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array_filter

(PHP 4 >= 4.0.6, PHP 5, PHP 7, PHP 8)

array_filter使用回调函数过滤数组的元素

说明

array_filter(array $array, ?callable $callback = null, int $mode = 0): array

遍历 array 数组中的每个值,并将每个值传递给 callback 回调函数。 如果 callback 回调函数返回 true,则将 array 数组中的当前值返回到结果 array 数组中。

返回结果 array 数组的键名(下标)会维持不变,如果 array 参数是索引数组,返回的结果 array 数组键名(下标)可能会不连续。 可以使用 array_values() 函数对数组重新索引。

参数

array

要遍历的数组

callback

使用的回调函数

如果没有提供 callback 回调函数,将删除数组中 array 的所有“空”元素。 有关 PHP 如何判定“空”元素,请参阅 empty()

mode

决定哪些参数发送到 callback 回调的标志:

  • ARRAY_FILTER_USE_KEY - 将键名作为 callback 回调的唯一参数,而不是值
  • ARRAY_FILTER_USE_BOTH - 将值和键都作为参数传递给 callback 回调,而不是仅传递值
默认值为 0 ,只传递值作为 callback 回调的唯一参数。

返回值

返回过滤后的数组。

更新日志

版本 说明
8.0.0 callback 回调现在可以为空(nullable)。

范例

示例 #1 array_filter() 示例

<?php
function odd($var)
{
    
// 返回输入整数是否为奇数(单数)
    
return $var 1;
}
function 
even($var)
{
    
// 返回输入整数是否为偶数
    
return !($var 1);
}
$array1 = ['a' => 1'b' => 2'c' => 3'd' => 4'e' => 5];
$array2 = [6789101112];
echo 
"Odd :\n";
print_r(array_filter($array1"odd"));
echo 
"Even:\n";
print_r(array_filter($array2"even"));
?>

以上例程会输出:

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

示例 #2 array_filter() 不使用 callback 参数时

<?php
$entry 
= [
    
=> 'foo',
    
=> false,
    
=> -1,
    
=> null,
    
=> '',
    
=> '0',
    
=> 0,
];
print_r(array_filter($entry));
?>

以上例程会输出:

Array
(
    [0] => foo
    [2] => -1
)

示例 #3 array_filter() 不使用 mode 参数时

<?php
$arr 
= ['a' => 1'b' => 2'c' => 3'd' => 4];
var_dump(array_filter($arr, function($k) {
    return 
$k == 'b';
}, 
ARRAY_FILTER_USE_KEY));
var_dump(array_filter($arr, function($v$k) {
    return 
$k == 'b' || $v == 4;
}, 
ARRAY_FILTER_USE_BOTH));
?>

以上例程会输出:

array(1) {
  ["b"]=>
  int(2)
}
array(2) {
  ["b"]=>
  int(2)
  ["d"]=>
  int(4)
}

注释

警告

不应该在回调函数中改变数组(如:添加、删除或销毁“unset”当前数组和里面的元素),否则此函数的行为将不可预测。

参见

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User Contributed Notes 4 notes

up
527
Anonymous
8 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
4 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
17 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
8
marc dot gray at gmail dot com
7 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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