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array_reduce — Iterative Reduktion eines Arrays zu einem Wert mittels einer Callbackfunktion
array_reduce() wendet die Funktion function iterativ auf die Elemente des Arrays input so an, dass das Array auf einen einzigen Wert reduziert wird.
Das Eingabe-Array.
Die Callbackfunktion.
Ist der optionale Parameter intial angegeben, wird er am Anfang des Prozesses benutzt oder als Resultat verwendet, sollte das Array leer sein.
Gibt den sich ergebenden Wert zurück.
Falls das Array leer ist und initial nicht angegeben wurde, so wird array_reduce() NULL zurückgeben.
| Version | Beschreibung |
|---|---|
| 5.3.0 | Der von initial akzeptierte Datentyp wurde zu Mixed geändert, vorher war dies Integer. |
Beispiel #1 array_reduce()-Beispiel
<?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", "Keine Daten für die Reduktion");
?>
Dies wird dazu führen, dass $b den Wert 15 beinhaltet, $c den Wert 1200 (= 10*1*2*3*4*5) und $d den Text Keine Daten für die Reduktion enthält.
Thanks to php at keith tyler dot com, knowing that "if you do not provide $initial, the first value used in the iteration is NULL", we can write callbacks that test for is_null() and take any necessary step.
Example:
<?php
function andFunc($a, $b) {
return (is_null($a) ? true : $a) && $b;
}
$foo = array(true, true, true);
var_dump(array_reduce($foo, "andFunc"));
?>
It returns bool(true), as expected.
It seems that $initial is a required value; if you do not provide $initial, the first value used in the iteration is NULL.
Perhaps this is not a problem for callback functions that treat NULL as an identity (e.g. addition), but is a problem for cases when NULL is not identity (such as boolean context).
Compare:
<?php
function andFunc($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.
The note by Hrobky immediately below applies only to PHP versions prior to 5.3.0; as noted in the function's version history, $initial is now a "mixed" parameter.
What a difference a year makes :)
The initial value have to be int. So it is impossible to implement non-recursive flatten like this:
<?php
$arr = array(array(1), array(2), array(3));
$result = array_reduce($arr, 'array_merge', array());
// expected: $result == array (1, 2, 3)
?>
I get: Warning: array_merge() : Argument #1 is not an array.
If $initial parameter is not an int, array_reduce passes 0 (zero) to the callback function. Hayley Watson's note suggests a solution:
<?php
$acc = array();
foreach($arr as $a)
$acc = array_merge($acc, $a);
// now: $acc == array (1, 2, 3)
?>
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 $acc;
}
?>
--REPOST, fixed some typos, please replace previous note--
In PHP ver. 4.3.1 the initial value parameter allowed for string type also. In PHP ver. 5.1.6 this param is now converted to int and callback function will receive number 0 when initial param is an empty string.
<?php
function arc ($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 $reduced;
}
?>
This code will reduce array deeply.
<?php
function print_s($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 : $b);
}
?>
The above code works better this way.
<?php
function reduceToTable($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";
?>
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...
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.
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
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.
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
?>
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
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 > 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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