a boolean function
even:{0=x!2}
even 1 2 3 4 5
0 1 0 1 0
 
/ filtering the even numbers
[email protected]&even'a:1+!10
2 4 6 8 10
 
/ as a function
evens:{[email protected]&even'x}
a:10?100
45 5 79 77 44 15 83 88 33 99
evens a
44 88

Alternative syntax:

   {x[&0=x!2]}
{x[&even x]}

Destructive:

   a:evens a
44 88

Kotlin[edit]

// version 1.0.5-2
 
fun main(args: Array<String>) {
val array = arrayOf(1, 2, 3, 4, 5, 6, 7, 8, 9)
println(array.joinToString(" "))
 
val filteredArray = array.filter{ it % 2 == 0 }
println(filteredArray.joinToString(" "))
 
val mutableList = array.toMutableList()
mutableList.retainAll { it % 2 == 0 }
println(mutableList.joinToString(" "))
}
Output:
1 2 3 4 5 6 7 8 9
2 4 6 8
2 4 6 8

Lambdatalk[edit]

 
 
{def filter
{lambda {:bool :a}
{if {S.empty? {S.rest :a}}
then {:bool {S.first :a}}
else {:bool {S.first :a}}
{filter :bool {S.rest :a}}}}}
 
{def even? {lambda {:w} {if {= {% :w 2} 0} then :w else}}}
{def odd? {lambda {:w} {if {= {% :w 2} 1} then :w else}}}
 
{filter even? {S.serie 1 20}}
-> 2 4 6 8 10 12 14 16 18 20
{filter odd? {S.serie 1 20}}
-> 1 3 5 7 9 11 13 15 17 19
 
 

Lang5[edit]

: filter  over swap execute select ;
10 iota "2 % not" filter . "\n" .
 
# [ 0 2 4 6 8 ]

langur[edit]

Using the where() function filters by a function and returns an array of values. Using wherekeys() filters the same way, but returns an array of keys instead.

Works with: langur version 0.8.1
val .arr = series 7
 
writeln " array: ", .arr
writeln "filtered: ", where f .x div 2, .arr
Output:
   array: [1, 2, 3, 4, 5, 6, 7]
filtered: [2, 4, 6]

Lasso[edit]

local(original = array(1,2,3,4,5,6,7,8,9,10))
local(evens = (with item in #original where #item % 2 == 0 select #item) -> asstaticarray)
#evens
staticarray(2, 4, 6, 8, 10)

Modifying the original array

local(original = array(1,2,3,4,5,6,7,8,9,10))
with item in #original where #item % 2 != 0 do #original ->removeall(#item)
#original
array(2, 4, 6, 8, 10)

Liberty BASIC[edit]

' 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)
next
 
'dim the extract and fill it
dim array2(count)
for i = 1 to 100
if not(array1(i) mod 2) then
n=n+1
array2(n)=array1(i)
end if
next
 
for n=1 to count
print array2(n)
next

Lisaac[edit]

+ 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;
};
};

[edit]

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]

Lua[edit]

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)))

The destructive version is even simpler, since tables are passed by reference:

function filter(t, func)
for i, v in ipairs(t) do
if not func(v) then table.remove(t, i) end
end
end
 
function even(a) return a % 2 == 0 end
 
local values = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10}
filter(values, even)
print(unpack(values))

M2000 Interpreter[edit]

Using Filter for arrays[edit]

 
Module Checkit {
Print (1,2,3,4,5,6,7,8)#filter(lambda ->number mod 2=0)
}
Checkit
 

Old style[edit]

Function GetEvenNumbers can get pointer to array or array and return a pointer to array.

Module Filter2EvenNumbers get an array by reference and first place numbers to stack and then make stack an array and then copy to array.

Module Filter2EvenNumbers change definition and now place numbers in A() and at the last statement A() change dimension, preserving values.

We can use Base 1 arrays too: Dim Base 1, A(5) : A(1)=10,3,6,7,11


 
Module CheckIt {
Function GetEvenNumbers (A as array){
If len(A)=0 then =(,) : exit
Flush ' empty current stack (of values)
n=each(A)
While n {
if array(n) mod 2 = 0 then data array(n)
}
\\ [] return a stack object, leave an empty stack as current stack
=Array([])
}
 
Dim A(5), B()
A(0)=10,3,6,7,11
B()=GetEvenNumbers(A())
Print B() ' print 10,6
Print GetEvenNumbers((1,2,3,4,5,6,7,8)) ' 2 4 6 8
 
Module Filter2EvenNumbers (&A()) {
If len(A())=0 then exit
Stack New {
Flush ' empty current stack (of values)
n=each(A())
While n {
if array(n) mod 2 = 0 then data array(n)
}
\\ [] return a stack object, leave an empty stack as current stack
A()=Array([])
}
}
A(0)=10,3,6,7,11
Filter2EvenNumbers &A()
Print A() ' 10 6
Module Filter2EvenNumbers (&A()) {
If len(A())=0 then exit
n=each(A())
x=Dimension(A(), 0)-1 ' base of array (0 or 1)
k=-x
While n {
if array(n) mod 2 = 0 then x++ : A(x)=Array(n)
}
Dim A(x+k)
}
Dim A(5)
A(0)=10,3,6,7,11
Filter2EvenNumbers &A()
Print A() ' 10 6
}
CheckIt
}
CheckIt
 

Maple[edit]

 
evennum:=proc(nums::list(integer))
return select(x->type(x, even), nums);
end proc;
 

Mathematica / Wolfram Language[edit]

Check for even integers:

Select[{4, 5, Pi, 2, 1.3, 7, 6, 8.0}, EvenQ]

gives:

{4, 2, 6}

To check also for approximate number (like 8.0 in the example above) a possible solution is:

Select[{4, 5, Pi, 2, 1.3, 7, 6, 8.0}, Mod[#, 2] == 0 &]

gives:

{4, 2, 6, 8.}

notice that the function returns 8. not 8 (the dot indicates that it is a float number, not an integer).

