array_reduce

(PHP 4 >= 4.0.5, PHP 5, PHP 7, PHP 8)

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

Description

array_reduce(array $array, callable $callback, mixed $initial = null): mixed

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

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

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
8.0.0	If `callback` expects a parameter to be passed by reference, this function will now emit an `E_WARNING`.

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"
?>

Found A Problem?

Learn How To Improve This Page • Submit a Pull Request • Report a Bug

＋add a note

User Contributed Notes 17 notes

down

135

Hayley Watson ¶

17 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;
}
?>

down

directrix1 at gmail dot com ¶

9 years 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"

down

souzacomprog at gmail dot com ¶

4 years ago

Sometimes we need to go through an array and group the indexes so that it is easier and easier to extract them in the iteration.

<?php

$people = [
    ['id' => 1, 'name' => 'Hayley'],
    ['id' => 2, 'name' => 'Jack', 'dad' => 1],
    ['id' => 3, 'name' => 'Linus', 'dad'=> 4],
    ['id' => 4, 'name' => 'Peter' ],
    ['id' => 5, 'name' => 'Tom', 'dad' => 4],
];

$family = array_reduce($people, function($accumulator, $item) {
    // if you don't have a dad you are probably a dad
    if (!isset($item['dad'])) {
        $id = $item['id'];
        $name = $item['name'];
        // take the children if you already have 
        $children = $accumulator[$id]['children'] ?? [];
        // add dad
        $accumulator[$id] = ['id' => $id, 'name' => $name,'children' => $children];
        return $accumulator;
    }

    // add a new dad if you haven't already 
    $dad = $item['dad'];
    if (!isset($accumulator[$dad])) {
        // how did you find the dad will first add only with children 
        $accumulator[$dad] = ['children' => [$item]];
        return $accumulator;
    }

    //  add a son to his dad who has already been added
    //  by the first or second conditional "if"
    
    $accumulator[$dad]['children'][] = $item;
    return $accumulator;
}, []);

var_export(array_values($family));

?>

OUTPUT

array (
  0 =>
  array (
    'id' => 1,
    'name' => 'Hayley',
    'children' =>
    array (
      0 =>
      array (
        'id' => 2,
        'name' => 'Jack',
        'dad' => 1,
      ),
    ),
  ),
  1 =>
  array (
    'id' => 4,
    'name' => 'Peter',
    'children' =>
    array (
      0 =>
      array (
        'id' => 3,
        'name' => 'Linus',
        'dad' => 4,
      ),
      1 =>
      array (
        'id' => 5,
        'name' => 'Tom',
        'dad' => 4,
      ),
    ),
  ),
)

<?php
$array = [
  [
    "menu_id" => "1",
    "menu_name" => "Clients",
    "submenu_name" => "Add",
    "submenu_link" => "clients/add"
  ],
  [
    "menu_id" => "1",
    "menu_name" => "Clients",
    "submenu_name" => "List",
    "submenu_link" => "clients"
  ],
  [
    "menu_id" => "2",
    "menu_name" => "Products",
    "submenu_name" => "List",
    "submenu_link" => "products"
  ],
];

//Grouping submenus to their menus

$menu =  array_reduce($array, function($accumulator, $item){
  $index = $item['menu_name'];

  if (!isset($accumulator[$index])) {
    $accumulator[$index] = [
      'menu_id' => $item['menu_id'],
      'menu_name' => $item['menu_name'],
      'submenu' => []    
    ];
  }

  $accumulator[$index]['submenu'][] = [
    'submenu_name' => $item['submenu_name'],
    'submenu_link' => $item['submenu_link']
  ];

  return $accumulator;
}, []);

var_export(array_values($menu));

?>

OUTPUT

array (
  0 =>
  array (
    'menu_id' => '1',
    'menu_name' => 'Clients',
    'submenu' =>
    array (
      0 =>
      array (
        'submenu_name' => 'Add',
        'submenu_link' => 'clients/add',
      ),
      1 =>
      array (
        'submenu_name' => 'List',
        'submenu_link' => 'clients',
      ),
    ),
  ),
  1 =>
  array (
    'menu_id' => '2',
    'menu_name' => 'Products',
    'submenu' =>
    array (
      0 =>
      array (
        'submenu_name' => 'List',
        'submenu_link' => 'products',
      ),
    ),
  ),
)

down

itsunclexo at gmail dot com ¶

2 years ago

Let's see an example of array_reduce() to get the frequency of letters.

