array_filter

(PHP 4 >= 4.0.6, PHP 5, PHP 7)

array_filter콜백 함수를 사용하여 배열 원소를 필터

설명

array array_filter ( array $input [, callback $callback ] )

array_filter()input 배열안의 각값에 callback 함수에 전달하는 작업을 반복한다. callback 함수가 true를 반환하면, input의 현재값이 결과 배열로 반환된다. 배열 키들은 보존된다.

인수

input

반복할 배열

callback

사용할 콜백 함수

callback을 지정하지 않으면, input에서 FALSE와 같은(논리형으로 변환 참고) 모든 원소는 제거됩니다.

반환값

필터한 배열을 반환합니다.

예제

Example #1 array_filter() 예제

<?php
function odd($var)
{
    return (
$var 1);
}

function 
even($var) {
    return (!(
$var 1));
}

$array1 = array("a"=>1"b"=>2"c"=>3"d"=>4"e"=>5);
$array2 = array(6789101112);

echo 
"홀수:\n";
print_r(array_filter($array1"odd"));
echo 
"짝수:\n";
print_r(array_filter($array2"even"));
?>

위 예제의 출력:

Odd :
Array
(
    [a] => 1
    [c] => 3
    [e] => 5
)
Even:
Array
(
    [0] => 6
    [2] => 8
    [4] => 10
    [6] => 12
)

Example #2 callback 없는 array_filter()

<?php

$entry 
= array(
             
=> 'foo',
             
=> 'false',
             
=> -1,
             
=> null,
             
=> ''
          
);

print_r(array_filter($entry));
?>

위 예제의 출력:

Array
(
    [0] => foo
    [1] => -1
)

주의

Caution

콜백 함수에서 배열을 변경할 때(원소 추가, 삭제, unset 등), 이 함수의 동작은 정의되지 않습니다.

참고

  • array_map() - Applies the callback to the elements of the given arrays
  • array_reduce() - 콜백 함수를 사용하여 배열을 반복적으로 단일 값으로 축소
  • array_walk() - 배열의 각 원소에 대해서 특정 함수를 적용

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User Contributed Notes 4 notes

up
528
Anonymous
9 years ago
If you want a quick way to remove NULL, FALSE and Empty Strings (""), but leave values of 0 (zero), you can use the standard php function strlen as the callback function:
eg:
<?php

// removes all NULL, FALSE and Empty Strings but leaves 0 (zero) values
$result = array_filter( $array, 'strlen' );

?>
up
17
nicolaj dot knudsen at gmail dot com
5 years ago
If you like me have some trouble understanding example #1 due to the bitwise operator (&) used, here is an explanation.

The part in question is this callback function:

<?php
function odd($var)
{
   
// returns whether the input integer is odd
   
return($var & 1);
}
?>

If given an integer this function returns the integer 1 if $var is odd and the integer 0 if $var is even.
The single ampersand, &, is the bitwise AND operator. The way it works is that it takes the binary representation of the two arguments and compare them bit for bit using AND. If $var = 45, then since 45 in binary is 101101 the operation looks like this:

45 in binary: 101101
1 in binary:  000001
              ------
result:       000001

Only if the last bit in the binary representation of $var is changed to zero (meaning that the value is even) will the result change to 000000, which is the representation of zero.
up
28
marc dot vanwoerkom at fernuni-hagen dot de
18 years ago
Some of PHP's array functions play a prominent role in so called functional programming languages, where they show up under a slightly different name:

<?php
  array_filter
() -> filter(),
 
array_map() -> map(),
 
array_reduce() -> foldl() ("fold left")
?>

Functional programming is a paradigm which centers around the side-effect free evaluation of functions. A program execution is a call of a function, which in turn might be defined by many other functions. One idea is to use functions to create special purpose functions from other functions.

The array functions mentioned above allow you compose new functions on arrays.

E.g. array_sum = array_map("sum", $arr).

This leads to a style of programming that looks much like algebra, e.g. the Bird/Meertens formalism.

E.g. a mathematician might state

  map(f o g) = map(f) o map(g)

the so called "loop fusion" law.

Many functions on arrays can be created by the use of the foldr() function (which works like foldl, but eating up array elements from the right).

I can't get into detail here, I just wanted to provide a hint about where this stuff also shows up and the theory behind it.
up
9
marc dot gray at gmail dot com
8 years ago
My favourite use of this function is converting a string to an array, trimming each line and removing empty lines:

<?php
$array
= array_filter(array_map('trim', explode("\n", $string)), 'strlen');
?>

Although it states clearly that array keys are preserved, it's important to note this includes numerically indexed arrays. You can't use a for loop on $array above without processing it through array_values() first.
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