MATLAB[edit]

function evens = selectEvenNumbers(list)
 
evens = list( mod(list,2) == 0 );
 
end
Output:
>> selectEvenNumbers([0 1 2 3 4 5 6 7 8 9 10])
 
ans =
 
0 2 4 6 8 10

Maxima[edit]

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]

MAXScript[edit]

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

min[edit]

Works with: min version 0.19.3
(1 2 3 4 5 6 7 8 9 10) 'even? filter print
Output:
(2 4 6 8 10)

MiniScript[edit]

We define a filter method on the list type that returns a new list containing elements filtered by the given function.

list.filter = function(f)
result = []
for item in self
if f(item) then result.push item
end for
return result
end function
 
isEven = function(x)
return x % 2 == 0
end function
 
nums = [1, 2, 3, 4, 5, 6, 7, 9, 12, 15, 18, 21]
print nums.filter(@isEven)

The in-place version is simpler, and even allows the use of an unnamed filter function, defined right on the method call.

list.filterInPlace = function(f)
for i in range(self.len-1, 0)
if not f(self[i]) then self.remove i
end for
end function
 
nums = [1, 2, 3, 4, 5, 6, 7, 9, 12, 15, 18, 21]
 
nums.filterInPlace function(x)
return x % 2 == 0
end function
 
print nums

ML[edit]

Standard ML[edit]

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

MLite[edit]

MLite is similar to Standard ML, though '=>' becomes '=' and 'List.' is elided:

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

MUMPS[edit]

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
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[edit]

Lists have a built-in method for filtering:

def original = $[1 .. 100];
def filtered = original.Filter(fun(n) {n % 2 == 0});
WriteLine($"$filtered");

The following would work for arrays:

Filter[T] (a : array[T], f : T -> bool) : array[T]
{
def b = $[x | x in a, (f(x))];
b.ToArray()
}

NetRexx[edit]

/* NetRexx */
options replace format comments java crossref symbols nobinary
numeric digits 5000
 
-- =============================================================================
class RFilter public
properties indirect
filter = RFilter.ArrayFilter
-- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
method main(args = String[]) public static
arg = Rexx(args)
RFilter().runSample(arg)
return
-- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
method runSample(arg) public
sd1 = Rexx[]
sd2 = Rexx[]
 
say 'Test data:'
sd1 = makeSampleData(100)
display(sd1)
setFilter(RFilter.EvenNumberOnlyArrayFilter())
say
say 'Option 1 (copy to a new array):'
sd2 = getFilter().filter(sd1)
display(sd2)
say
say 'Option 2 (replace the original array):'
sd1 = getFilter().filter(sd1)
display(sd1)
return
-- ---------------------------------------------------------------------------
method display(sd = Rexx[]) public static
say '-'.copies(80)
loop i_ = 0 to sd.length - 1
say sd[i_] '\-'
end i_
say
return
-- ---------------------------------------------------------------------------
method makeSampleData(size) public static returns Rexx[]
sd = Rexx[size]
loop e_ = 0 to size - 1
sd[e_] = (e_ + 1 - size / 2) / 2
end e_
return sd
 
-- =============================================================================
class RFilter.ArrayFilter abstract
-- ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~
method filter(array = Rexx[]) public abstract returns Rexx[]
-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = =
class RFilter.EvenNumberOnlyArrayFilter extends RFilter.ArrayFilter
-- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
method filter(array = Rexx[]) public returns Rexx[]
clist = ArrayList(Arrays.asList(array))
li = clist.listIterator()
loop while li.hasNext()
e_ = Rexx li.next
if \e_.datatype('w'), e_ // 2 \= 0 then li.remove()
end
ry = Rexx[] clist.toArray(Rexx[clist.size()])
return ry
 
Output:
Test data:
--------------------------------------------------------------------------------
-24.5 -24 -23.5 -23 -22.5 -22 -21.5 -21 -20.5 -20 -19.5 -19 -18.5 -18 -17.5 -17 -16.5 -16 -15.5 -15 -14.5 -14 -13.5 -13 -12.5 -12 -11.5 -11 -10.5 -10 -9.5 -9 -8.5 -8 -7.5 -7 -6.5 -6 -5.5 -5 -4.5 -4 -3.5 -3 -2.5 -2 -1.5 -1 -0.5 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 6.5 7 7.5 8 8.5 9 9.5 10 10.5 11 11.5 12 12.5 13 13.5 14 14.5 15 15.5 16 16.5 17 17.5 18 18.5 19 19.5 20 20.5 21 21.5 22 22.5 23 23.5 24 24.5 25

Option 1 (copy to a new array):
--------------------------------------------------------------------------------
-24 -22 -20 -18 -16 -14 -12 -10 -8 -6 -4 -2 0 2 4 6 8 10 12 14 16 18 20 22 24

Option 2 (replace the original array):
--------------------------------------------------------------------------------
-24 -22 -20 -18 -16 -14 -12 -10 -8 -6 -4 -2 0 2 4 6 8 10 12 14 16 18 20 22 24

NewLISP[edit]

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

NGS[edit]

F even(x:Int) x % 2 == 0
 
evens = Arr(1...10).filter(even)

Nial[edit]

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

Nim[edit]

import sequtils
 
let values = toSeq(0..9)
 
# Filtering by returning a new sequence.
# - using an explicit filtering procedure.
echo "Even values: ", values.filter(proc(x: int): bool = x mod 2 == 0)
# - using a predicate.
echo "Odd values: ", values.filterIt(it mod 2 == 1)
 
# Filtering by modifying the sequence.
# - using an explicit filtering procedure.
var v1 = toSeq(0..9)
v1.keepIf(proc(x: int): bool = x mod 2 == 0)
echo "Even values: ", v1
# - using a predicate.
var v2 = toSeq(0..9)
v2.keepItIf(it mod 2 != 0)
echo "Odd values: ", v2
Output:
Even values: @[0, 2, 4, 6, 8]
Odd values: @[1, 3, 5, 7, 9]
Even values: @[0, 2, 4, 6, 8]
Odd values: @[1, 3, 5, 7, 9]

Objeck[edit]

 
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;
}
}
}
 

Objective-C[edit]

Works with: Cocoa version Mac OS X 10.6+
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; } ]];
Works with: Cocoa version Mac OS X 10.5+
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];
Works with: GNUstep
#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;
}

