array_merge is useful - but with a caveat

October 7th, 2008 by Aaron

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So, the other day, I saw a horrible thing. I saw two PHP associative arrays that needed to be combined into one, and the worst example of NOT using PHP’s built in functions to combine them. They weren’t using array_merge - instead they were looping through each value.

That’s what I thought until I did some testing. There is a legitimate difference in the looping method vs the array_merge method. This could be by design in your application, so don’t get over-eager optimizing. Lets take a look:

Example Arrays

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$ar1 = array('a'=>'ay', 'b'=>'bee', 'c'=>'see');
$ar2 = array('d'=>'dee', 'e'=>'ee', 'f'=>'ef');

Well, first off, lets try my way - with array_merge:

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$ar2 = array_merge($ar1, $ar2);
var_dump($ar2);

array(6) { ["a"]=>  string(2) "ay" ["b"]=>  string(3) "bee"
["c"]=>  string(3) "see" ["d"]=>  string(3) "dee"
["e"]=>  string(2) "ee" ["f"]=>  string(2) "ef" }

Ok - decent. Now lets try it their way:

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foreach ($ar1 as $k=>$v) {
    $ar2[$k]=$v;
}
var_dump($ar2);

array(6) { ["d"]=>  string(3) "dee" ["e"]=>  string(2) "ee"
["f"]=>  string(2) "ef" ["a"]=>  string(2) "ay"
["b"]=>  string(3) "bee" ["c"]=>  string(3) "see" }

The array is in a different order.

Special Thanks to Sjan and James for commenting on my original version of this story and explaining that I was totally running in circles!

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Tags: PHP

This entry was posted on Tuesday, October 7th, 2008 at 2:58 pm and is filed under PHP. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.

3 Responses to “array_merge is useful - but with a caveat”

1. Sjan Evardsson Says:
   October 7th, 2008 at 3:05 pm

   Or, by changing the merge order from

   $ar2 = array_merge($ar1, $ar2);

   to
   $ar2 = array_merge($ar2, $ar1);

   you can come up with the same result.

2. James Rodenkirch Says:
   October 7th, 2008 at 3:07 pm

   From the PHP docs:
   “Merges the elements of one or more arrays together so that the values of one are appended to the end of the previous one. It returns the resulting array. ”

   So, if you want the second array first, why wouldn’t you just do array_merge($ar2, $ar1)?

3. Aaron Says:
   October 7th, 2008 at 3:34 pm

   Thanks guys for the comments! What you suggested would also get the desired effect.

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