array_reduce

(PHP 4 >= 4.0.5, PHP 5, PHP 7)

array_reduce — 用回调函数迭代地将数组简化为单一的值

说明

mixed array_reduce ( array $input , callable $function [, mixed $initial = NULL ] )

array_reduce() 将回调函数 function 迭代地作用到 input 数组中的每一个单元中，从而将数组简化为单一的值。

参数

input: The input array.
function: The callback function.

mixed callback ( mixed &$result , mixed $item )
initial: 如果指定了可选参数 initial，该参数将被当成是数组中的第一个值来处理，或者如果数组为空的话就作为最终返回值。

返回值

返回结果值。

initial 参数， array_reduce() 返回 NULL 。

更新日志

版本	说明
5.3.0	Changed `initial` to allow mixed , previously integer .

范例

Example #1 array_reduce() 例子

  <?php
 function  rsum ( $v ,  $w )
{
     $v  +=  $w ;
    return  $v ;
}

function  rmul ( $v ,  $w )
{
     $v  *=  $w ;
    return  $v ;
}

 $a  = array( 1 ,  2 ,  3 ,  4 ,  5 );
 $x  = array();
 $b  =  array_reduce ( $a ,  "rsum" );
 $c  =  array_reduce ( $a ,  "rmul" ,  10 );
 $d  =  array_reduce ( $x ,  "rsum" ,  "No data to reduce" );
 ?>

这将使 $b 的值为 15， $c 的值为 1200（= 10*1*2*3*4*5），以及 $d 的值为 1。

参见

array_filter() - 用回调函数过滤数组中的单元
array_map() - 将回调函数作用到给定数组的单元上
array_unique() - 移除数组中重复的值
array_count_values() - 统计数组中所有的值出现的次数

用户评论:

[#1] directrix1 at gmail dot com [2015-11-03 16:39:02]

So, if you were wondering how to use this where key and value are passed in to the function. I've had success with the following (this example generates formatted html attributes from an associative array of attribute => value pairs):

<?php // Attribute List $attribs = [ 'name' => 'first_name', 'value' => 'Edward' ]; // Attribute string formatted for use inside HTML element $formatted_attribs = array_reduce( array_keys($attribs), // We pass in the array_keys instead of the array here function ($carry, $key) use ($attribs) { // ... then we 'use' the actual array here return $carry . ' ' . $key . '="' . htmlspecialchars( $attribs[$key] ) . '"'; }, '' ); echo $formatted_attribs; ?>

This will output:
name="first_name" value="Edward"

[#2] cwu at nolo dot com [2015-09-18 19:34:26]

The single value returned by array_reduce() can be an array -- as illustrated in the following example:
<?php # calculate the average of an array function calculate_sum_and_count($sum_and_count, $item) { list($sum, $count) = $sum_and_count; $sum += $item; $count += 1; return [$sum, $count]; } $a = array(1, 2, 3, 4, 5, 6, 7, 8, 9, 10); $initial_sum_and_count = [0, 0]; list($sum, $count) = array_reduce($a, "calculate_sum_and_count", $initial_sum_and_count); echo $sum / $count; ?>

[#3] aiadfaris at yahoo dot de [2014-05-29 20:16:21]

notice to function array_reduce()
I suppose the function rsum in the example 1 so it is not correct,
but
$ v + = $ w;
will output 15

[#4] aiadfaris at yahoo dot de [2014-05-29 20:14:40]

notice to function array_reduce()
I suppose the function rsum in the example 1 so it is not correct,
but
$ v + = $ w;
will output 15

[#5] magnesium dot oxide dot play+php at gmail dot com [2014-04-11 05:27:28]

You can reduce a two-dimensional array into one-dimensional using array_reduce and array_merge. (PHP>=5.3.0)

<?php

$two_dimensional = array();
$two_dimensional['foo'] = array(1, 2, 3);
$two_dimensional['bar'] = array(4, 5, 6);

$one_dimensional = array_reduce($two_dimensional, 'array_merge', array());
# becomes array(1, 2, 3, 4, 5, 6)

[#6] Altreus [2014-02-26 14:30:19]

You can effectively ignore the fact $result is passed into the callback by reference. Only the return value of the callback is accounted for.

<?php $arr = [1,2,3,4]; var_dump(array_reduce( $arr, function(&$res, $a) { $res += $a; }, 0 )); # NULL ?>

<?php $arr = [1,2,3,4]; var_dump(array_reduce( $arr, function($res, $a) { return $res + $a; }, 0 )); # int(10) ?>

Be warned, though, that you *can* accidentally change $res if it's not a simple scalar value, so despite the examples I'd recommend not writing to it at all.

[#7] kon [2012-12-03 13:12:53]

Walking down related object's properties using array_reduce:

<?php $a=new stdClass; $a->b=new stdClass; $a->b->c="Hello World!\n"; $reductionPath=array("b","c"); print_r( array_reduce( $reductionPath, function($result, $item){ return $result->$item; }, $a ) ); ?>

[#8] ruslan dot zavackiy at gmail dot com [2012-07-06 08:29:44]

If you want something elegant in your code, when dealing with reducing array, just unshift first element, and use it as initial, because if you do not do so, you will + first element with first element:

<?php $arr = array( array('min' => 1.5456, 'max' => 2.28548, 'volume' => 23.152), array('min' => 1.5457, 'max' => 2.28549, 'volume' => 23.152), array('min' => 1.5458, 'max' => 2.28550, 'volume' => 23.152), array('min' => 1.5459, 'max' => 2.28551, 'volume' => 23.152), array('min' => 1.5460, 'max' => 2.28552, 'volume' => 23.152), ); $initial = array_shift($arr); $t = array_reduce($arr, function($result, $item) { $result['min'] = min($result['min'], $item['min']); $result['max'] = max($result['max'], $item['max']); $result['volume'] += $item['volume']; return $result; }, $initial); ?>

[#9] php at keith tyler dot com [2010-04-18 15:30:09]

If you do not provide $initial, the first value used in the iteration is NULL. This is not a problem for callback functions that treat NULL as an identity (e.g. addition), but is a problem for cases when NULL is not identity (such as boolean context).