OCaml[edit]

It is easier to do it with a list:

let lst = [1;2;3;4;5;6]
let even_lst = List.filter (fun x -> x mod 2 = 0) lst

Octave[edit]

arr = [1:100];
evennums = arr( mod(arr, 2) == 0 );
disp(evennums);

Oforth[edit]

100 seq filter(#isEven)

Ol[edit]

 
(filter even? '(1 2 3 4 5 6 7 8 9 10))
 

ooRexx[edit]

 Call random ,,1234567
a=.array~new
b=.array~new
Do i=1 To 10
a[i]=random(1,9999)
End
Say 'Unfiltered values:' a~makestring(line,' ')
/* copy even numbers to array b */
j=0
Do i=1 to 10
If filter(a[i]) Then Do
j = j + 1
b[j]=a[i]
End
end
Say 'Filtered values (in second array): ' b~makestring(line,' ')
/* destructive filtering: copy within array a */
j=0
Do i=1 to 10
If filter(a[i]) Then Do
j = j + 1
a[j]=a[i]
End
end
/* destructive filtering: delete the remaining elements */
Do i=10 To j+1 By -1
a~delete(i)
End
Say 'Filtered values (destructive filtering):' a~makestring(line,' ')
Exit
filter: Return arg(1)//2=0
Output:
Unfiltered values: 1412 2244 6778 4002 439 3335 5877 8273 7882 1469
Filtered values (in second array):       1412 2244 6778 4002 7882
Filtered values (destructive filtering): 1412 2244 6778 4002 7882

Oz[edit]

It is easier to do it with a list:

declare
Lst = [1 2 3 4 5]
LstEven = {Filter Lst IsEven}

PARI/GP[edit]

Works with: PARI/GP version 2.4.3 and above
This code uses the select() function, which was added in PARI version 2.4.2. The order of the arguments changed between versions; to use in 2.4.2 change select(function, vector) to select(vector, function).
iseven(n)=n%2==0
select(iseven, [2, 3, 4, 5, 7, 8, 9, 11, 13, 16, 17])

Or in anonymous form

select(n -> n%2==0, [2, 3, 4, 5, 7, 8, 9, 11, 13, 16, 17])

Pascal[edit]

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

Works with: Delphi

Works with: Turbo 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.');

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

Peloton[edit]

Fixed length English dialect

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

Perl[edit]

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

Phix[edit]

Library: Phix/basics

basic task[edit]

function even(integer i)
    return remainder(i,2)=0
end function
?filter(tagset(10),even)
Output:
{2,4,6,8,10}

extra credit[edit]

The following discusses possible destructive/in situ behaviours (in excruciatingly painstaking detail). Phix is reference counted so the distinction between destructive and non-destructive is somewhat subtle. The following code (builtin filter routine) acts both ways.

function even(integer i)
    return remainder(i,2)=0
end function
procedure main()
sequence s = tagset(10),
         t = filter(s,even) -- copy-on-write here...
    ?s -- still all 10
    ?t -- 5
    s = filter(s,even)      -- ...but automatic pass by reference occurs here
    ?s -- 5
end procedure
main()
Output:
{1,2,3,4,5,6,7,8,9,10}
{2,4,6,8,10}
{2,4,6,8,10}

It will help to explain what is going on by looking at a couple of longhand (and greatly simplified) versions, first an explicitly non-destructive one

function lhnd_filter(sequence a, integer fn)
    sequence res = {}
    for i=1 to length(a) do
        if fn(a[i]) then
            res = append(res,a[i])
        end if
    end for
    return res
end function

Clearly the above is non-destructive. It makes no attempt to modify a, but builds a new result, and it is fair to say that in some cases the above may be the fastest approach due to fewer reference count updates. However the following may or may not be destructive:

function lhd_filter(sequence a, integer fn)
    integer l = 0
    for i=1 to length(a) do
        if fn(a[i]) then
            l += 1
            a[l] = a[i]
        end if
    end for
    a = a[1..l] -- (can occur in situ)
    return a
end function
procedure main()
sequence s = tagset(10),
         t = lhd_filter(s,even) -- copy on write here...
    ?s -- still all 10
    ?t -- 5
    s = lhd_filter(s,even)      -- ...but automatic pass by reference occurs here
    ?s -- 5
end procedure
main()

In the t = lhd_filter(s) call, s is preserved because of copy-on-write semantics. Modifying a does not modify s, because it has a reference count of 2 the first attempt to modify it triggers copy-on-write and safely makes a top-level copy. In the s = lhd_filter(s) call however, s is automatically passed by reference, ie the local s is <no value> over the duration of the call and parameter a of lhd_filter() contains the only reference to the previous content of s, and no copy-on-write occurs. Technically modifying a is still not modifying s, but since it has a reference count of 1 it modifies the data that used to be referenced by s, and will again rsn, in situ. Note: adding t = s before the s = lhd_filter(s) call would make it non-destructive again, as t must be preserved and there is now a reference count >1 on that data. Also note that automatic pass-by-reference only occurs for routine-local variables.

There is one case in the interpreter (pEmit2.e/rebuild_callback()) where it needs to circumvent this behaviour. For performance reasons it does not populate the symbol table with actual names until a fatal error or trace event occurs. At that time, the symbol table may have a reference count>1, so it deliberately patches it to 1 over the name population call to switch off the copy-on-write semantics, and later restores the reference count before carrying on. Not that you really needed to know that.

PHL[edit]

module var;
 
extern printf;
 
@Integer main [
var arr = 1..9;
var evens = arr.filter(#(i) i % 2 == 0);
printf("%s\n", evens::str);
 
return 0;
]

PHP[edit]

Using a standard loop

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

Picat[edit]

List comprehension is probably the best way of filtering:

[I : I in 1..20, I mod 2 == 0]

A more general version of filtering is to use call/1 with a defined predicate (here p/1):

go => 
L = 1..20,
A = filter(L,p).
 
p(N) => N mod 2 == 0.
 
filter(A,F) = [N : N in A, call(F,N)].

This general version might be slower since using call/1 has some overhead.


