php create a third array with an extra value where the keys match from two existing arrays

Question

I have two arrays:

$graph1 = Array ( 
[0] => Array ( [0] => 202101 [1] => 2 ) 
[1] => Array ( [0] => 202102 [1] => 3 ) 
[2] => Array ( [0] => 202103 [1] => 5 ) 
[3] => Array ( [0] => 202104 [1] => 2 ) 
[4] => Array ( [0] => 202105 [1] => 3 ) 
[5] => Array ( [0] => 202106 [1] => 4 ) 
[6] => Array ( [0] => 202107 [1] => 14 ) )

$graph2 = Array ( 
[0] => Array ( [0] => 202105 [1] => 3 ) 
[1] => Array ( [0] => 202107 [1] => 1 ) 
[2] => Array ( [0] => 202108 [1] => 6 ) 
)

and want to create a third array with an extra value where value[0] matches, and an extra row and extra value if there is no matching value[0]

I have tried array_merge and array_push but my understanding of arrays is not up to the task The output I want is like this:

$graph3 = Array ( 
[0] => Array ( [0] => 202101 [1] => 2  [2] => 0 ) 
[1] => Array ( [0] => 202102 [1] => 3  [2] => 0 ) 
[2] => Array ( [0] => 202103 [1] => 5  [2] => 0 ) 
[3] => Array ( [0] => 202104 [1] => 2  [2] => 0 ) 
[4] => Array ( [0] => 202105 [1] => 3  [2] => 3 ) 
[5] => Array ( [0] => 202106 [1] => 4  [2] => 0 ) 
[6] => Array ( [0] => 202107 [1] => 14 [2] => 1 )
[7] => Array ( [0] => 202108 [1] => 0  [2] => 6 )  )

Answers and explanation would be great - thank you

You need to use foreach loop along with array merge. Do you know array_intersect()? — mighty_dev
– mighty_dev, Commented Aug 18, 2021 at 9:17
Does this answer your question? PHP merge arrays by value and merge it's sub arrays — mighty_dev
– mighty_dev, Commented Aug 18, 2021 at 9:19
thank you @nice_dev - I did try this but couldn't work out how to have it add the 3rd value to the array - please see answer below which does that — paul724
– paul724, Commented Aug 18, 2021 at 10:51

Levi Cole · Accepted Answer · 2021-08-18 09:55:33Z

This function should do the trick...

function combine_graphs( ...$arrays ) {
    $output = [];

    // First create keyed array
    foreach ( $arrays as $row ) {
        foreach ( $row as $columns ) {
            $key = $columns[ 0 ];
            if ( !array_key_exists( $key, $output ) ) {
                $output[ $key ] = [ $key ];
            }
        }
    }

    // Now add values
    foreach ( $output as $key => $values ) {
        foreach ( $arrays as $row ) {
            $index = array_search( $key, array_column( $row, 0 ) );
            $output[ $key ][] = $index !== false ? $row[ $index ][ 1 ] : 0;
        }
    }

    return array_values( $output );
}

Usage would be...

$graph1 = [
    [ 202101, 2 ],
    [ 202102, 3 ],
    [ 202103, 5 ],
    [ 202104, 2 ],
    [ 202105, 3 ],
    [ 202106, 4 ],
    [ 202107, 14 ],
];

$graph2 = [
    [ 202105, 3 ],
    [ 202107, 1 ],
    [ 202108, 6 ],
];

$graph3 = combine_graphs( $graph1, $graph2 );

The function allows any number of graphs to be combined for example...

$graph1 = [
    [ 202101, 2 ],
    [ 202102, 3 ],
    [ 202103, 5 ],
    [ 202104, 2 ],
    [ 202105, 3 ],
    [ 202106, 4 ],
    [ 202107, 14 ],
];

$graph2 = [
    [ 202105, 3 ],
    [ 202107, 1 ],
    [ 202108, 6 ],
];

$graph3 = [
    [ 202105, 3 ],
    [ 202107, 4 ],
    [ 202108, 9 ],
    [ 202107, 12 ],
    [ 202104, 2 ],
];

$graph4 = combine_graphs( $graph1, $graph2, $graph3 );

The above would output...

Array
(
    [0] => Array
        (
            [0] => 202101
            [1] => 2
            [2] => 0
            [3] => 0
        )

    [1] => Array
        (
            [0] => 202102
            [1] => 3
            [2] => 0
            [3] => 0
        )

    [2] => Array
        (
            [0] => 202103
            [1] => 5
            [2] => 0
            [3] => 0
        )

    [3] => Array
        (
            [0] => 202104
            [1] => 2
            [2] => 0
            [3] => 2
        )

    [4] => Array
        (
            [0] => 202105
            [1] => 3
            [2] => 3
            [3] => 3
        )

    [5] => Array
        (
            [0] => 202106
            [1] => 4
            [2] => 0
            [3] => 0
        )

    [6] => Array
        (
            [0] => 202107
            [1] => 14
            [2] => 1
            [3] => 4
        )

    [7] => Array
        (
            [0] => 202108
            [1] => 0
            [2] => 6
            [3] => 9
        )

)

sorry for mis-typing @LeviCole - thank you - exactly and precisely the result needed and great that the function will combine many arrays - appreciated

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