Difference between array_map, array_walk and array_filter

03/22/2020 21:00:01

What exactly is the the difference between array_map, array_walk and array_filter. What I could see from documentation is that you could pass a callback function to perform an action on the supplied array. But I don't seem to find any particular difference between them.

Do they perform the same thing?
Can they be used interchangeably?

I would appreciate your help with illustrative example if they are different at all.

Verified Answer (551 Votes)

08/08/2010 03:22:35
  • Changing Values:
  • Array Keys Access:
  • Return Value:
    • array_map returns a new array, array_walk only returns true. Hence, if you don't want to create an array as a result of traversing one array, you should use array_walk.
  • Iterating Multiple Arrays:
    • array_map also can receive an arbitrary number of arrays and it can iterate over them in parallel, while array_walk operates only on one.
  • Passing Arbitrary Data to Callback:
    • array_walk can receive an extra arbitrary parameter to pass to the callback. This mostly irrelevant since PHP 5.3 (when anonymous functions were introduced).
  • Length of Returned Array:
    • The resulting array of array_map has the same length as that of the largest input array; array_walk does not return an array but at the same time it cannot alter the number of elements of original array; array_filter picks only a subset of the elements of the array according to a filtering function. It does preserve the keys.

Example:

<pre>
<?php

$origarray1 = array(2.4, 2.6, 3.5);
$origarray2 = array(2.4, 2.6, 3.5);

print_r(array_map('floor', $origarray1)); // $origarray1 stays the same

// changes $origarray2
array_walk($origarray2, function (&$v, $k) { $v = floor($v); }); 
print_r($origarray2);

// this is a more proper use of array_walk
array_walk($origarray1, function ($v, $k) { echo "$k => $v", "\n"; });

// array_map accepts several arrays
print_r(
    array_map(function ($a, $b) { return $a * $b; }, $origarray1, $origarray2)
);

// select only elements that are > 2.5
print_r(
    array_filter($origarray1, function ($a) { return $a > 2.5; })
);

?>
</pre>

Result:

Array
(
    [0] => 2
    [1] => 2
    [2] => 3
)
Array
(
    [0] => 2
    [1] => 2
    [2] => 3
)
0 => 2.4
1 => 2.6
2 => 3.5
Array
(
    [0] => 4.8
    [1] => 5.2
    [2] => 10.5
)
Array
(
    [1] => 2.6
    [2] => 3.5
)
551

Answer #2 (90 Votes)

08/08/2010 08:46:51

The idea of mapping an function to array of data comes from functional programming. You shouldn't think about array_map as a foreach loop that calls a function on each element of the array (even though that's how it's implemented). It should be thought of as applying the function to each element in the array independently.

In theory such things as function mapping can be done in parallel since the function being applied to the data should ONLY affect the data and NOT the global state. This is because an array_map could choose any order in which to apply the function to the items in (even though in PHP it doesn't).

array_walk on the other hand it the exact opposite approach to handling arrays of data. Instead of handling each item separately, it uses a state (&$userdata) and can edit the item in place (much like a foreach loop). Since each time an item has the $funcname applied to it, it could change the global state of the program and therefor requires a single correct way of processing the items.

Back in PHP land, array_map and array_walk are almost identical except array_walk gives you more control over the iteration of data and is normally used to "change" the data in-place vs returning a new "changed" array.

array_filter is really an application of array_walk (or array_reduce) and it more-or-less just provided for convenience.

90

Answer #3 (40 Votes)

08/08/2010 03:24:04

From the documentation,

bool array_walk ( array &$array , callback $funcname [, mixed $userdata ] ) <-return bool

array_walk takes an array and a function F and modifies it by replacing every element x with F(x).

array array_map ( callback $callback , array $arr1 [, array $... ] )<-return array

array_map does the exact same thing except that instead of modifying in-place it will return a new array with the transformed elements.

array array_filter ( array $input [, callback $callback ] )<-return array

array_filter with function F, instead of transforming the elements, will remove any elements for which F(x) is not true

40
3
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