PicoLisp[edit]

(filter '((N) (not (bit? 1 N)))
(1 2 3 4 5 6 7 8 9) )
Output:
-> (2 4 6 8)

PL/I[edit]

(subscriptrange):
filter_values: procedure options (main); /* 15 November 2013 */
declare a(20) fixed, b(*) fixed controlled;
declare (i, j, n) fixed binary;
 
a = random()*99999; /* fill the array with random elements from 0-99998 */
put list ('Unfiltered values:');
put skip edit (a) (f(6));
/* Loop to count the number of elements that will be filtered */
n = 0;
do i = 1 to hbound(a);
n = n + filter(a(i));
end;
allocate b(n);
j = 0;
do i = 1 to hbound(a);
if filter(a(i)) then do; j = j + 1; b(j) = a(i); end;
end;
put skip list ('Filtered values:');
put skip edit (b) (f(6));
 
filter: procedure (value) returns (bit(1));
declare value fixed;
 
return (iand(abs(value), 1) = 0);
end filter;
 
end filter_values;

Results:

Unfiltered values:
 44270  6008 80477 17004 91587 48669 29623 74640 29841 20019 77833 59865 49647  2272 54781
 36154 40114 71893 25960 76863
Filtered values:
 44270  6008 17004 74640  2272 36154 40114 25960

Pop11[edit]

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

;;; 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) =>

PostScript[edit]

Library: initlib
 
[1 2 3 4 5 6 7 8 9 10] {2 mod 0 eq} find
 

PowerShell[edit]

$array = -15..37
$array | Where-Object { $_ % 2 -eq 0 }

Prolog[edit]

findall[edit]

evens(D, Es) :- findall(E, (member(E, D), E mod 2 =:= 0), Es).

Usage:

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

Anonymous functions[edit]

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

?- 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].

filter and anonymous functions[edit]

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

:- 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).
 

Usage :

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

PureBasic[edit]

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)
Next
 
;- 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
Next
 
;- 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
Output:
List of Randoms
32 35 89 91 11 33 12 22 42 43
List of Even(s)
32 12 22 42

Python[edit]

Works with: Python version 2.4
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)

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

values = range(10)
evens = values[::2]

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:

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


Or in functional terms, by descending generality and increasing brevity:

Works with: Python version 3
'''Functional filtering - by descending generality and increasing brevity'''
 
from functools import (reduce)
from itertools import (chain)
import inspect
import re
 
 
def f1(xs):
'''Catamorphism: fold / reduce.
See [The expressiveness and universality of fold]
(http://www.cs.nott.ac.uk/~pszgmh/fold.pdf)'''

return reduce(lambda a, x: a + [x] if even(x) else a, xs, [])
 
 
def f2(xs):
'''List monad bind/inject operator (concatMap combined with
an (a -> [b]) function which wraps its result in a
possibly empty list). This is the universal abstraction
which underlies list comprehensions.'''

return concatMap(lambda x: [x] if even(x) else [])(xs)
 
 
def f3(xs):
'''Built-in syntactic sugar for list comprehensions.
Convenient, and encouraged as 'Pythonic',
but less general and expressive than a fold.'''

return (x for x in xs if even(x))
 
 
def f4(xs):
'''Built-in filter function'''
return filter(even, xs)
 
 
def main():
'''Tests'''
xs = enumFromTo(0)(10)
print(
tabulated(showReturn)(
'By descending generality and increasing brevity:\n'
)(
lambda f: list(f(xs))
)([f1, f2, f3, f4])
)
 
 
# GENERIC -------------------------------------------------
 
 
# concatMap :: (a -> [b]) -> [a] -> [b]
def concatMap(f):
'''Concatenated list over which a function has been mapped.
The list monad can be derived by using a function of the type
(a -> [b]) which wraps its output in list
(using an empty list to represent computational failure).'''

return lambda xs: list(
chain.from_iterable(
map(f, xs)
)
)
 
 
# enumFromTo :: (Int, Int) -> [Int]
def enumFromTo(m):
'''Integer enumeration from m to n.'''
return lambda n: list(range(m, 1 + n))
 
 
# even :: Int -> Bool
def even(x):
'''Predicate'''
return 0 == x % 2
 
 
# showReturn :: (a -> b) -> String
def showReturn(f):
'''Stringification of final (return) expression in function body.'''
return re.split('return ', inspect.getsource(f))[-1].strip()
 
 
# tabulated :: (a -> String) -> String -> (a -> b) -> [a] -> String
def tabulated(fShow):
'''heading -> function -> input List -> tabulated output string'''
def go(s, f, xs):
w = max(len(fShow(x)) for x in xs)
return s + '\n' + '\n'.join([
fShow(x).rjust(w, ' ') +
' -> ' + str(f(x)) for x in xs
])
return lambda s: lambda f: lambda xs: go(s, f, xs)
 
 
if __name__ == '__main__':
main()
Output:
By descending generality and increasing brevity:

reduce(lambda a, x: a + [x] if even(x) else a, xs, []) -> [0, 2, 4, 6, 8, 10]
       concatMap(lambda x: [x] if even(x) else [])(xs) -> [0, 2, 4, 6, 8, 10]
                            (x for x in xs if even(x)) -> [0, 2, 4, 6, 8, 10]
                                      filter(even, xs) -> [0, 2, 4, 6, 8, 10]

Quackery[edit]

  [ [] ]'[ rot
witheach
[ tuck over do iff
[ dip [ nested join ] ]
else nip ]
drop ] is only ( [ --> [ )
 
[ 1 & not ] is even ( n --> b )
 
[] 10 times [ 10 random join ]
say "Ten arbitrary digits: " dup echo cr
say "Only the even digits: " only even echo cr
Output:
Ten arbitrary digits: [ 1 6 1 2 2 1 8 5 5 4 ]
Only the even digits: [ 6 2 2 8 4 ]

Destructively[edit]

  [ ]'[ over size times
[ over i peek
over do if
[ dip [ i pluck drop ] ] ]
drop ] is without ( [ --> [ )
 
[ 1 & ] is odd ( n --> b )
 
