<s>
In	O
functional	B-Language
programming	I-Language
,	O
filter	O
is	O
a	O
higher-order	B-Language
function	I-Language
that	O
processes	O
a	O
data	B-General_Concept
structure	I-General_Concept
(	O
usually	O
a	O
list	O
)	O
in	O
some	O
order	O
to	O
produce	O
a	O
new	O
data	B-General_Concept
structure	I-General_Concept
containing	O
exactly	O
those	O
elements	O
of	O
the	O
original	O
data	B-General_Concept
structure	I-General_Concept
for	O
which	O
a	O
given	O
predicate	B-Algorithm
returns	O
the	O
boolean	O
value	O
true	O
.	O
</s>
<s>
evaluates	O
to	O
the	O
list	O
2	O
,	O
4	O
,	O
…,	O
10	O
by	O
applying	O
the	O
predicate	B-Algorithm
even	O
to	O
every	O
element	O
of	O
the	O
list	O
of	O
integers	O
1	O
,	O
2	O
,	O
…,	O
10	O
in	O
that	O
order	O
and	O
creating	O
a	O
new	O
list	O
of	O
those	O
elements	O
for	O
which	O
the	O
predicate	B-Algorithm
returns	O
the	O
boolean	O
value	O
true	O
,	O
thereby	O
giving	O
a	O
list	O
containing	O
only	O
the	O
even	O
members	O
of	O
that	O
list	O
.	O
</s>
<s>
evaluates	O
to	O
the	O
list	O
1	O
,	O
3	O
,	O
…,	O
9	O
by	O
collecting	O
those	O
elements	O
of	O
the	O
list	O
of	O
integers	O
1	O
,	O
2	O
,	O
…,	O
10	O
for	O
which	O
the	O
predicate	B-Algorithm
even	O
returns	O
the	O
boolean	O
value	O
false	O
(	O
with	O
.	O
</s>
<s>
being	O
the	O
function	B-Application
composition	I-Application
operator	I-Application
)	O
.	O
</s>
<s>
This	O
function	O
express	O
that	O
if	O
is	O
even	O
the	O
return	O
value	O
is	O
,	O
otherwise	O
it	O
's	O
.	O
</s>
<s>
This	O
is	O
the	O
predicate	B-Algorithm
.	O
</s>
<s>
Haskell	B-Language
,	O
</s>
<s>
OCaml	B-Language
,	O
</s>
<s>
Standard	B-Language
ML	I-Language
,	O
</s>
<s>
or	O
Erlang	B-Operating_System
.	O
</s>
<s>
Common	B-Language
Lisp	I-Language
provides	O
the	O
functions	O
remove-if	B-Application
and	O
remove-if-not	B-Application
.	O
</s>
<s>
Scheme	B-Language
Requests	I-Language
for	I-Language
Implementation	I-Language
(	O
SRFI	O
)	O
1	O
provides	O
an	O
implementation	O
of	O
filter	O
for	O
the	O
language	O
Scheme	B-Language
.	O
</s>
<s>
C++	B-Language
provides	O
the	O
algorithms	B-Language
remove_if	O
(	O
mutating	O
)	O
and	O
remove_copy_if	O
(	O
non-mutating	O
)	O
;	O
C++11	B-Language
additionally	O
provides	O
copy_if	O
(	O
non-mutating	O
)	O
.	O
</s>
<s>
Smalltalk	B-Application
provides	O
the	O
select	O
:	O
method	O
for	O
collections	O
.	O
</s>
<s>
Filter	O
can	O
also	O
be	O
realized	O
using	O
list	B-Language
comprehensions	I-Language
in	O
languages	O
that	O
support	O
them	O
.	O
</s>
<s>
In	O
Haskell	B-Language
,	O
filter	O
can	O
be	O
implemented	O
like	O
this	O
:	O
</s>
<s>
+	O
Filter	O
in	O
various	O
languages	O
Language	O
Filter	O
Notes	O
APL	B-Language
(	O
pred	O
array	O
)	O
/arrayorpredarray	O
The	O
second	O
example	O
is	O
an	O
APL	B-Language
dop	O
.	O
</s>
<s>
