Filter: Difference between revisions
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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: |
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: |
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<lang Dyalect> |
<lang Dyalect>var xs = [1..20] |
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| ⚫ | |||
for x in this when pred(x) { |
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yield x |
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} |
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} |
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func Iterator.Select(proj) { |
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for x in this { |
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yield proj(x) |
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} |
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} |
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var xs = [1..20] |
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| ⚫ | |||
print(arr.ToArray())</lang> |
print(arr.ToArray())</lang> |
||
Revision as of 22:25, 20 May 2022
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.
11l
<lang 11l>V array = Array(1..10) V even = array.filter(n -> n % 2 == 0) print(even)</lang>
- Output:
[2, 4, 6, 8, 10]
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>
Action!
<lang Action!>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</lang>
- 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
<lang actionscript>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]);
}</lang>
Actionscript 3 <lang actionscript>var arr:Array = new Array(1, 2, 3, 4, 5); arr = arr.filter(function(item:int, index:int, array:Array) {
return item % 2 == 0;
}); </lang>
Ada
<lang ada>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;</lang> Here is a non-recursive solution: <lang ada>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;</lang>
Aime
<lang aime>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;
}</lang>
- Output:
0 2 4 6 8
ALGOL 68
<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
ALGOL W
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.
<lang algolw>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. </lang>
- Output:
2 4 6 8 10
AmigaE
<lang amigae>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</lang>
AntLang
<lang AntLang>x:range[100] {1- x mod 2}hfilter x</lang>
Apex
<lang Apex>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');</lang>
APL
<lang APL> (0=2|x)/x←⍳20 2 4 6 8 10 12 14 16 18 20</lang>
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>
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:
- The index of the current element
- A reference to the whole list.
This allows for context-sensitive filters, which can take account of following or preceding elements in a sequence.
<lang AppleScript>-------------------------- 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</lang>
- Output:
{0, 2, 4, 6, 8, 10}
Arturo
<lang rebol>arr: [1 2 3 4 5 6 7 8 9 10]
print select arr [x][even? x]</lang>
- Output:
2 4 6 8 10
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>
AWK
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: <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}'</lang>
- Output:
4 6 8 2
Regular script: <lang 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
} </lang> Same output.
Batch File
<lang dos> @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 </lang>
- 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
<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>
BCPL
<lang bcpl>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')
$)</lang>
- Output:
Numbers: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Even numbers: 2 4 6 8 10 12 14 Even multiples of 3: 6 12
Bracmat
<lang bracmat>( :?odds & ( 1 2 3 4 5 6 7 8 9 10 16 25 36 49 64 81 100:? (=.!sjt*1/2:/&!odds !sjt:?odds)$() ()
| !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>
Burlesque
<lang burlesque> blsq ) 1 13r@{2.%n!}f[ {2 4 6 8 10 12} </lang>
BQN
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.