[] 10 times [ i join ] shuffle
say "Ten shuffled digits: " dup echo cr
say "Less the odd digits: " without odd echo cr
Output:
Ten shuffled digits: [ 5 6 8 2 1 4 0 3 9 7 ]
Less the odd digits: [ 6 8 2 4 0 ]

Q[edit]

x where 0=x mod 2

R[edit]

a <- 1:100
evennums <- a[ a%%2 == 0 ]
print(evennums)

Racket[edit]

The classic way:

 
-> (filter even? '(0 1 2 3 4 5 6 7 8 9))
'(0 2 4 6 8)
 

getting the list of non-evens too:

 
-> (partition even? '(0 1 2 3 4 5 6 7 8 9))
'(0 2 4 6 8)
'(1 3 5 7 9)
 

Finally, using a for loop, similar to list comprehension:

 
-> (for/list ([x '(0 1 2 3 4 5 6 7 8 9)] #:when (even? x)) x)
'(0 2 4 6 8)
 

Raku[edit]

(formerly Perl 6)

Works with: Rakudo version 2018.03
my @a = 1..6;
my @even = grep * %% 2, @a;

Alternatively:

my @a = 1..6;
my @even = @a.grep(* %% 2);

Destructive:

my @a = 1..6;
@a .= grep(* %% 2);

Raven[edit]

[ 0 1 2 3 4 5 6 7 8 9 ] as nums
group nums each
dup 1 & if drop
list as evens

REBOL[edit]

a: []  repeat i 100 [append a i] ; Build and load array.
 
evens: [] repeat element a [if even? element [append evens element]]
 
print mold evens
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]

Red[edit]

Red []
orig: [] repeat i 10 [append orig i]
?? orig
cpy: [] forall orig [if even? orig/1 [append cpy orig/1]]
;; or - because we know each second element is even :- )
;; cpy: extract next orig 2
?? cpy
remove-each ele orig [odd? ele]  ;; destructive
?? orig
 
Output:
orig: [1 2 3 4 5 6 7 8 9 10]
cpy: [2 4 6 8 10]
orig: [2 4 6 8 10]
>>

REXX[edit]

using two arrays[edit]

This example uses two arrays.   The   random   BIF is used to generate the numbers.

/*REXX program selects all  even numbers  from an array and puts them  ──►  a new array.*/
parse arg N seed . /*obtain optional arguments from the CL*/
if N=='' | N=="," then N= 50 /*Not specified? Then use the default.*/
if datatype(seed,'W') then call random ,,seed /*use the RANDOM seed for repeatability*/
old.= /*the OLD array, all are null so far. */
new.= /* " NEW " " " " " " */
do i=1 for N /*generate N random numbers ──► OLD */
old.i= random(1, 99999) /*generate random number 1 ──► 99999*/
end /*i*/
#= 0 /*number of elements in NEW (so far).*/
do j=1 for N /*process the elements of the OLD array*/
if old.j//2 \== 0 then iterate /*if element isn't even, then skip it.*/
#= # + 1 /*bump the number of NEW elements. */
new.#= old.j /*assign the number to the NEW array.*/
end /*j*/
 
do k=1 for # /*display all the NEW numbers. */
say right('new.'k, 20) "=" right(new.k, 9) /*display a line (an array element). */
end /*k*/ /*stick a fork in it, we're all done. */

Programming note:   the REXX statement

      if old.j//2 \== 0  then iterate

could've been replaced with

      if old.j//2        then iterate

but that would've assumed the numbers are integers   (no matter what form they're expressed in).
As it happens, the REXX program uses the numbers generated from the   random   BIF,   which are integers.

output   when using the input of:     ,   1234567

The 1234567 is the random BIF   seed   so that the random numbers can be repeated when re-running the REXX program.

               new.1 =     17520
               new.2 =     77326
               new.3 =     36128
               new.4 =     19124
               new.5 =       202
               new.6 =     82314
               new.7 =     96140
               new.8 =      4066
               new.9 =      3254
              new.10 =     91178
              new.11 =     18806
              new.12 =     60646
              new.13 =     26428
              new.14 =     16790
              new.15 =     24868
              new.16 =     61954
              new.17 =     63424
              new.18 =     97538
              new.19 =     82278
              new.20 =     33360
              new.21 =     74026
              new.22 =     48472
              new.23 =     44360

using one array with a control array[edit]

This version uses a control array, which isn't fully populated   (in REXX terms, a sparse array.)

/*REXX program finds all  even  numbers from an array,  and  marks a control array.     */
parse arg N seed . /*obtain optional arguments from the CL*/
if N=='' | N=="," then N= 50 /*Not specified? Then use the default.*/
if datatype(seed,'W') then call random ,,seed /*use the RANDOM seed for repeatability*/
 
do i=1 for N /*generate N random numbers ──► OLD */
@.i= random(1, 99999) /*generate random number 1 ──► 99999*/
end /*i*/
!.= 0 /*number of elements in NEW (so far).*/
do j=1 for N /*process the OLD array elements. */
if @.j//2 \==0 then !.j= 1 /*mark the  ! array that it's ¬even. */
end /*j*/
 
do k=1 for N /*display all the @ even numbers. */
if !.k then iterate /*if it's marked as not even, skip it.*/
say right('array.'k, 20) "=" right(@.k, 9) /*display a even number, filtered array*/
end /*k*/ /*stick a fork in it, we're all done. */

For the following input:     , 1234567

output   Output is the same as the 1st REXX version   (using two arrays).


using one array, destructive[edit]

This version just uses one array to perform the filtering instead of creating a   new   array.

This method doesn't need as much memory to hold the sparse array.

/*REXX program finds all  even  numbers from an array, and marks the  not even  numbers.*/
parse arg N seed . /*obtain optional arguments from the CL*/
if N=='' | N=="," then N= 50 /*Not specified? Then use the default.*/
if datatype(seed,'W') then call random ,,seed /*use the RANDOM seed for repeatability*/
 
do i=1 for N /*generate N random numbers ──► OLD */
@.i= random(1, 99999) /*generate a random number 1 ──► 99999 */
end /*i*/
 
do j=1 for N /*process the OLD array elements. */
if @.j//2 \==0 then @.j= /*mark the @ array that it's not even*/
end /*j*/
 
do k=1 for N /*display all the @ even numbers. */
if @.k=='' then iterate /*if it's marked not even, then skip it*/
say right('array.'k, 20) "=" right(@.k, 9) /*display a line (an array element). */
end /*k*/ /*stick a fork in it, we're all done. */

For the following input:     , 1234567

output   is the same as the 1st REXX version   (using two arrays).


