Computing differences in PHP array

Calculating first order differences

[technossomy]

Hi all

I frequently perform difference calculations on arrays. Example, I have

$a = array(1, 8, 5, 7, 12);

and need

$b = array(7, -3, 2, 5);

What would be the fastest PHP implementation of this for arrays having several 1000s of just floating numbers?

Thanks in advance

Tech

[sned]

PHP has a lot of built in array [us3.php.net] functions .. I'm not sure which one you would use in your case - perhaps array_walk or array_map.

-sned

[ChadSEO]

Tech,

The following should do that for you:

for($i=1;$i<count($a);$i++) {
$b[$i-1] = $a[$i] - $a[$i-1];
}

Chad

[dmorison]

$a = array(1, 8, 5, 7, 12); 
foreach($a as $n) 
{ 
 if ($p) 
 { 
 $b[] = ($n - $p); 
 } 
 $p = $n; 
} 
print_r($b); 

...is the logical solution (off the top of my head, not tested).

One optimisation is that with 1000's of elements you could leave out the test for a previous value and then instead shift element 1 off the output array when you've finished...

$a = array(1, 8, 5, 7, 12); 
$p = 0; 
foreach($a as $n) 
{ 
 $b[] = ($n - $p); 
 $p = $n; 
} 
array_shift($b); 
print_r($b); 

[technossomy]

Thanks all for your swift responses and I will try your suggestions.

Tech

[dmorison]

If performance is key you could have a look at profiling the various options using the microtime() [uk.php.net] function.

[dmorison]

Regarding the following from Chad's solution...

for($i=1;$i<count($a);$i++)

..what are the optimisation implications of this? Is count($a) evaluated every itteration, and if so, is it an expensive operation?

If it is you could of course evaluate it once and use a variable instead, but i'm just curious - i've wondered about this before.

[ChadSEO]

Good call dmorison with only getting the count once, that would definitely help.

I tested each of the 3 methods listed here, in the order listed, on an array of 300,000 items. Each test was done 10 times; here are the times it took for each:

Average / Min / Max times
Test 1: 5.206/ 5.011/ 5.632
Test 2: 6.656/ 4.928/ 12.447
Test 3: 5.634/ 4.804/ 12.028

Chad