Calculate average percentage difference in a php array

Question

Array
(
    [0] => 1
    [1] => 2
    [2] => 3
    [3] => 4
    [4] => 5
    [5] => 3
    [6] => 1
)

I was wondering how one would go about working out the average percentage difference between the current value in an array and the next value. If the next value were a larger one, it would perform like so. (ie keys [0]-[1] 1/2 * 100 = 50). If it were a smaller value it would perform like so. (ie keys [4]-[5] = 3/5 * 100 = -60).

The following will represent what I am aiming to do with these percentage calculations.

1/2 * 100

2/3 * 100

3/4 * 100

4/5 * 100

3/5 * 100 (negative)

1/3 * 100 (negative)

---

Total : total/count

This will iterate through the list and then work out the average from the count. I have looked into splitting arrays but don't see how else I could do this.

$count = count($num); 
foreach ($num as $value) {
    array_chunk($num, 1);
    if($value<$value){
    $total1 = $total1 + ($value/$value)*100;
    }
    if($value>$value){
    $total2 = $total2 + ($value/$value)*100;
    }
}
$average = (($total1-$total2)/$count);
print($average);

I understand the above code is incorrect, but I hope it reveals where I am getting at with this.

Any help would be greatly appreciated.

Answer 1

You don't want to use foreach as you'll always be needing two array elements. Note that this snippet does not protect you from 0 values. These will make your script fail.

$num   = array(1, 2, 3, 4, 5, 3, 1); 
$total = 0;
// The number of percent changes is one less than
// the size of your array.
$count = count($num) - 1;
// Array indexes start at 0
for ($i = 0; $i < $count; $i++) {
    // The current number is $num[$i], and the
    // next is $num[$i + 1]; knowing that it's
    // pretty easy to compare them.
    if ($num[$i] < $num[$i + 1]) {
        $total += (100 * $num[$i] / $num[$i + 1]);
    }   
    else {
        $total += (-100 * $num[$i + 1] / $num[$i]);
    };  
};
echo ($total / $count);