C#	B-Application
3.0	O
ienum.Where(pred )	O
orThe	O
where	O
clause	O
Where	O
is	O
an	O
extension	O
method	O
ienum	O
is	O
an	O
IEnumerable	O
Similarly	O
in	O
all	O
.NET	O
languages	O
CFML	B-Language
obj.filter(func )	O
Where	O
obj	O
is	O
an	O
array	O
or	O
a	O
structure	O
.	O
</s>
<s>
The	O
func	O
receives	O
as	O
an	O
argument	O
each	O
element	O
's	O
value	O
.	O
</s>
<s>
Clojure	B-Language
(	O
filter	O
predicate	B-Algorithm
list	O
)	O
clojure.core/filter	O
on	O
ClojureDocs	O
Or	O
,	O
via	O
list	B-Language
comprehension	I-Language
:	O
(	O
for	O
[	O
x	O
list	O
:when	O
(	O
pred	O
x	O
)	O
]	O
x	O
)	O
Common	B-Language
Lisp	I-Language
(	O
remove-if	B-Application
inverted-pred	O
list	O
)	O
(	O
remove-if	B-Application
(	O
complement	O
pred	O
)	O
list	O
)	O
(	O
remove-if-not	B-Application
pred	O
list	O
)	O
The	O
function	O
remove-if-not	B-Application
has	O
been	O
deprecated	O
in	O
favor	O
of	O
the	O
equivalent	O
remove-if	B-Application
where	O
the	O
predicate	B-Algorithm
is	O
complemented.Function	O
COMPLEMENT	O
in	O
the	O
Common	B-Language
Lisp	I-Language
HyperSpec	I-Language
Thus	O
the	O
filter	O
should	O
be	O
written	O
or	O
more	O
simply	O
:	O
where	O
evenp	O
returns	O
the	O
inverted	O
value	O
of	O
oddp.Function	O
EVENP	O
,	O
ODDP	O
in	O
the	O
Common	B-Language
Lisp	I-Language
HyperSpec	I-Language
C++	B-Language
std::remove_copy_if(begin, end, result, prednot )	O
std::copy_if(begin, end, result, pred )	O
(	O
C++11	B-Language
)	O
in	O
header	O
<algorithm>	O
begin	O
,	O
end	O
,	O
result	O
are	O
iterators	O
predicate	B-Algorithm
is	O
reversed	O
D	B-Application
std.algorithm.filter	O
!	O
</s>
<s>
(	O
pred	O
)	O
(	O
list	O
)	O
Erlang	B-Operating_System
lists:filter(Fun, List )	O
Or	O
,	O
via	O
list	B-Language
comprehension	I-Language
:	O
[	O
X	O
||	O
X	O
<	O
-	O
List	O
,	O
Fun(X )	O
]	O
Groovy	B-Application
list.findAll(pred )	O
Haskell	B-Language
filter	O
pred	O
list	O
Or	O
,	O
via	O
list	B-Language
comprehension	I-Language
:	O
[	O
x	O
|	O
x	O
<	O
-	O
list	O
,	O
pred	O
x ]	O
Haxe	B-Language
list.filter(pred )	O
Lambda.filter(list, pred )	O
Or	O
,	O
via	O
list	B-Language
comprehension	I-Language
:	O
[	O
x	O
|	O
x	O
<	O
-	O
list	O
,	O
pred	O
x ]	O
J	B-Language
(#~	O
pred	O
)	O
list	O
An	O
example	O
of	O
a	O
monadic	O
hook	O
.	O
</s>
<s>
(	O
f	O
g	O
)	O
y	O
=	O
y	O
f	O
(	O
g	O
y	O
)	O
Julia	B-Application
filter(pred, array )	O
The	O
filter	O
function	O
also	O
accepts	O
dict	O
datatype	O
.	O
</s>
<s>
Or	O
,	O
via	O
list	B-Language
comprehension	I-Language
:	O
[	O
x	O
for	O
x	O
in	O
array	O
if	O
pred(x )	O
]	O
Java	B-Language
8+	O
stream.filter(pred )	O
JavaScript	B-Language
1.6	I-Language
array.filter(pred )	O
Kotlin	B-Language
array.filter(pred )	O
Mathematica	B-Language
Select[ list	O
,	O
pred ]	O
Objective-C	B-Language
(	O
Cocoa	B-Operating_System
