Merging two multidimensional arrays on specific key

Question

Let's say I have following arrays:

Array
    (
        [0] => Array
            (
                [id] => 5
                [name] => Education
            )

        [1] => Array
            (
                [id] => 4
                [name] => Computers
            )

        [3] => Array
            (
                [id] => 7
                [name] => Science

        [4] => Array
            (
                [id] => 1
                [name] => Sports
            )
    )

And the second one:

Array
    (
        [0] => Array
            (
                [id] => 1
                [title] => Sport
            )

        [1] => Array
            (
                [id] => 7
                [title] => Sci
            )

        [3] => Array
            (
                [id] => 4
                [title] => Comp

        [4] => Array
            (
                [id] => 5
                [title] => Edu
            )
    )

And desired output is:

Array
    (
        [0] => Array
            (
                [id] => 5
                [name] => Education
                [title] => Edu
            )

        [1] => Array
            (
                [id] => 4
                [name] => Computers
                [title] => Comp
            )

        [3] => Array
            (
                [id] => 7
                [name] => Science
                [title] => Sci

        [4] => Array
            (
                [id] => 1
                [name] => Sports
                [title] => Sport
            )
    )

I have managed to merge these arrays with simply:

foreach($first as $key => $value){
    $result[$key] = array_merge($first[$key], $second[$key]);
}

But the output is not combined correctly:

Array
    (
        [0] => Array
            (
                [id] => 5
                [name] => Education
                [title] => Sport
            )

        [1] => Array
            (
                [id] => 4
                [name] => Computers
                [title] => Sci
            )

        [3] => Array
            (
                [id] => 7
                [name] => Science
                [title] => Comp

        [4] => Array
            (
                [id] => 1
                [name] => Sports
                [title] => Edu
            )
    )

The problem is I would like to merge these arrays on the same id. Desired output sorting should be same as in the first array.

How can I achieve this? Any help is much appreciated.

Answer 1

This can be easily done using the 3-parameter form of array_column to re-index your second array by the id value and then looping over the first array, merging the contents of the matching second value (where it exists):

$second = array_column($second, null, 'id');
foreach ($first as &$subject) {
    $subject = array_merge($subject, $second[$subject['id']] ?? []);
}

Output:

Array
(
    [0] => Array
        (
            [id] => 5
            [name] => Education
            [title] => Edu
        )
    [1] => Array
        (
            [id] => 4
            [name] => Computers
            [title] => Comp
        )
    [3] => Array
        (
            [id] => 7
            [name] => Science
            [title] => Sci
        )
    [4] => Array
        (
            [id] => 1
            [name] => Sports
            [title] => Sport
        )
)

Demo on 3v4l.org

This has the advantage over Will B.'s answer of not re-indexing the first array. It's also slightly faster (my tests show ~10%) due to only having one call to array_column and none to array_values.

Update

This code can actually be sped up even more by using the array union operator (+) (thanks @mickmackusa):

$second = array_column($second, null, 'id');
foreach ($first as &$subject) {
    $subject += $second[$subject['id']] ?? [];
}

The output of this code is the same as above, but it runs about 10% faster again.

Demo on 3v4l.org

Answer 2

You can just do a nested loop and check if the id values match, then add title to $first (or name to $second)

foreach($first as $key => $value){
    foreach($second as $value2){
        if($value['id'] === $value2['id']){
            $first[$key]['title'] = $value2['title'];
        }               
    }
}

Answer 3

To provide an alternative approach available in PHP 5.5+.

Since the ordering of the two arrays is not identical, first use array_column to index the arrays by the id.

This will allow you to use array_replace_recusrive on the id indexed arrays.

array_replace_recursive will merge the values in the arrays matching the index association of both array sets.

Optionally use array_values to reindex the array, removing the id index association.

Example https://3v4l.org/ndv2j

$first = array_column($first, null, 'id');
$second = array_column($second, null, 'id');
$result = array_values(array_replace_recursive($first, $second));

Result

Array
(
    [0] => Array
        (
            [id] => 5
            [name] => Education
            [title] => Edu
        )

    [1] => Array
        (
            [id] => 4
            [name] => Computers
            [title] => Comp
        )

    [2] => Array
        (
            [id] => 7
            [name] => Science
            [title] => Sci
        )

    [3] => Array
        (
            [id] => 1
            [name] => Sports
            [title] => Sport
        )

)