PHP Array addition on matching elements

Question

I need a function that can add the elements of x arrays that have matching elements.

Here is an example of a source array with two elements.

Array
(
    [0] => Array
        (
            [0] => Array
                ([kwh] => 313.9799,[time_read] => 1311825600)

            [1] => Array
                ([kwh] => 312.3098,[time_read] => 1311826500)

            [2] => Array
                ([kwh] => 302.0525,[time_read] => 1311827400)

            [3] => Array
                ([kwh] => 312.2946,[time_read] => 1311828300)
        )
    [1] => Array
        (
            [0] => Array
                ([kwh] => 723.4205,[time_read] => 1311825600)

            [1] => Array
                ([kwh] => 686.9258,[time_read] => 1311826500)

            [2] => Array
                ([kwh] => 714.3203,[time_read] => 1311827400)

            [3] => Array
                ([kwh] => 707.8232,[time_read] => 1311828300)
        )
)

And I'd like to see output like the following. What throws me is that the indexes of the arrays might not match up. One array might have 10 elements while the other might have 20. The time_read value has to be used to find matching array elements.

    Array
        (
            [0] => Array
                ([kwh] => 1036,[time_read] => 1311825600)

            [1] => Array
                ([kwh] => 998,[time_read] => 1311826500)

            [2] => Array
                ([kwh] => 1016,[time_read] => 1311827400)

            [3] => Array
                ([kwh] => 1019,[time_read] => 1311828300)
        )

Would I be correct in assuming you're dealing with a smart meter? Just curious :) — Breakthrough
– Breakthrough, Commented Jul 28, 2011 at 16:32
Yep. Bought a TED5000 and hooked it up to my house meter. Instead of using a third party service I'm writing my own little php script for monitoring. — Chris
– Chris, Commented Jul 28, 2011 at 16:50
Google Power Meter thought of it a long time ago, but their service is discontinued and I need a hobby :P — Chris
– Chris, Commented Jul 28, 2011 at 18:54

Chris Baker · Accepted Answer · 2011-07-28 16:59:18Z

4

Use the timestamp as an array key, use nested loops to grab the internal values and add them up

$output = array();
foreach ($source_array as $group) {
  foreach ($group as $a) {
    if (!isset($output[$a['time_read']]))
      $output[$a['time_read']] = array('time_read'=>$a['time_read'], 'kwh'=>0);
    $output[$a['time_read']]['kwh'] += $a['kwh'];
  }
}

print_r($output);
/* output

Array
    (
        [1311825600] => Array
            ([kwh] => 1036,[time_read] => 1311825600)

        [1311826500] => Array
            ([kwh] => 998,[time_read] => 1311826500)

        [1311827400] => Array
            ([kwh] => 1016,[time_read] => 1311827400)

        [1311828300] => Array
            ([kwh] => 1019,[time_read] => 1311828300)
    ) */

Also nice because then you can use ksort to order them chronologically

edited Jul 28, 2011 at 16:59

answered Jul 28, 2011 at 16:35

Chris Baker

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