<lang bqn>_filter ← {(𝔽𝕩)/𝕩} Odd ← 2⊸|
Odd _filter 1‿2‿3‿4‿5</lang> <lang>⟨ 1 3 5 ⟩</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#
<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>
<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>
<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>
<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>
CoffeeScript
<lang coffee>[1..10].filter (x) -> not (x%2)</lang>
- Output:
[ 2, 4, 6, 8, 10 ]
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>
Cowgol
<lang cowgol>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();</lang>
- Output:
2 4 6 8 10 2 4 6
D
<lang d>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]));
}</lang>
Tango Version
<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 .. $].sort ).newline; // Unlike odd elements
}</lang>
- Output:
Evens - [ 2, 4, 6, 8, 10 ] Odds - [ 1, 3, 5, 7, 9 ]
Delphi
Hand-coded version
<lang Delphi>program FilterEven;
{$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>
Using Boost.Int library
Alternative using Boost.Int[1]:
<lang Delphi>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. </lang>
- Output:
[2,4,6,8] [2,4,6,8]
Dyalect
Non-destructively
<lang Dyalect>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)</lang>
- Output:
[2, 4, 6, 8, 10, 12, 14, 16, 18, 20]
Destructively
<lang Dyalect>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)</lang>
- Output:
[2, 4, 6, 8, 10, 12, 14, 16, 18, 20]
Idiomatic approach
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:
<lang Dyalect>var xs = [1..20] var arr = xs.Iterate().Filter(x => x % 2 == 0).Map(x => x.ToString()) print(arr.ToArray())</lang>
- Output:
["2", "4", "6", "8", "10", "12", "14", "16", "18", "20"]
Déjà Vu
Non-destructively
<lang dejavu>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 ]</lang>
- Output:
[ 0 2 4 6 8 ]
Destructively
<lang dejavu>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</lang>
- Output:
[ 0 2 4 6 8 ]
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>
EasyLang
<lang>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[]</lang>
EchoLisp
<lang scheme> (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)
</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>
Elena
ELENA 5.0 : <lang elena>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
}</lang> Using strong typed collections and extensions: <lang elena>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) }
}</lang>
- Output:
2 4
Elixir
<lang elixir>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]</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>
FreeBASIC
<lang freebasic>' 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</lang>
- 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
<lang frink> b = array[1 to 100] c = select[b, {|x| x mod 2 == 0}] </lang>
Futhark
<lang Futhark> fun main(as: []int): []int =
filter (fn x => x%2 == 0) as
</lang>
Gambas
Click this link to run this code <lang gambas>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</lang> 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
<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>
A Java 8 solution with stream and generic types: <lang java>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));
}</lang>
Methodcall:
<lang java>Integer[] array = {1, 2, 3, 4, 5};
Integer[] result = filter(array, i -> (i % 2) == 0);</lang>
Warning: This solution works not with primitive types!
For arrays with a primitive type use the wrapper class.
JavaFX Script
<lang javafx>def array = [1..100]; def evens = array[n | n mod 2 == 0];</lang>
JavaScript
ES5
The standard way is to use the Array.prototype.filter function (
):
<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>
<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>
<lang javascript>Functional.select("+1&1", [1,2,3,4]) // [2, 4]</lang>
ES6
<lang JavaScript>(() => {
'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]
})();</lang>
- Output:
<lang JavaScript>[2, 4, 6, 8]</lang>
jq
jq's "select" filter is designed to make it easy to filter both arrays and streams: <lang jq>(1,2,3,4,5,6,7,8,9) | select(. % 2 == 0)</lang>
- Output:
2 4 6 8
<lang jq> [range(1;10)] | map( select(. % 2 == 0) ) </lang>
- Output:
[2,4,6,8]
Julia
<lang julia>@show filter(iseven, 1:10)</lang>
- Output:
filter(iseven, 1:10) = [2, 4, 6, 8, 10]
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>
Kotlin
<lang scala>// 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(" "))
}</lang>
- Output:
1 2 3 4 5 6 7 8 9 2 4 6 8 2 4 6 8
Lambdatalk
<lang scheme>
{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
</lang>
Lang5
<lang lang5>: filter over swap execute select ; 10 iota "2 % not" filter . "\n" .
- [ 0 2 4 6 8 ]</lang>
langur
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.