<?php

$items = "Hello";

$frequencies = array_reduce(str_split($items), 
    function($result, $item) {
        if (isset($result[$item])) {
            $result[$item] += 1;
        } else { 
            $result[$item] = 1;
        }
        return $result;
    },
    [] // note the initial is an array
);

print_r($frequencies);

?>

and output should be like:

Array
(
    [H] => 1
    [e] => 1
    [l] => 2
    [o] => 1
)

down

Julian Sawicki ¶

5 years ago

Array reduce offers a way to transform data. 
Please look at the array below. The array has 4 nested array's.
The nested array's have the same keys. Only the value is different. 

This code transforms the whole array. See below.  

$array = array(  
    0 => array('id' => '100', 'name' => 'Henk', 'age' => '30'),   
    1 => array('id' => '101', 'name' => 'Piet', 'age' => '33'),   
    2 => array('id' => '102', 'name' => 'Wim', 'age' => '43'),   
    3 => array('id' => '103', 'name' => 'Jaap', 'age' => '53'),  
);  
 
$arr = array_reduce($array, function($carry, $item){ 
 
    $arr = array(  
        'id' => $item['id'],  
        'value' => $item['name'],  
    );  
      
    $id = $item['id'];  
    $carry[$id] = $arr;  
      
    return $carry;  
}, array());  
 
var_dump($arr);
 
 
// OUTPUT

array (size=4)  
100 => array (size=2)         
   'id' => string '100' (length=3)         
   'value' => string 'Henk' (length=4)  
101 => array (size=2)          
   'id' => string '101' (length=3)          
   'value' => string 'Piet' (length=4)  
102 => array (size=2)          
   'id' => string '102' (length=3)          
   'value' => string 'Wim' (length=3)  
103 => array (size=2)          
   'id' => string '103' (length=3)          
   'value' => string 'Jaap' (length=4)

down

849330489 at qq dot com ¶

5 years ago

The first parameter $array can be also be functions, which produces very interesting and powerful result, which can be used to make an union of middlewares.

<?php

$f1 = function($x, $f){
    echo 'middleware 1 begin.'.PHP_EOL;
    $x += 1;
    $x = $f($x);
    echo 'middleware 1 end.'.PHP_EOL;
    return $x;    
};
    

$f2 = function($x, $f){
    echo 'middleware 2 begin: '.PHP_EOL;
    $x += 2;
    $x = $f($x);
    echo 'middleware 2 end.'.PHP_EOL;
    return $x;
};

$respond = function($x){
    echo 'Generate some response.'.PHP_EOL;
    return $x;
};

$middlewares = [$f1, $f2];
$initial = $respond;
$foo = array_reduce($middlewares, function($stack, $item){
    return function($request) use ($stack, $item){
        return $item($request, $stack);
    };
}, $initial);

$x = 1;
echo $foo($x);

?>

//output:
middleware 2 begin: 
middleware 1 begin.
Generate some response.
middleware 1 end.
middleware 2 end.
4

down

magnesium dot oxide dot play+php at gmail dot com ¶

10 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)

down

Altreus ¶

10 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.

down

ruslan dot zavackiy at gmail dot com ¶

12 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);

?>

down

php at keith tyler dot com ¶

14 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.

down

cwu at nolo dot com ¶

9 years 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;
?>

down

bdechka at yahoo dot ca ¶

17 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";
?>

down

yuki [dot] kodama [at] gmail [dot] com ¶

17 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);
}
?>

down

Seanj.jcink.com ¶

18 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.

down

kon ¶

12 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
    )
  );
?>

down

xyz at record dot contact ¶

1 year ago

If you want to reindex an array, say to add an ID to each item as a key, you can use the following:

$array = [['id' => 3, 'name' => 'bob'], ['id' => 4, 'name' => 'alice']];
array_reduce($array, fn($new_array, $item) => $new_array + [$item['id'] => $item], []);

Result:

[ 3 => ["id" => 3, "name" => "bob"], 4 => ['id' => 4, 'name' => 'alice'] ]

down

wallfur at gmail dot com ¶

4 months ago

If you need access to the keys of the array in your callback, this alternative uses the same signatures as array_reduce() and its callback, with the addition of a $key argument:
<?php
function array_reduce_assoc(array $array, callable $callback, mixed $initial = null): mixed
{
    foreach ($array as $key => $item)
    {
        $initial = call_user_func($callback, $initial, $item, $key);
    }
    return $initial;
}

＋add a note

array_reduce

Description

Parameters

Return Values

Changelog

Examples

See Also

Found A Problem?

User Contributed Notes 17 notes