Compare:

<?php function andFunc($a, $b) { return $a && $b; } $foo = array(true, true, true); var_dump(array_reduce($foo, "andFunc")); ?>

returns false! One would expect that it would return true because `true && true && true == true`!

Adding diagnostic output to andFunc() shows that the first call to andFunc is with the arguments (NULL, true). This resolves to false (as `(bool) null == false`) and thereby corrupts the whole reduction.

So in this case I have to set `$initial = true` so that the first call to andFunc() will be (true, true). Now, if I were doing, say, orFunc(), I would have to set `$initial = false`. Beware.

Note that the "rmul" case in the example sneakily hides this defect! They use an $initial of 10 to get `10*1*2*3*4*5 = 12000`. So you would assume that without an initial, you would get `1200/10 = 120 = 1*2*3*4*5`. Nope! You get big fat zero, because `int(null)==0`, and `0*1*2*3*4*5 = 0`!

I don't honestly see why array_reduce starts with a null argument. The first call to the callback should be with arguments ($initial[0],$initial[1]) [or whatever the first two array entries are], not (null,$initial[0]). That's what one would expect from the description.

Incidentally this also means that under the current implementation you will incur `count($input)` number of calls to the callback, not `count($input) - 1` as you might expect.

[#10] Hayley Watson [2007-10-23 14:49:03]

To make it clearer about what the two parameters of the callback are for, and what "reduce to a single value" actually means (using associative and commutative operators as examples may obscure this).

The first parameter to the callback is an accumulator where the result-in-progress is effectively assembled. If you supply an $initial value the accumulator starts out with that value, otherwise it starts out null.
The second parameter is where each value of the array is passed during each step of the reduction.
The return value of the callback becomes the new value of the accumulator. When the array is exhausted, array_reduce() returns accumulated value.

If you carried out the reduction by hand, you'd get something like the following lines, every one of which therefore producing the same result:
<?php array_reduce(array(1,2,3,4), 'f', 99 ); array_reduce(array(2,3,4), 'f', f(99,1) ); array_reduce(array(3,4), 'f', f(f(99,1),2) ); array_reduce(array(4), 'f', f(f(f(99,1),2),3) ); array_reduce(array(), 'f', f(f(f(f(99,1),2),3),4) ); f(f(f(f(99,1),2),3),4) ?>

If you made function f($v,$w){return "f($v,$w)";} the last line would be the literal result.

A PHP implementation might therefore look something like this (less details like error checking and so on):
<?php function array_reduce($array, $callback, $initial=null) { $acc = $initial; foreach($array as $a) $acc = $callback($acc, $a); return $acc; } ?>

[#11] yuki [dot] kodama [at] gmail [dot] com [2007-01-31 08:56:17]

This code will reduce array deeply.

<?php function print_s($s) { return is_null($s) ? "NULL" : (is_array($s) ? "Array" : ($s ? "TRUE" : "FALSE")); } function r_and_dp($a, $b) { echo "phase1:" . print_s($a) . "," . print_s($b) . "<br>\n"; if(is_array($a)) { $a = array_reduce($a, "r_and_dp"); } if(is_array($b)) { $b = array_reduce($b, "r_and_dp"); } echo "phase2:" . print_s($a) . "," . print_s($b) . "<br>\n"; $a = is_null($a) ? TRUE : $a; $b = is_null($b) ? TRUE : $b; echo "phase3:" . print_s($a) . "," . print_s($b) . "<br>\n"; return $a && $b; } $bools = array(TRUE, array(FALSE, TRUE), TRUE); echo print_s(array_reduce($bools, "r_and_dp")) . "<br>\n"; // result: FALSE ?>

When using boolean, you have to carefully set an "initial" argument.

<?php function r_or_dp($a, $b) { if(is_array($a)) { $a = array_reduce($a, "r_or_dp"); } if(is_array($b)) { $b = array_reduce($b, "r_or_dp"); } return (is_null($a) ? FALSE : $a) || (is_null($b) ? FALSE : $b); } ?>

[#12] bdechka at yahoo dot ca [2007-01-10 00:57:36]

The above code works better this way.

<?php function reduceToTable($html, $p) { $html .= "<TR><TD><a href=\"$p.html\">$p</a></td></tr>\n"; return $html; } $list = Array("page1", "page2", "page3"); $tab = array_reduce($list, "reduceToTable"); echo "<table>".$tab . "</table>\n"; ?>

[#13] Seanj.jcink.com [2006-01-04 18:23:57]

The code posted below by bishop to count the characters of an array is simply... erm... well useless to me...

$array=Array("abc","de","f");
strlen(implode("",$array)); //6

works; and is much smaller. Probably much faster too.