<lang langur>val .arr = series 7
writeln " array: ", .arr writeln "filtered: ", where f .x div 2, .arr</lang>
- Output:
array: [1, 2, 3, 4, 5, 6, 7] filtered: [2, 4, 6]
Lasso
<lang Lasso>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</lang>
staticarray(2, 4, 6, 8, 10)
Modifying the original array <lang lasso>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</lang>
array(2, 4, 6, 8, 10)
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)
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</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>
The destructive version is even simpler, since tables are passed by reference:
<lang lua>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))</lang>
M2000 Interpreter
Using Filter for arrays
<lang M2000 Interpreter> Module Checkit {
Print (1,2,3,4,5,6,7,8)#filter(lambda ->number mod 2=0)
} Checkit </lang>
Old style
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
<lang M2000 Interpreter> 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 </lang>
Maple
<lang Maple> evennum:=proc(nums::list(integer)) return select(x->type(x, even), nums); end proc; </lang>
Mathematica / Wolfram Language
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>
- 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>
min
<lang min>(1 2 3 4 5 6 7 8 9 10) 'even? filter print</lang>
- Output:
(2 4 6 8 10)
MiniScript
We define a filter method on the list type that returns a new list containing elements filtered by the given function. <lang MiniScript>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)</lang>
The in-place version is simpler, and even allows the use of an unnamed filter function, defined right on the method call. <lang MiniScript>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</lang>
ML
Standard ML
<lang sml>val ary = [1,2,3,4,5,6]; List.filter (fn x => x mod 2 = 0) ary</lang>
MLite
MLite is similar to Standard ML, though '=>' becomes '=' and 'List.' is elided: <lang ocaml>val ary = [1,2,3,4,5,6]; filter (fn x = x mod 2 = 0) ary;</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>
NetRexx
<lang NetRexx>/* 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
</lang>
- 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
<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>
Nim
<lang nim>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</lang>
- 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
<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
<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>
<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>
<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>
Oforth
<lang Oforth>100 seq filter(#isEven)</lang>
Ol
<lang scheme> (filter even? '(1 2 3 4 5 6 7 8 9 10)) </lang>
ooRexx
<lang oorexx> 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</lang>
- 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
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
- 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)toselect(vector, function).
<lang parigp>iseven(n)=n%2==0 select(iseven, [2, 3, 4, 5, 7, 8, 9, 11, 13, 16, 17])</lang>
Or in anonymous form <lang parigp>select(n -> n%2==0, [2, 3, 4, 5, 7, 8, 9, 11, 13, 16, 17])</lang>
Pascal
Arrays are supported in all versions of pascal so this simple example will cover the entire gamut.
<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().
Peloton
Fixed length English dialect <lang sgml><@ LETCNWLSTLIT>numbers|1 2 3 4 5 6 7 8 9 10 11 12</@> <@ DEFLST>evens</@> <@ ENULSTLIT>numbers| <@ TSTEVEELTLST>...</@> <@ IFF> <@ LETLSTELTLST>evens|...</@> </@> </@></lang>
Perl
<lang perl>my @a = (1, 2, 3, 4, 5, 6); my @even = grep { $_%2 == 0 } @a;</lang>
Phix
basic task
function even(integer i) return remainder(i,2)=0 end function ?filter(tagset(10),even)
- Output:
{2,4,6,8,10}
extra credit
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
<lang phl>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; ]</lang>
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>
Picat
List comprehension is probably the best way of filtering: <lang Picat>[I : I in 1..20, I mod 2 == 0]</lang>
A more general version of filtering is to use call/1 with a defined predicate (here p/1):
<lang Picat>go =>
L = 1..20, A = filter(L,p).
p(N) => N mod 2 == 0.
filter(A,F) = [N : N in A, call(F,N)].</lang>
This general version might be slower since using call/1 has some overhead.
PicoLisp
<lang PicoLisp>(filter '((N) (not (bit? 1 N)))
(1 2 3 4 5 6 7 8 9) )</lang>
- Output:
-> (2 4 6 8)
PL/I
<lang pli>(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;</lang> 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
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
<lang postscript> [1 2 3 4 5 6 7 8 9 10] {2 mod 0 eq} find </lang>
PowerShell
<lang powershell>$array = -15..37 $array | Where-Object { $_ % 2 -eq 0 }</lang>
Prolog
findall
<lang prolog>evens(D, Es) :- findall(E, (member(E, D), E mod 2 =:= 0), Es).</lang>
Usage:
<lang prolog>?- evens([1,2,3,4,5,6,7,8,9,10],E). E = [2, 4, 6, 8, 10]</lang>
Anonymous functions
Works with SWI-Prolog and module(lambda) written by Ulrich Neumerkel, "lambda.pl" can be found there : http://www.complang.tuwien.ac.at/ulrich/Prolog-inedit/lambda.pl <lang Prolog>?- use_module(library(lambda)). true.
