array_reduce

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

array_reduceコールバック関数を用いて配列を普通の値に変更することにより、配列を再帰的に減らす

説明

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

array_reduce() は、配列 array の各要素に callback 関数を繰り返し適用し、 配列を一つの値に減らします。

パラメータ

array

入力の配列。

callback
mixed callback ( mixed $carry , mixed $item )
carry

前回の反復処理の結果を保持します。最初の反復処理のときには、 initial の値を保持しています。

item

現在の反復処理の値を保持します。

initial

オプションの intial が利用可能な場合、処理の最初で使用されたり、 配列が空の場合の最終結果として使用されます。

返り値

結果の値を返します。

配列が空で initial が渡されなかった場合は、 array_reduce()NULL を返します。

変更履歴

バージョン 説明
5.3.0 initialmixed を指定できるようになりました。 これまでは integer のみでした。

例1 array_reduce() の例

<?php
function sum($carry$item)
{
    
$carry += $item;
    return 
$carry;
}

function 
product($carry$item)
{
    
$carry *= $item;
    return 
$carry;
}

$a = array(12345);
$x = array();

var_dump(array_reduce($a"sum")); // int(15)
var_dump(array_reduce($a"product"10)); // int(1200), because: 10*1*2*3*4*5
var_dump(array_reduce($x"sum""No data to reduce")); // string(17) "No data to reduce"
?>

参考

  • array_filter() - コールバック関数を使用して、配列の要素をフィルタリングする
  • array_map() - 指定した配列の要素にコールバック関数を適用する
  • array_unique() - 配列から重複した値を削除する
  • array_count_values() - 配列の値の数を数える

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User Contributed Notes 14 notes

up
67
Hayley Watson
9 years ago
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;
}
?>
up
22
directrix1 at gmail dot com
1 year ago
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"
up
18
php at keith tyler dot com
7 years ago
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.
up
3
cwu at nolo dot com
1 year ago
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;
?>
up
7
Altreus
3 years ago
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.
up
4
bdechka at yahoo dot ca
10 years ago
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";
?>
up
3
yuki [dot] kodama [at] gmail [dot] com
10 years ago
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);
}
?>
up
3
ruslan dot zavackiy at gmail dot com
5 years ago
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);
?>
up
2
Seanj.jcink.com
11 years ago
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.
up
2
kon
4 years ago
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
   
)
  );
?>
up
1
magnesium dot oxide dot play+php at gmail dot com
3 years ago
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)
up
0
galley dot meng at gmail dot com
5 months ago
If your array has string keys, you can reduce a two-dimensional array into one-dimensional using array_reduce, array_merge and array_values. (PHP>=5.3.0)

<?php

$two_dimensional
= array();

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

$one_dimensional = array_reduce($two_dimensional, 'array_merge', array());

$one_dimensional = array_reduce($two_dimensional, function ($one_dimensional, $value) {
    return
array_merge($one_dimensional, array_values($value));
}, array());

# becomes array(1, 2, 3, 4, 5, 6)
up
-4
aiadfaris at yahoo dot de
3 years ago
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
up
-4
aiadfaris at yahoo dot de
3 years ago
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
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