How can I efficiently split an array into its associative key arrays? - php

Question

How can I split a single array into it's sub-keys?
$arr = array(
0 => array(
'foo' => '1',
'bar' => 'A'
),
1 => array(
'foo' => '2',
'bar' => 'B'
),
2 => array(
'foo' => '3',
'bar' => 'C'
)
);
What is the most efficient way to return an array of foo and bar separately?
I need to get here:
$foo = array('1','2','3');
$bar = array('A','B','C');
I'm hoping there's a clever way to do this using array_map or something similar. Any ideas?
Or do I have to loop through and build each array that way? Something like:
foreach ($arr as $v) {
$foo[] = $v['foo'];
$bar[] = $v['bar'];
}

Answer 1

In a lucky coincidence, I needed to do almost the exact same thing earlier today. You can use array_map() in combination with array_shift():
$foo = array_map('array_shift', &$arr);
$bar = array_map('array_shift', &$arr);
Note that $arr is passed by reference! If you don't do that, then each time it would return the contents of $arr[<index>]['foo']. However, again because of the reference - you won't be able to reuse $arr, so if you need to do that - copy it first.
The downside is that your array keys need to be ordered in the same way as in your example, because array_shift() doesn't actually know what the key is. It will NOT work on the following array:
$arr = array(
0 => array(
'foo' => '1',
'bar' => 'A'
),
1 => array(
'bar' => 'B',
'foo' => '2'
),
2 => array(
'foo' => '3',
'bar' => 'C'
)
);
Update:
After reading the comments, it became evident that my solution triggers E_DEPRECATED warnings for call-time-pass-by-reference. Here's the suggested (and accepted as an answer) alternative by #Baba, which takes advantage of the two needed keys being the first and last elements of the second-dimension arrays:
$foo = array_map('array_shift', $arr);
$bar = array_map('array_pop', $arr);

Answer 2

$n = array();
foreach($arr as $key=>$val) {
foreach($val as $k=>$v) {
$n[$k][] = $v;
}
}

Answer 3

array_merge_recursive will combine scalar values with the same key into an array. e.g.:
array_merge_recursive(array('a',1), array('b',2)) === array(array('a','b'),array(1,2));
You can use this property to simply apply array_merge_recursive over each array in your array as a separate argument:
call_user_func_array('array_merge_recursive', $arr);
You will get this result:
array (
'foo' =>
array (
0 => '1',
1 => '2',
2 => '3',
),
'bar' =>
array (
0 => 'A',
1 => 'B',
2 => 'C',
),
)
It won't even be confused by keys in different order.
However, every merged value must be scalar! Arrays will be merged instead of added as a sub-array:
array_merge_recursive(array(1), array(array(2)) === array(array(1,2))
It does not produce array(array(1, array(2)))!