?- include((\X^(X mod 2 =:= 0)), [1,2,3,4,5,6,7,8,9], L). L = [2,4,6,8].</lang>
filter and anonymous functions
Works with SWI-Prolog and module(lambda) written by Ulrich Neumerkel, "lambda.pl" can be found there : http://www.complang.tuwien.ac.at/ulrich/Prolog-inedit/lambda.pl <lang prolog>:- use_module(lambda).
%% filter(Pred, LstIn, LstOut) %% filter(_Pre, [], []).
filter(Pred, [H|T], L) :- filter(Pred, T, L1), ( call(Pred,H) -> L = [H|L1]; L = L1). </lang> Usage : <lang prolog> ?- filter(\X^(X mod 2 =:= 0), [1,2,3,4,5,6,7,8,9], L). L = [2,4,6,8] . </lang>
PureBasic
<lang PureBasic>Dim Tal.i(9) Dim Evens.i(0)
- - Set up an array with random numbers
For i=0 To ArraySize(Tal())
Tal(i)=Random(100)
Next
- - Pick out all Even and save them
j=0 For i=0 To ArraySize(Tal())
If Tal(i)%2=0 ReDim Evens(j) ; extend the Array as we find new Even's Evens(j)=tal(i) j+1 EndIf
Next
- - Display the result
PrintN("List of Randoms") For i=0 To ArraySize(Tal())
Print(Str(Tal(i))+" ")
Next PrintN(#CRLF$+#CRLF$+"List of Even(s)") For i=0 To ArraySize(Evens())
Print(Str(Evens(i))+" ")
Next</lang>
- Output:
List of Randoms 32 35 89 91 11 33 12 22 42 43
List of Even(s) 32 12 22 42
Python
<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>
Or in functional terms, by descending generality and increasing brevity:
<lang python>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()</lang>
- 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
<lang Quackery> [ [] ]'[ 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</lang>
- Output:
Ten arbitrary digits: [ 1 6 1 2 2 1 8 5 5 4 ] Only the even digits: [ 6 2 2 8 4 ]
Destructively
<lang Quackery> [ ]'[ 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</lang>
- Output:
Ten shuffled digits: [ 5 6 8 2 1 4 0 3 9 7 ] Less the odd digits: [ 6 8 2 4 0 ]
Q
<lang q>x where 0=x mod 2</lang>
R
<lang R>a <- 1:100 evennums <- a[ a%%2 == 0 ] print(evennums)</lang>
Racket
The classic way: <lang Racket> -> (filter even? '(0 1 2 3 4 5 6 7 8 9)) '(0 2 4 6 8) </lang> getting the list of non-evens too: <lang Racket> -> (partition even? '(0 1 2 3 4 5 6 7 8 9)) '(0 2 4 6 8) '(1 3 5 7 9) </lang> Finally, using a for loop, similar to list comprehension: <lang Racket> -> (for/list ([x '(0 1 2 3 4 5 6 7 8 9)] #:when (even? x)) x) '(0 2 4 6 8) </lang>
Raku
(formerly Perl 6)
<lang perl6>my @a = 1..6; my @even = grep * %% 2, @a;</lang>
Alternatively:
<lang perl6>my @a = 1..6; my @even = @a.grep(* %% 2);</lang>
Destructive:
<lang perl6>my @a = 1..6; @a .= grep(* %% 2);</lang>
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]
Red
<lang Red>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 </lang>
- 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
using two arrays
This example uses two arrays. The random BIF is used to generate the numbers. <lang rexx>/*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. */</lang>
Programming note: the REXX statement
<lang rexx> if old.j//2 \== 0 then iterate</lang>
could've been replaced with
<lang rexx> if old.j//2 then iterate</lang>
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
This version uses a control array, which isn't fully populated (in REXX terms, a sparse array.) <lang rexx>/*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. */</lang>
For the following input: , 1234567
- output Output is the same as the 1st REXX version (using two arrays).
using one array, destructive
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. <lang rexx>/*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. */</lang>
For the following input: , 1234567
- output is the same as the 1st REXX version (using two arrays).
