PHP running average of multiple array value differences - php

Question

First, I have been programming in PHP for all of two weeks. So, please excuse my ignorance.
Here is my problem. I am trying to find essentially the running average of multiple difference values within an array. I think I have figured out how to get what I want, but the code is really inefficient and takes several seconds in a small test array (n = 20). My production array will be around n = 1000. I suspect that there is a much more efficient way to get what I want.
Here is what I am conceptually trying to do:
array = (20, 19, 18, 17, 16, 15, 14, 13, 12, 11, 10, 9, 8, 7, 6, 5, 4, 3, 2, 1);
newarray = (20-19, 19-18, 18-17, 17-16, 16-15, ... 2-1);
newarray2 = (20-18, 19-17, 18-16, 17-15, 16-14 ... 3-1);
newarray3 = (20-17, 19-16, 18-15, 17-14, ... 4-1);
differenceaverage = (sum of (20-19, 20-18, 20-17)/3, sum of (19-18, 19-17, 19-16)/3, etc.)
Here is the code that I used to generate the average:
$n=count($appArrayqb);
for($i = 1; $i<=($n-1); $i++){
$diff[]= $appArrayqb[$i-1]-$appArrayqb[$i];
}
for($i = 1; $i<=($n-2); $i++){
$diff2[]= $appArrayqb[$i-1]-$appArrayqb[$i+1];
}
for($i = 1; $i<=($n-3); $i++){
$diff3[]= $appArrayqb[$i-1]-$appArrayqb[$i+2];
}
$VOcomb = array($diff,$diff2,$diff3)
$sumVO = array_map ("sum", $VOcomb[0], $VOcomb[1], $VOcomb[2])
function sum ($arr1, $arr2, $arr3){
return($arr1+$arr2+$arr3);
}
$counts = array();
foreach($VOcomb as $item){
foreach($item as $k=>$v){
if(isset($item[$k])){
$counts[$k]++;
}
}
}
$VOarray=array($sumVO,$counts);
$VO = array_map("VO", $VOarray[0],$VOarray[1]);
function VO($arr1, $arr2) {
return($arr1/$arr2);
}
Incidentally, while the code works, I get an undefined offset notice when I run the count loop because it is trying to count values that don't exist in $item array. So I have three related questions:
1.) Is there a better way to calculate the differences?
2.) Is there a better way to get the count of the differences so I don't wind up with an undefined offset?
3.) Is the array_map function the best function to use to get the sums and averages?
Thanks!

Answer 1

Try this simplified code that seems to give the same result as yours:
It works by building the result array element by element, calculating the differences and averages as needed instead of creating arrays of differences and mapping over them.
// Calculate average differences between E(n) and E(n+1), E(n+2), and E(n+3)
// in an array
function diffaverages($appArrayqb) {
$n = count($appArrayqb);
$diffAvg = array();
for($i = 0; $i+1 < $n; $i++){
$diffsum = 0;
$count = 0;
for($j = 1; $j <= 3 && $i+$j < $n; $j++){
$diffsum += $appArrayqb[$i] - $appArrayqb[$i+$j];
$count++;
}
$diffAvg[] = $diffsum / $count;
}
return $diffAvg;
}