Inserting calculated values during a for each loop - php

Question

I have an array that looks like this.
Array
(
[0] => Array
(
[0] => 1
[1] => 500
[2] => 800
)
[1] => Array
(
[0] => 1
[1] => 100
[2] => 200
)
[2] => Array
(
[0] => 1
[1] => 300
[2] => 400
)
)
I want to use a foreach loop (not a function) to do several things. First I just want to subtract subelement [1] from subelement [2] to generate sub element [3] and then insert subelement3 back into the output array. See below:
$output=
Array
(
[0] => Array
(
[0] => 1
[1] => 500
[2] => 800
[3] => 300
)
[1] => Array
(
[0] => 1
[1] => 100
[2] => 200
[3] => 100
)
[2] => Array
(
[0] => 1
[1] => 400
[2] => 250
[3] => 150
)
)
To do the above I am using the for each loop below to generate the values of subelement 3 and it works ok. My problem is inserting the values back into the output array in the right place. I have commented out my last attempt which failed. Sorry if I am missing the obvious here.
foreach($result as $sub)
{
//get values
$sub[3]=$sub[1]-$sub[2];
echo "<difference>".$sub[3]."<br>";//works ok
//insert values back into array
//$result[$sub[0]][3] = $sub[3];
}
print "<pre>";
print_r($result);
print "</pre>";
die();

Answer 1

The problem:
$result[$sub[0]][3] = $sub[3]; //= $result[1][3] = $sub[3]
$sub[0] always return 1 (according to your input array) so it's not what you're looking for.
The solution:
Use the $array as $key => $val format in your foreach loop
and then access the sub array according to the relevant key.
foreach($result as $key => $sub)
{
//get values
$result[$key][3]=$sub[1]-$sub[2];

Answer 2

You can use array_map too, it's more elegant:
$output = aray_map( function ($item) {
$item[] = $sub[2] - $sub[1];
return $item;
}, $arr);
Demo