Group subarrays based on column value

Question

I have an array:

[['a', 1], ['b', 1], ['c',2], ['d',2]]

How can I group the subarrays based on the second column value like this:

[[['a', 1], ['b', 1]], [['c',2], ['d',2]]]

I have an idea to solve this with a foreach, but it may have a way with the built-in functions?

Sketch with foreach:

$in = [['a', 1], ['b', 1], ['c',2], ['d',2]];
$out = [];

foreach($in as $i) {
    $out[$i[1]][] = $i;
}

[Here's the list of PHP built-in array functions](http://www.php.net/manual/en/ref.array.php) — Matthemattics, Apr 10 '12 at 16:19
@Lübnah About the list of built-in functions, I know. But in this task, I do not know how to use them. And is it possible at all. Can you rephrase your first question? Unfortunately, I do not understand. — Ticksy, Apr 10 '12 at 16:24
If the 2nd element in the inner arrays always refer to the index of the "column" it belongs to, then your solution is fine. I don't really think there's anything built in for this. — Kasapo, Apr 10 '12 at 16:25
@Ticksy Sorry, what I meant was, are you in control of the way that data is originally formatted? E.g., do you have control of the data source? — Matthemattics, Apr 10 '12 at 16:32
This is an array grouping problem, your approach is correct assuming you are not building `$in` (if you are, just build it grouped). See more @ http://stackoverflow.com/questions/2189626/php-how-to-group-a-multidimensional-array-by-a-particular-value — connec, Apr 10 '12 at 16:55

score 1 · Accepted Answer · answered Apr 10 '12 at 16:28

You more or less have what I would have arrived at for a solution.

As long as the data that you wish to use as an index will always be in the same position in the array, you will be able to hardcode the index as you have done in the collector array.

If not, I would think about moving to an associative array and assigning a name to the value that you want to use as an index when creating the array.

[['a', 'index'=>1] [etc...]]

$out[$i[index]][] = $i;

Is it necessary to preserve the value that you use as an index, incidentally? — Malovich, Apr 10 '12 at 16:30

score 0 · Answer 2 · answered Apr 04 '21 at 00:52

There isn't a native php function that will group on the column value of an indexed array of indexed arrays. If you were talking about associative arrays, the array_merge_recursive() might be of interest.

(Demo)

Basic loop syntax: (more concise, usually faster)

$result = [];
foreach ($array as $row) {
    $result[$row[1]][] = $row;
}
var_export($result);

Functional syntax: (more declarative, no new global variable declared)

var_export(
    array_reduce(
        $array,
        function ($carry, $row) {
            $carry[$row[1]][] = $row;
            return $carry;
        }
    )
);

If you wish to re-index the result (remove the group keys), then call array_values() on the result.

score -1 · Answer 3 · answered Apr 10 '12 at 16:30

The only PHP built-in that's specific to multi-dimensional arrays seems to be array_multisort, which won't be of much help unless there are assumptions we can make about the data contained within the input array.

In the case of a small dataset, your brute-force example seems fine to me.

Group subarrays based on column value

3 Answers3

Linked