Does php have flip function?

(PHP 4, PHP 5, PHP 7, PHP 8)

array_flip — Exchanges all keys with their associated values in an array

Description

array_flip(array $array): array

Note that the values of array need to be valid keys, i.e. they need to be either int or string. A warning will be emitted if a value has the wrong type, and the key/value pair in question will not be included in the result.

If a value has several occurrences, the latest key will be used as its value, and all others will be lost.

Parameters

An array of key/value pairs to be flipped.

Return Values

Returns the flipped array.

Examples

Example #1 array_flip() example

$input = array("oranges", "apples", "pears");
$flipped = array_flip($input);print_r($flipped);
?>

The above example will output:

Array
(
    [oranges] => 0
    [apples] => 1
    [pears] => 2
)

Example #2 array_flip() example : collision

$input = array("a" => 1, "b" => 1, "c" => 2);
$flipped = array_flip($input);print_r($flipped);
?>

The above example will output:

Array
(
    [1] => b
    [2] => c
)

See Also

• array_values() - Return all the values of an array
• array_keys() - Return all the keys or a subset of the keys of an array
• array_reverse() - Return an array with elements in reverse order

Final ¶

10 years ago

I find this function vey useful when you have a big array and you want to know if a given value is in the array. in_array in fact becomes quite slow in such a case, but you can flip the big array and then use isset to obtain the same result in a much faster way.

Tony H ¶

9 years ago

This function is useful when parsing a CSV file with a heading column, but the columns might vary in order or presence:

$f

Bob Ray ¶

5 years ago

array_flip will remove duplicate values in the original array when you flip either an associative or numeric array. As you might expect it's the earlier of two duplicates that is lost:

    $a = array('one', 'two', 'one');
    print_r($a); $b = array_flip($a);
    print_r($b);
?>

Result:

array(3) {
  [0]  => string(3) "one"
  [1]  =>  string(3) "two"
  [2]  =>  string(3) "one"
}

array(2) {
  'one' => int(2)
  'two' => int(1)
}

This may be good or bad, depending on what you want, but no error is thrown.

snaury at narod dot ru ¶

17 years ago

When you do array_flip, it takes the last key accurence for each value, but be aware that keys order in flipped array will be in the order, values were first seen in original array. For example, array:

    [1] => 1
    [2] => 2
    [3] => 3
    [4] => 3
    [5] => 2
    [6] => 1
    [7] => 1
    [8] => 3
    [9] => 3

After flipping will become:
(first seen value -> first key)

    [1] => 7
    [2] => 5
    [3] => 9

And not anything like this:
(last seen value -> last key)

    [2] => 5
    [1] => 7
    [3] => 9

In my application I needed to find five most recently commented entries. I had a sorted comment-id => entry-id array, and what popped in my mind is just do array_flip($array), and I thought I now would have last five entries in the array as most recently commented entry => comment pairs. In fact it wasn't (see above, as it is the order of values used). To achieve what I need I came up with the following (in case someone will need to do something like that):

First, we need a way to flip an array, taking the first encountered key for each of values in array. You can do it with:

  $array = array_flip(array_unique($array));

Well, and to achieve that "last comments" effect, just do:

  $array = array_reverse($array, true);
  $array = array_flip(array_unique($array));
  $array = array_reverse($array, true);

In the example from the very beginning array will become:

    [2] => 5
    [1] => 7
    [3] => 9

Just what I (and maybe you?) need. =^_^=

Prabhas Gupte ¶

7 years ago

array_flip() does not retain the data type of values, when converting them into keys. :(

$arr = array('one' => '1', 'two' => '2', 'three' => '3');
var_dump($arr);
$arr2 = array_flip($arr);
var_dump($arr2);
?>

This code outputs this:
array(3) {
  ["one"]=>
  string(1) "1"
  ["two"]=>
  string(1) "2"
  ["three"]=>
  string(1) "3"
}
array(3) {
  [1]=>
  string(3) "one"
  [2]=>
  string(3) "two"
  [3]=>
  string(5) "three"
}

It is valid expectation that string values "1", "2" and "3" would become string keys "1", "2" and "3".

pinkgothic at gmail dot com ¶

15 years ago

In case anyone is wondering how array_flip() treats empty arrays:

print_r(array_flip(array()));
?>

results in:

Array
(
)

I wanted to know if it would return false and/or even chuck out an error if there were no key-value pairs to flip, despite being non-intuitive if that were the case. But (of course) everything works as expected. Just a head's up for the paranoid.

kjensen at iaff106 dot com ¶

10 years ago

I needed a way to flip a multidimensional array and came up with this function to accomplish the task.  I hope it helps someone else.

