array_reverse

array_rand

Last updated: Fri, 21 Nov 2008

array_reduce

(PHP 4 >= 4.0.5, PHP 5)

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

Description

mixed array_reduce ( array $input , callback $function [, int $initial ] )

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

Parameters

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

Examples

Example #1 array_reduce() example


<?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", 1);
?>

This will result in $b containing 15, $c containing 1200 (= 10*1*2*3*4*5), and $d containing 1.


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;

}

?>

Janez R.
30-Jul-2007 08:27


--REPOST, fixed some typos, please replace previous note--

In PHP ver. 4.3.1 the initial value parameter allowed for string type also. In PHP ver. 5.1.6 this param is now converted to int and callback function will receive number 0 when initial param is an empty string.



<?php

function arc ($reduced, $item)

{

  $reduced = $item.$reduced;

  return $reduced;

}



array_reduce( array(a,b,c), "arc", "" );

?>



Output in PHP 4.3.1: cba

Output in PHP 5.1.6: cba0



Possible solution:

<?php

function arc ($reduced, $item)

{

  if ($reduced === 0)  $reduced = "";

  $reduced = $item.$reduced;

  return $reduced;

}

?>

yuki [dot] kodama [at] gmail [dot] com
31-Jan-2007 08:56


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);

}

?>

bdechka at yahoo dot ca
10-Jan-2007 12:57


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";

?>

marcel dot oehler at kubusmedia dot com
08-Feb-2006 07:26


I've just experienced some really strange behaviour of array_reduce in PHP 5.0.4:



$result = array( 0, 17, 0, 0, 33, 0, 0, 0, 0, 50);

$total = array_reduce( $result, "sumCalc", 0);



function sumCalc( $a, $b){

    return $a + $b;

}



and $total equals to 83!



I know, this could be done easier, but it should work nevertheless. Has anybody experienced something similar? I will avoid using array_reduce in the future...

Seanj.jcink.com
04-Jan-2006 06:23


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.

ildar [DASH] sh [AT] mail [DOT] ru
29-Nov-2005 10:02


in rare cases when an array is a set of numeric values and result is one of sum or product of numbers the next examples may be useful



<?php



// sum of array items

echo eval('return ' . implode('+', $nums) . ';');



// product of array items

echo eval('return ' . implode('*', $nums) . ';');



?>



the reason of these codes is omitting of single used per script of callbacks

david dot tulloh at infaze dot com dot au
23-Jun-2005 01:18


The code supplied by cuntbubble is unfortunately incorrect.



Running it I got the output:

0<TR><TD><a href="page1.html">page1</a></td>

<TR><TD><a href="page2.html">page2</a></td>

<TR><TD><a href="page3.html">page3</a></td>

</table>



So php, not finding an integer, used int(0) to start the process.  I've tested to confirm this.

bishop
30-Apr-2004 08:19


Count the total number of characters in an array of strings:



<?php

$lines = array ('abc', 'd', 'ef');

$totalChars = array_reduce($lines, create_function('$v,$w','return $v + strlen($w);'), 0);

// $totalChars === 6

?>

tonicpeddler at aol dot com
01-Nov-2002 07:17


in response to php dot net at cuntbubble dot com



actually when you pass a value to a function that accepts a specific data type, php automatically evaluates that value as the data type expected

php dot net at cuntbubble dot com
17-Jan-2002 08:08


There is an error/misleading item in the documentation





[, int initial]





int is not constrained to an integer, it can be any data type (although I've not tested ALL data types)





and $v is the cumulative part, the current value of the reduction.





and I'll take the liberty to add another example, as used in my code





<?php


function reduceToTable($html, $p) {


    $html .= "<TR><TD><a href=\"$p.html\">$p</a></td>\n";


    return $html;


}





$list = Array("page1", "page2", "page3");





$tab = array_reduce($list, "reduceToTable", "<table>\n");


echo $tab . "</table>\n";


?>





hmm, getting stuff on one line sure is tricky, it get's wordwrapped on the char count in html so &gt; counts as 4 chars not one so by the time you've counted "< you've used up 8 chars


If it get's through moderation could someone please make it look ok :)

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