PHP 7.0.6 Released

    array_reduce

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

    array_reduce — Iteratively reduce the array to a single value using a callback function

    Description

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

    array_reduce() applies iteratively the callback function to the elements of the array, so as to reduce the array to a single value.

    Parameters

    array

    The input array.

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

    Holds the return value of the previous iteration; in the case of the first iteration it instead holds the value of initial.

    item

    Holds the value of the current iteration.

    initial

    If the optional initial is available, it will be used at the beginning of the process, or as a final result in case the array is empty.

    Return Values

    Returns the resulting value.

    If the array is empty and initial is not passed, array_reduce() returns NULL.

    Changelog

    Version Description
    5.3.0 Changed initial to allow mixed, previously integer.

    Examples

    Example #1 array_reduce() example

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

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

    $a = array(1, 2, 3, 4, 5);
    $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"
    ?>

    See Also

    User Contributed Notes

    Hayley Watson
    8 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;
    }
    ?>
    Altreus
    2 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.
    php at keith tyler dot com
    6 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.
    bdechka at yahoo dot ca
    9 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";
    ?>
    yuki [dot] kodama [at] gmail [dot] com
    9 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);
    }
    ?>
    Seanj.jcink.com
    10 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.
    kon
    3 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
            )
      );
    ?>
    magnesium dot oxide dot play+php at gmail dot com
    2 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)
    directrix1 at gmail dot com
    5 months 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"
    cwu at nolo dot com
    7 months 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;
    ?>
    ruslan dot zavackiy at gmail dot com
    3 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);
    ?>
    aiadfaris at yahoo dot de
    1 year 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
    aiadfaris at yahoo dot de
    1 year 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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