Ring
<lang> 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 </lang>
Ruby
Enumerable#select is the filter that returns a new Array.
<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>
Destructive
Array#select! is the destructive version which modifies the original Array.
<lang ruby>ary = [1, 2, 3, 4, 5, 6] ary.select! {|elem| elem.even?} p ary # => [2, 4, 6]</lang>
Shorthand:
<lang ruby>ary = [1, 2, 3, 4, 5, 6] ary.select!(&:even?) p ary # => [2, 4, 6]</lang>
Run BASIC
<lang runbasic>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</lang>
Rust
<lang rust>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);
}</lang>
- 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
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
Filter function definition: <lang scheme> (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)))))))
</lang> Usage 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>
SequenceL
Filters are primarily written in SequenceL using partial Indexed Functions.
<lang sequencel>evens(x(1))[i] := x[i] when x[i] mod 2 = 0;</lang>
- Output:
cmd:>evens(1...5) [2,4]
Sidef
<lang ruby>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]</lang>
Slate
<lang slate>#(1 2 3 4 5) select: [| :number | number isEven].</lang>
Smalltalk
Creates a new array: <lang smalltalk>#(1 2 3 4 5) select: [:number | number even]</lang> or for short: <lang smalltalk>#(1 2 3 4 5) select:#even</lang> 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
Task: Select certain elements from an Array into a new Array in a generic way. To demonstrate, select all even numbers from an Array.
<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>
'
<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>
Stata
<lang stata>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</lang>
Swift
<lang swift>let numbers = [1,2,3,4,5,6] let even_numbers = numbers.filter { $0 % 2 == 0 } println(even_numbers)</lang>
- Output:
[2, 4, 6]
Tcl
<lang tcl>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}]</lang>
- Output:
56 62 76 82 94 28 90 52 44
Inplace way, quite the inefficient contraption compared to mapping:
<lang tcl>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</lang>
- 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:
<lang tcl>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}]"
}</lang>
- 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
<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
<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[$i]}"
done</lang>
Or, using grep:
<lang bash>a=(1 2 3 4 5) read -a e -d\n < <(printf '%s\n' "${a[@]}" | grep '[02468]$')</lang>
Either way, to display the results:
<lang bash>echo "${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>
- Output:
<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.
V
<lang v>[even? dup 2 / >int 2 * - zero?].
[1 2 3 4 5 6 7 8 9] [even?] filter =[2 4 6 8]</lang>
VBA
<lang vb> 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</lang>
- 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
<lang vb> 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</lang>
- 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
<lang vbnet>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 </lang>
- 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
<lang vlang>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')
}</lang>
- 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
<lang WDTE>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
- </lang>
- Output:
[2; 4; 6; 8; 10]
Doing this in a destructive manner is not possible normally in WDTE as everything is immutable.
Wrapl
<lang wrapl>VAR a <- ALL 1:to(10);</lang> a will be the list [1, 2, 3, 4, 5, 6, 7, 8, 9, 10] <lang wrapl>VAR e <- ALL a:values \ $ % 2 = 0;</lang> e will be the list [2, 4, 6, 8, 10]
Wren
<lang ecmascript>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)")</lang>
- 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
This uses the kludge of making the first element of an array its size. There is no 'sizeof' operator, unfortunately.
<lang XPL0>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); ]</lang>
- Output:
4 2 6 4 4 2 6 4
XQuery
<lang xquery> (: 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>
</lang>
- Output:
<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
<lang xml><xsl:for-each select="nodes[@value mod 2 = 0]">
<xsl:value-of select="@value" />
</xsl:for-each></lang>
Z80 Assembly
<lang z80>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</lang>
- 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
<lang zkl>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)</lang>
ZX Spectrum Basic
<lang zxbasic>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</lang>
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