Ring[edit]

 
aList = [1, 2, 3, 4, 5, 6]
bArray = list(3)
see evenSelect(aList)
 
func evenSelect aArray
i = 0
for n = 1 to len(aArray)
if (aArray[n] % 2) = 0
i = i + 1
bArray[i] = aArray[n] ok
next
return bArray
 

Ruby[edit]

Enumerable#select is the filter that returns a new Array.

# 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]

Destructive[edit]

Array#select! is the destructive version which modifies the original Array.

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

Shorthand:

ary = [1, 2, 3, 4, 5, 6]
ary.select!(&:even?)
p ary # => [2, 4, 6]

Run BASIC[edit]

dim a1(100)
count = 100
for i = 1 to 100
a1(i) = int(rnd(0)*100)+1
count = count - (a1(i) mod 2)
next
 
'dim the extract and fill it
dim a2(count)
for i = 1 to 100
if not(a1(i) mod 2) then
n = n+1
a2(n) = a1(i)
end if
next
 
for i = 1 to count
print a2(i)
next

Rust[edit]

fn main() {
println!("new vec filtered: ");
let nums: Vec<i32> = (1..20).collect();
let evens: Vec<i32> = nums.iter().cloned().filter(|x| x % 2 == 0).collect();
println!("{:?}", evens);
 
// Filter an already existing vector
println!("original vec filtered: ");
let mut nums: Vec<i32> = (1..20).collect();
nums.retain(|x| x % 2 == 0);
println!("{:?}", nums);
}
Output:
new vec filtered:
[2, 4, 6, 8, 10, 12, 14, 16, 18]
original vec filtered:
[2, 4, 6, 8, 10, 12, 14, 16, 18]

Salmon[edit]

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.

iterate(x; comprehend(y; [1...10]; y % 2 == 0) (y))
x!;

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

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!;

Sather[edit]

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;

Scala[edit]

(1 to 100).filter(_ % 2 == 0)

Scheme[edit]

Filter function definition:

 
(define filter
(lambda (fn lst)
(let iter ((lst lst) (result '()))
(if (null? lst)
(reverse result)
(let ((item (car lst))
(rest (cdr lst)))
(if (fn item)
(iter rest (cons item result))
(iter rest result)))))))
 

Usage in the interactive prompt:

> (filter even? '(1 2 3 4 5 6 7 8 9 10))
(2 4 6 8 10)

Or as a function:

(define (select-even lst)
(filter even? lst))
 
(select-even '(1 2 3 4 5 6 7 8 9 10))

Seed7[edit]

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;

SequenceL[edit]

Filters are primarily written in SequenceL using partial Indexed Functions.

evens(x(1))[i] := x[i] when x[i] mod 2 = 0;
Output:
cmd:>evens(1...5)
[2,4]

Sidef[edit]

var arr = [1,2,3,4,5];
 
# Creates a new array
var new = arr.grep {|i| i %% 2};
say new.dump; # => [2, 4]
 
# Destructive (at variable level)
arr.grep! {|i| i %% 2};
say arr.dump; # => [2, 4]

Slate[edit]

#(1 2 3 4 5) select: [| :number | number isEven].

Smalltalk[edit]

Creates a new array:

#(1 2 3 4 5) select: [:number | number even]

or for short:

#(1 2 3 4 5) select:#even

Destructive modification is not possible for literal constants (these are immutable); in addition, Arrays are fix sized collections. It is also considered very bad style, to modify passed in arguments this way. Thus constructing a new Array object (as above) is the only correct solution.

SQL[edit]

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
--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
'
Works with: MySQL
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;

Or to be shorter, you could create the table evens directly from the query result :

CREATE TEMPORARY TABLE evens SELECT * FROM nos WHERE v%2=0;

Stata[edit]

mata
a=2,9,4,7,5,3,6,1,8
 
// Select even elements of a
select(a,mod(a,2):==0)
 
// Select the indices of even elements of a
selectindex(mod(a,2):==0)
end

Swift[edit]

let numbers = [1,2,3,4,5,6]
let even_numbers = numbers.filter { $0 % 2 == 0 }
println(even_numbers)
Output:
[2, 4, 6]

Tcl[edit]

set l {56 21 71 27 39 62 87 76 82 94 45 83 65 45 28 90 52 44 1 89}
 
puts [lmap x $l {if {$x % 2} continue; set x}]
Output:
56 62 76 82 94 28 90 52 44


Inplace way, quite the inefficient contraption compared to mapping:

proc lreplaceip {_list args} {
upvar 1 $_list list
set list [lreplace $list[set list {}] {*}$args]
}
 
set l {56 21 71 27 39 62 87 76 82 94 45 83 65 45 28 90 52 44 1 89}
 
for {set i 0} {$i < [llength $l]} {} {
if {[lindex $l $i] % 2 == 1} {
lreplaceip l $i $i
} else {
incr i
}
}
 
puts $l
Output:
56 62 76 82 94 28 90 52 44

Explanation: https://wiki.tcl-lang.org/page/lreplace, section "Performance: Modifying a List In-Place"
Proof by timing removal of the end element of lists of different lengths:

proc lreplaceip {_list args} {
upvar 1 $_list list
set list [lreplace $list[set list {}] {*}$args]
}
 
proc iota {n {start 0}} {
set res {}
set end [expr {$start + $n}]
for {set i $start} {$i <= $end} {incr i} {
lappend res $i
}
return $res
}
 
foreach e {5 6 7} {
set l [iota 1e$e]
puts 1e$e
puts " lreplace: [time {set l [lreplace $l end end]}]"
puts " lreplaceip: [time {lreplaceip l end end}]"
}
Output:
1e5
    lreplace:   564 microseconds per iteration
    lreplaceip: 19 microseconds per iteration
1e6
    lreplace:   15215 microseconds per iteration
    lreplaceip: 6 microseconds per iteration
1e7
    lreplace:   148028 microseconds per iteration
    lreplaceip: 6 microseconds per iteration