function multi_array_flip($arrayIn, $DesiredKey, $DesiredKey2=false, $OrigKeyName=false) {
$ArrayOut=array();
foreach ($arrayIn as $Key=>$Value)
    {
        // If there is an original key that need to be preserved as data in the new array then do that if requested ($OrigKeyName=true)
        if ($OrigKeyName) $Value[$OrigKeyName]=$Key;
        // Require a string value in the data part of the array that is keyed to $DesiredKey
        if (!is_string($Value[$DesiredKey])) return false; // If $DesiredKey2 was specified then assume a multidimensional array is desired and build it
        if (is_string($DesiredKey2))
        {
            // Require a string value in the data part of the array that is keyed to $DesiredKey2
            if (!is_string($Value[$DesiredKey2])) return false; // Build NEW multidimensional array
            $ArrayOut[$Value[$DesiredKey]][$Value[$DesiredKey2]]=$Value;
        } // Build NEW single dimention array
        else $ArrayOut[$Value[$DesiredKey]][]=$Value;
    }
return $ArrayOut;
}//end multi_array_flip
?>

mmulej at gmail dot com ¶

11 months ago

If you don't want to lose duplicates, and you're ok, with having the values in the flipped array in an array as well, you may use this:

PHP 7.4 - ^8

function array_flip_safe(array $array) : array
{
    return array_reduce(array_keys($array), function ($carry, $key) use (&$array) {
        $carry[$array[$key]] ??= [];
        $carry[$array[$key]][] = $key;
        return $carry;
    }, []);
}?>

PHP 7.0 - ^7.3 (Time to upgrade to PHP 8 ^^)

function array_flip_safe(array $array) : array
{
    return array_reduce(array_keys($array), function ($carry, $key) use (&$array) {
        $carry[$array[$key]] = $carry[$array[$key]] ?? [];
        $carry[$array[$key]][] = $key;
        return $carry;
    }, []);
}?>

PHP 5.4 - ^5.6 (Just don't)

function array_flip_safe(array $array)
{
    return array_reduce(array_keys($array), function ($carry, $key) use (&$array) {
        if (!isset($carry[$array[$key]])
                $carry[$array[$key]] = [];
        $carry[$array[$key]][] = $key;
        return $carry;
    }, []);
}?>

Hayley Watson ¶

4 years ago

Don't use this function for filtering or searching an array - PHP already has functions for exactly those purposes. If nothing else, array_flip will trash the array's elements if they're anything other than integers or non-decimal-integer strings.

Ahammar Yassine ¶

3 years ago

$arr

Anonymous ¶

10 years ago

Similarly, if you want the last value without affecting the pointer, you can do:

$array

Hayley Watson ¶

13 years ago

Finding the longest string in an array?

function longest_string_in_array($array)
{
    $mapping = array_combine($array, array_map('strlen', $array));
    return array_keys($mapping, max($mapping));
}
?>

Differences are obvious: returns an array of [i]all[/i] of the longest strings, instead of just picking one arbitrarily. Doesn't do the stripslashing or magic stuff because that's another job for for another function.

dan at aoindustries dot com ¶

13 years ago

From an algorithmic efficiency standpoint, building an entire array of lengths to then sort to only retrieve the longest value is unnecessary work.  The following should be O(n) instead of O(n log n).  It could also be:

function get_longest_value($array) {
    // Some don't like to initialize, I do
    $longest = NULL;
    $longestLen = -1;
    foreach ($array $value) {
        $len = strlen($value);
        if($len>$longestLen) {
            $longest = $value;
            $longestLen = $len;
        }
    }
    $longest = str_replace("\r\n", "\n", $longest);
    if (get_magic_quotes_gpc()) { return stripslashes($longest); }
    return $longest;
}
?>

dash ¶

3 years ago

Notice : array_flip can turn string into integer

znailz at yahoo dot com ¶

19 years ago

I know a lot of people want a function to remove a key by value from an array. I saw solutions that iterate(!) though the whole array comparing value by value and then unsetting that value's key. PHP has a built-in function for pretty much everything (heard it will even cook you breakfast), so if you think "wouldn't it be cool if PHP had a function to do that...", odds are it already has. Check out this example. It takes a value, gets all keys for that value if it has duplicates, unsets them all, and returns a reindexed array.

$arr = array(11,12,13,12);        // sample array
$arr = array_flip($arr);
unset($arr[12]);
$arr = array(array_keys($arr));
?>

$arr contains:

Array
(
    [0] => Array
        (
            [0] => 11
            [1] => 13
        )
?>

)

info at sabastore dot net ¶

6 years ago

note :: array_flip is a changer for key and value and a auto unique like array_unique :

/*
sabastore
*/
$intArray1 = array(-4,1,1,3);
print_r($intArray1);
$intArray1 = array_flip($intArray1);
print_r($intArray1);
?>

grimdestripador at hotmail dot com ¶

8 years ago

function array_flip_into_subarray($input){
$output = array();
foreach ($input as $key=>$values){
    foreach ($values as $value){
        $output[$value][] = $key;
    }
}
return $output;
}

h3x ¶

12 years ago

this function can be used to remove null elements form an array:

$ar = array(null,'1','2',null,'3',null);
print_r($ar);
/*
result:
Array
(
    [0] =>
    [1] => 1
    [2] => 2
    [3] =>
    [4] => 3
    [5] =>
)
*/print_r(array_flip(array_flip($ar)));
/*
result:
Array
(
    [1] => 1
    [2] => 2
    [4] => 3
)
*/
?>

corz at corz dot org ¶

13 years ago

/*
    Fun function to return the longest physical *value* from an array.

    Culled from a small script designed to capture the longest $_POST variable,
    usually the textarea, which would then be dumped to a "emergency post dump file".

    corz at corz dot org
*/

Longest value: ',
                                            get_longest_value($array),'