Question

I have a loop that adds an array of two values to a main array.

How can I merge all of the arrays in the main array that have the same first values while at the same time add up the values of the second?

$mainData = array() ;
//Loop...

$cRes = $dbh->query("SELECT overdue FROM _credit_control_overdue WHERE entityID = $entityId") ;
        $currentOwed = $cRes->fetchColumn() ;
        $dbh->exec("REPLACE INTO _credit_control_overdue (entityID, overdue) VALUES ('$entityId', '$remaining')") ;
        $totalRemaining += $remaining ;

        array_push($mainData, array($entityId, $remaining)) ;
//End of loop

In many cases $entityId will be the same, and $remaining will be different.

Now I need a function similar to array_unique that will leave me with a unique $entityId but with all the $remaining values added up, so I am left with e.g. 2339, 83572.60.

Hope I have explained this clearly!

---

This is the output I desire:

Array ( [0] => Array ( [0] => 2499 [1] => 5314.50 ) [1] => Array ( [0] => 639 [1] => 75.00 ))

i.e Array ( [0] => UNIQUEID [1] => SUM )

Answer 1

The best way to do this is probably to first build an associative array (hash) using the $entityId as the keys (which will give you a unique entry for each one) and the accumulated totals of $remaining as the values:

// Initialise the hash before looping
$perEntityTotals = array();

/* Loop ... */

// Add the $remaining value to the appropriate entity's total
// If the key doesn't yet exist, it will be created with the value $remaining
$perEntityTotals[$entityId] += $remaining;

/* End Loop */

To re-format this as you originally asked for, you’ll then need to manipulate the hash:

$mainData = array();
foreach ( $perEntityTotals as $entityId => $total )
{
    $mainData[] = array($entityId, $total);
}

The below is equivalent, but somewhat harder to read:

$mainData = array_map(
    NULL,             // see Example #4 on http://php.net/array_merge
    array_keys($perEntityTotals),
    array_values($perEntityTotals) 
);