Toka[edit]

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

TUSCRIPT[edit]

 
$$ 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
 
Output:
4'16'36'64'100

UNIX Shell[edit]

Works with: 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

Or, using grep:

a=(1 2 3 4 5)
read -a e -d\n < <(printf '%s\n' "${a[@]}" | grep '[02468]$')

Either way, to display the results:

echo "${a[@]}"
echo "${e[@]}"
Output:
1 2 3 4 5
2 4

UnixPipes[edit]

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

Ursala[edit]

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[edit]

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

#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
Output:
<36,92,16,52,88,54,88>

Binary predicates[edit]

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.

z = (not remainder)~| (36,<1,2,3,4,5,6,7,8,9,10,11,12>)

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[edit]

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

shortcut = ~&ihBF x

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.

V[edit]

[even? dup 2 / >int 2 * - zero?].
 
[1 2 3 4 5 6 7 8 9] [even?] filter
=[2 4 6 8]

VBA[edit]

 
Option Explicit
 
Sub Main()
Dim evens() As Long, i As Long
Dim numbers() As Long
 
For i = 1 To 100000
ReDim Preserve numbers(1 To i)
numbers(i) = i
Next i
 
evens = FilterInNewArray(numbers)
 
Debug.Print "Count of initial array : " & UBound(numbers) & ", first item : " & numbers(LBound(numbers)) & ", last item : " & numbers(UBound(numbers))
Debug.Print "Count of new array : " & UBound(evens) & ", first item : " & evens(LBound(evens)) & ", last item : " & evens(UBound(evens))
 
FilterInPlace numbers
 
Debug.Print "Count of initial array (filtered): " & UBound(numbers) & ", first item : " & numbers(LBound(numbers)) & ", last item : " & numbers(UBound(numbers))
End Sub
 
Private Function FilterInNewArray(arr() As Long) As Long()
Dim i As Long, t() As Long, cpt As Long
For i = LBound(arr) To UBound(arr)
If IsEven(arr(i)) Then
cpt = cpt + 1
ReDim Preserve t(1 To cpt)
t(cpt) = i
End If
Next i
FilterInNewArray = t
End Function
 
Private Sub FilterInPlace(arr() As Long)
Dim i As Long, cpt As Long
For i = LBound(arr) To UBound(arr)
If IsEven(arr(i)) Then
cpt = cpt + 1
arr(cpt) = i
End If
Next i
ReDim Preserve arr(1 To cpt)
End Sub
 
Private Function IsEven(Number As Long) As Boolean
IsEven = (CLng(Right(CStr(Number), 1)) And 1) = 0
End Function
Output:
Count of initial array : 100000, first item : 1, last item : 100000
Count of new array : 50000, first item : 2, last item : 100000
Count of initial array (filtered): 50000, first item : 2, last item : 100000

VBScript[edit]

 
test_arr_1 = Array(1,2,3,4,5,6,7,8,9,10)
test_arr_2 = Array(1,2,3,4,5,6,7,8,9,10)
 
WScript.StdOut.Write "Scenario 1: Create a new array"
WScript.StdOut.WriteLine
WScript.StdOut.Write "Input: " & Join(test_arr_1,",")
WScript.StdOut.WriteLine
WScript.StdOut.Write "Output: " & filter_create(test_arr_1)
WScript.StdOut.WriteBlankLines(2)
 
WScript.StdOut.Write "Scenario 2: Destructive approach"
WScript.StdOut.WriteLine
WScript.StdOut.Write "Input: " & Join(test_arr_2,",")
WScript.StdOut.WriteLine
WScript.StdOut.Write "Output: " & filter_destruct(test_arr_2)
WScript.StdOut.WriteBlankLines(2)
 
Function filter_create(arr)
ReDim arr_new(0)
For i = 0 To UBound(arr)
If arr(i) Mod 2 = 0 Then
If arr_new(0) = "" Then
arr_new(0) = arr(i)
Else
ReDim Preserve arr_new(UBound(arr_new)+1)
arr_new(UBound(arr_new)) = arr(i)
End If
End If
Next
filter_create = Join(arr_new,",")
End Function
 
Function filter_destruct(arr)
count = 0
For i = 0 To UBound(arr)
If arr(i) Mod 2 <> 0 Then
count = count + 1
For j = i To UBound(arr)
If j + 1 <= UBound(arr) Then
arr(j) = arr(j+1)
End If
Next
End If
Next
ReDim Preserve arr(UBound(arr)-count)
filter_destruct = Join(arr,",")
End Function
Output:
Scenario 1: Create a new array
Input: 1,2,3,4,5,6,7,8,9,10
Output: 2,4,6,8,10

Scenario 2: Destructive approach
Input: 1,2,3,4,5,6,7,8,9,10
Output: 2,4,6,8,10

Visual Basic .NET[edit]

Works with: Visual Basic .NET version 9.0+
Module Filter
 
Sub Main()
Dim array() As Integer = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10}
Dim newEvenArray() As Integer
 
Console.WriteLine("Current Array:")
For Each i As Integer In array
Console.WriteLine(i)
Next
 
newEvenArray = filterArrayIntoNewArray(array)
 
Console.WriteLine("New Filtered Array:")
For Each i As Integer In newEvenArray
Console.WriteLine(i)
Next
 
array = changeExistingArray(array)
 
Console.WriteLine("Orginal Array After Filtering:")
For Each i As Integer In array
Console.WriteLine(i)
Next
End Sub
 
Private Function changeExistingArray(array() As Integer) As Integer()
Return filterArrayIntoNewArray(array)
End Function
 
Private Function filterArrayIntoNewArray(array() As Integer) As Integer()
Dim result As New List(Of Integer)
For Each element As Integer In array
If element Mod 2 = 0 Then
result.Add(element)
End If
Next
Return result.ToArray
End Function
 
End Module
 
Output:
Current Array:
1
2
3
4
5
6
7
8
9
10
New Filtered Array:
2
4
6
8
10
Orginal Array After Filtering:
2
4
6
8
10

