Merge array according to key in PHP

Question

I've got a huge list of arrays with 2 members which looks like that:

<?php
$aData = array ();
$aData[0] = array ( 14400, 'Item1' );
$aData[1] = array ( 14400, 'Item4' );
$aData[2] = array ( 14401, 'Item2' );
$aData[3] = array ( 14402, 'Item4' );
$aData[4] = array ( 14402, 'Item5' );
$aData[5] = array ( 14402, 'Item9' );

and I'd like to convert this into arrays like that:

$aData[0] = array ( 14400, 'Item1,Item4' );
$aData[1] = array ( 14401, 'Item4' );
$aData[2] = array ( 14402, 'Item4,Item5,Item9' );

Whereas the 2nd one could be a string or an array. Now I was trying to implement some logic - but is there a function that could do that at least partly? array_map or array_merge_recursive don't seem to fit exactly?

Answer 1

Use a Map of (int -> string list)

for each (key,value) pair
    map.get(key).add(value)

What language are you using?

Answer 2

Because you don't know what you you want. You can easy receive such result'

$arr = [
[14400,Item1],
[14400,Item4],
[14401,Item2],
[14402,Item4],
[14402,Item5],
[14402,Item9]
];

$new = [];
foreach($arr as $item) 
  $new[$item[0]][] = $item[1];

print_r($new);

result

Array
(
[14400] => Array
        (
        [0] => Item1
        [1] => Item4
    )

[14401] => Array
    (
        [0] => Item2
    )

[14402] => Array
    (
        [0] => Item4
        [1] => Item5
        [2] => Item9
    )
)