How can I flip an array when each value is also an array?

Question

How would I go about flipping an array and establishing relationships between all the values and keys? For example:

I am trying to turn this:

Array ( 
[11913] => Array ( 
            [0] => 4242 
            [1] => 3981 
    ) 
[9878] => Array ( 
            [0] => 2901 
            [1] => 3981 
    ) 
[11506] => Array ( 
            [0] => 3981 
            [1] => 2901 
    ) 
 )

Into this:

Array ( 
    [3981] => Array ( 
            [0] => 11506 
            [1] => 9878 
            [2] => 11913 
        ) 
    [2901] => Array ( 
            [0] => 11506 
            [1] => 9878 
        ) 
    [4242] => Array ( 
            [0] => 11913 
        ) 
     )

Are there any PHP functions that will already do this automatically? If not what would be a way of going about this? Can't seem to wrap my head around it.

I think you're going to have to brute-force this approach. With any luck, I am wrong about this. — Brad
– Brad, Commented Feb 6, 2012 at 22:04
try some combination of array_walk and array_flip, or check out the multi_array_flip function given in the comments at php.net/manual/en/function.array-flip.php — Daan
– Daan, Commented Feb 6, 2012 at 22:06

Grexis · Accepted Answer · 2012-02-06 22:06:35Z

6

Here you go.

$final_array = array();
foreach($initial_array as $key => $val){
    foreach($val as $v){
        $final_array[$v][] = $key;
    }
}

answered Feb 6, 2012 at 22:06

Grexis

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3 Comments

bzx Over a year ago

the $val is an array in his $initial_array example, isn't it?

Grexis Over a year ago

$val are the second level arrays in the $initial_array, yes.

bzx Over a year ago

oh when you don't specify that in foreach it will just grab the value, as we don't need those keys.. i think i get it :)

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