Append column values from a 2d array to the rows of another 2d array based on shared column values

Question

Let's say I have following arrays:

$first = [
    ['id' => 5, 'name' => 'Education'],
    ['id' => 4, 'name' => 'Computers'],
    ['id' => 7, 'name' => 'Science'],
    ['id' => 1, 'name' => 'Sports'],
];

$second = [
    ['id' => 1, 'title' => 'Sport'],
    ['id' => 7, 'title' => 'Sci'],
    ['id' => 4, 'title' => 'Comp'],
    ['id' => 5, 'title' => 'Edu'],
];

And desired output is:

[
    ['id' => 5, 'name' => 'Education', 'title' => 'Edu'],
    ['id' => 4, 'name' => 'Computers', 'title' => 'Comp'],
    ['id' => 7, 'name' => 'Science', 'title' => 'Sci'],
    ['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 combined by first level index instead of their id value:

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
            )
    )

I would like to append the title values from the second array to the first array where there is a matching id value and maintain the sorting of the first array.

How can I achieve this?

Answer 1

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 2

You code works fine. Your expectations are simply incorrect. For example in one array 4th element id holds 1 but in another array, 4th element id is 5, so your "merge these arrays on the same id" makes no sense as by merging 4th elements into one you also merge their children, and since id is used in both arrays, once value HAVE TO be gone as there cannot be two equal keys in array.

EDIT

you have to merge manually as PHP functions merge based on keys while you want to merge based on content:

$result = array();
foreach( $arrayA as $keyA => $valA ) {
  foreach( $arrayB as $keyB => $valB ) {
     if( $valA['id'] == $valB['id'] ) {
       $result[$keyA] = $valA + $valB;

       // or if you do not care output keys, just
       // $result[] = $valA + $valB;
     }
  }
}

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
        )

)

Answer 4

Do not use foreach in foreach,that might be too slow when the array so big.

$idArray = array_column($secondArray,'title','id');
foreach($firstArray as $key => $val){
  $firstArray[$key]['title'] = (isset($idArray[$val['id']])) ? $idArray[$val['id']] : 'some title';
}

Answer 5

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 6

Sort both arrays by your 'id' field, then let php do the merge with array_replace_recursive.

function cmp($a, $b) {
  return ((int) $a['id'] < (int) $b['id']) ? -1 : 1;
}

usort($array1, 'cmp');
usort($array2, 'cmp');

$result = array_replace_recursive($array1, $array2);

Answer 7

Make sure that the items are in the same order then:

$items = array_map(function($itemFirstArray, $itemSecondArray) {
  return array_merge($itemFirstArray, $itemSecondArray);
}, $firstArray, $secondArray);