Vlang[edit]

fn reduce(mut a []int){
mut last := 0
for e in a {
if e%2==0 {
a[last] = e
last++
}
}
a = a[..last]
}
fn main() {
mut nums := [5,4,8,2,4,6,5,6,34,12,21]
even := nums.filter(it%2==0)
println('orig: $nums')
println('even: $even')
reduce(mut nums)
println('dest: $nums')
}
Output:
orig: [5,4,8,2,4,6,5,6,34,12,21]
even: [4, 8, 2, 4, 6, 6, 34, 12]
dest: [4, 8, 2, 4, 6, 6, 34, 12]

WDTE[edit]

let a => import 'arrays';
let s => import 'stream';
 
a.stream [1; 2; 3; 4; 5; 6; 7; 8; 9; 10]
-> s.filter (@ even n => % n 2 -> == 0)
-> s.collect
-- io.writeln io.stdout
;
Output:
[2; 4; 6; 8; 10]

Doing this in a destructive manner is not possible normally in WDTE as everything is immutable.

Wrapl[edit]

VAR a <- ALL 1:to(10);

a will be the list [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

VAR e <- ALL a:values \ $ % 2 = 0;

e will be the list [2, 4, 6, 8, 10]

Wren[edit]

var a = [1, 4, 17, 8, -21, 6, -11, -2, 18, 31]
System.print("The original array is  : %(a)")
 
System.print("\nFiltering to a new array  :-")
var evens = a.where { |e| e%2 == 0 }.toList
System.print("The even numbers are  : %(evens)")
System.print("The original array is still : %(a)")
 
// Destructive filter, permanently remove even numbers.
evens.clear()
for (i in a.count-1..0) {
if (a[i]%2 == 0) {
evens.add(a[i])
a.removeAt(i)
}
}
evens = evens[-1..0]
System.print("\nAfter a destructive filter  :-")
System.print("The even numbers are  : %(evens)")
System.print("The original array is now  : %(a)")
Output:
The original array is       : [1, 4, 17, 8, -21, 6, -11, -2, 18, 31]

Filtering to a new array    :-
The even numbers are        : [4, 8, 6, -2, 18]
The original array is still : [1, 4, 17, 8, -21, 6, -11, -2, 18, 31]

After a destructive filter  :-
The even numbers are        : [4, 8, 6, -2, 18]
The original array is now   : [1, 17, -21, -11, 31]

XPL0[edit]

This uses the kludge of making the first element of an array its size. There is no 'sizeof' operator, unfortunately.

include c:\cxpl\codes;          \intrinsic 'code' declarations
 
proc Filter(A, B, Option); \Select all even numbers from array A
int A, B, Option; \ and return them in B, unless Option = true
int I, J;
[J:= 0;
for I:= 1 to A(0) do
if (A(I)&1) = 0 then
[J:= J+1;
if Option then
A(J):= A(I)
else B(J):= A(I);
];
if Option then A(0):= J else B(0):= J;
];
 
int Array, Evens(11), I;
[Array:= [10, 3, 1, 4, 1, 5, 9, 2, 6, 5, 4];
Filter(Array, Evens, false);
for I:= 1 to Evens(0) do
[IntOut(0, Evens(I)); ChOut(0, ^ )];
CrLf(0);
 
Filter(Array, Evens \not used\, true);
for I:= 1 to Array(0) do
[IntOut(0, Array(I)); ChOut(0, ^ )];
CrLf(0);
]
Output:
4 2 6 4
4 2 6 4

XQuery[edit]

 
(: Sequence of numbers from 1 to 10 :)
let $array := (1 to 10)
 
(: Short version :)
let $short := $array[. mod 2 = 0]
 
(: 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>{$short}</short>
<long>{$long}</long>
</result>
 
Output:
 
<?xml version="1.0" encoding="UTF-8"?>
<result>
<short>2 4 6 8 10</short>
<long>2 4 6 8 10</long>
</result>
 

XSLT[edit]

<xsl:for-each select="nodes[@value mod 2 = 0]">
<xsl:value-of select="@value" />
</xsl:for-each>

Z80 Assembly[edit]

This example is untested. Please check that it's correct, debug it as necessary, and remove this message.


TestArray_Metadata:
byte 4,4 ;4 rows, 4 columns.
TestArray:
byte 0,1,2,3
byte 4,5,6,7
byte 8,9,10,11
byte 12,13,14,15
 
OutputArray_Metadata:
byte 2,4
OutputArray:
ds 8,0 ;16 bytes each equaling zero
 
FilterEvenValues:
ld hl,TestArray_Metadata
ld a,(hl)
inc hl
ld b,(hl)
inc hl ;LD HL,TestArray
call mul_A_times_B ;unimplemented multiplication routine, multiplies A by B and returns product in A.
ld b,a ;we'll use this product as a loop counter.
 
ld de,OutputArray
 
loop_filterEvenValues:
ld a,(hl)
ld c,a
rrc c ;destructively test if odd or even. (That's why it was copied into C first.)
jr c,skipThis
ld (de),a
inc de
 
skipThis:
inc hl
djnz loop_filterEvenValues
 
ret ;return to basic
Output:

There was no code showing the contents of OutputArray to the screen, so here's a hexdump instead:

00 02 04 06 08 0A 0C 0E

zkl[edit]

T(1,4,9,16,25,36,"37",49,64,81,100, True,self)
.filter(fcn(n){(0).isType(n) and n.isOdd})
//-->L(1,9,25,49,81)

ZX Spectrum Basic[edit]

10 LET items=100: LET filtered=0
20 DIM a(items)
30 FOR i=1 TO items
40 LET a(i)=INT (RND*items)
50 NEXT i
60 FOR i=1 TO items
70 IF FN m(a(i),2)=0 THEN LET filtered=filtered+1: LET a(filtered)=a(i)
80 NEXT i
90 DIM b(filtered)
100 FOR i=1 TO filtered
110 LET b(i)=a(i): PRINT b(i);" ";
120 NEXT i
130 DIM a(1): REM To free memory (well, almost all)
140 DEF FN m(a,b)=a-INT (a/b)*b