in	O
Mac	O
OS	O
X	O
10.4	O
+	O
)	O
[	O
array	O
filteredArrayUsingPredicate:pred	O
]	O
pred	O
is	O
an	O
NSPredicate	O
object	O
,	O
which	O
may	O
be	O
limited	O
in	O
expressiveness	O
F#	B-Operating_System
,	O
OCaml	B-Language
,	O
Standard	B-Language
ML	I-Language
List.filter	O
pred	O
list	O
PARI/GP	B-Language
select(expr, list )	O
The	O
order	O
of	O
arguments	O
is	O
reversed	O
in	O
v	O
.	O
2.4.2	O
.	O
</s>
<s>
Perl	B-Language
grep	O
block	O
list	O
grep	O
expr	O
,	O
list	O
PHP	B-Application
array_filter(array, pred )	O
Prolog	B-Language
filter( +Closure	O
,	O
+List	O
,	O
-List	O
)	O
Since	O
ISO/IEC	O
13211-1:1995/Cor.2:2012ISO/IEC	O
13211-1:1995/Cor	O
2:2012	O
the	O
core	O
standard	O
contains	O
closure	O
application	O
via	O
call/N	O
Python	B-Language
filter(func, list )	O
Or	O
,	O
via	O
list	B-Language
comprehension	I-Language
:	O
[	O
x	O
for	O
x	O
in	O
list	O
if	O
pred(x )	O
]	O
.	O
</s>
<s>
In	O
Python	B-Language
3	O
,	O
filter	O
was	O
changed	O
to	O
return	O
an	O
iterator	O
rather	O
than	O
a	O
list	O
.	O
</s>
<s>
The	O
complementary	O
functionality	O
,	O
returning	O
an	O
iterator	O
over	O
elements	O
for	O
which	O
the	O
predicate	B-Algorithm
is	O
false	O
,	O
is	O
also	O
available	O
in	O
the	O
standard	O
library	O
as	O
filterfalse	O
in	O
the	O
itertools	O
module	O
.	O
</s>
<s>
Ruby	B-Language
enum.find_all	O
 { block } 	O
enum.select	O
 { block } 	O
enum	O
is	O
an	O
Enumeration	O
Rust	B-Application
iterator.filter(pred )	O
iterator	O
is	O
an	O
Iterator	O
and	O
the	O
filter	O
method	O
returns	O
a	O
new	O
iterator	O
;	O
pred	O
is	O
a	O
function	O
(	O
specifically	O
FnMut	O
)	O
that	O
receives	O
the	O
iterator	O
's	O
item	O
and	O
returns	O
a	O
bool	O
S	B-Language
,	O
R	B-Language
Filter(pred,array )	O
array[ 	O
pred(array )	O
]	O
In	O
the	O
second	O
case	O
,	O
pred	O
must	O
be	O
a	O
vectorized	O
function	O
Scala	B-Application
list.filter(pred )	O
Or	O
,	O
via	O
for-comprehension	O
:	O
for( x	O
<	O
-	O
list	O
;	O
if	O
pred	O
)	O
yield	O
x	O
Scheme	B-Language
R6RS	O
(	O
filter	O
pred	O
list	O
)	O
(	O
remove	O
inverted	O
pred	O
list	O
)	O
(	O
partition	O
pred	O
list	O
list	O
)	O
Smalltalk	B-Application
aCollection	O
select	O
:	O
aBlock	O
Swift	B-Application
array.filter(pred )	O
filter(sequence, pred'' )	O
XPath	B-Language
,	O
XQuery	B-Language
list[block]	O
filter(list, func )	O
In	O
block	O
the	O
context	O
item	O
.	O
</s>
<s>
Other	O
variants	O
of	O
filter	O
(	O
e.g.	O
,	O
Haskell	B-Language
dropWhile	O
and	O
partition	O
)	O
are	O
also	O
common	O
.	O
</s>
<s>
A	O
common	O
memory	O
optimization	O
for	O
purely	O
functional	B-Language
programming	I-Language
languages	I-Language
is	O
to	O
have	O
the	O
input	O
list	O
and	O
filtered	O
result	O
share	O
the	O
longest	O
common	O
tail	O
(	O
tail-sharing	B-Data_Structure
)	O
.	O
</s>
