Updating a value in a multidimensionnal array regarding another value and using array_walk (PHP)

Question

I would like to update the values of a multidimensionnal php array : $a

array(1) { 
["test"]=> array(4) { 
[0]=> int(5) 
[1]=> int(7) 
[2]=> int(10) 
[3]=> int(14) 
} 
}

For $a["test"][$i], I would like to obtain this new value $a["test"][$i] - $a["test"][$i-1].

In this case the resulting array would be :

array(1) { 
["test"]=> array(4) { 
[0]=> int(5) 
[1]=> int(2) 
[2]=> int(3) 
[3]=> int(4) 
} 
}

Of course, I could do it with a foreach loop and a new array

$new = array();
foreach($a as $k=>$v){
for($i=0;$i<=3;$i++){
$new[$k][$i] = $v[$i] - $v[$i-1];
}
}
$a = $new;
var_dump($a);

but is this the best way? Just out of curiosity, I was wondering if using array_walk to make it could be nicer and, generally, how/if array_walk could access the previous/next value in the array.

Thank you a lot !

When using `array_walk` you can use a closure like `function($a)use(&$carray){}` to carray along prev./needed data — JustOnUnderMillions, Apr 10 '17 at 15:24

score 0 · Answer 1 · answered Apr 10 '17 at 15:35

PHP code demo

<?php

$array = array(
    "test" => array(5, 7, 10, 14)
);
$new=array();
array_walk($array["test"], function($value,$key) use (&$new,&$array){
    if($key>0)
    {
        $new["test"][]=$array["test"][$key]-$array["test"][$key-1];
    }
    else
    {
        $new["test"][]=$value;
    }
});
print_r($new);

Or:

PHP code demo

<?php
$array = array(
    "test" => array(5, 7, 10, 14)
);
$newArray=array();
foreach($array["test"] as $key => $value)
{
    if($key>0)
    {
        $newArray["test"][]=$array["test"][$key]-$array["test"][$key-1];
    }
    else
    {
        $newArray["test"][]=$value;
    }
}
print_r($newArray);

score 0 · Answer 2 · answered Apr 10 '17 at 15:43

Since you're overwriting the original array with the new values in the end anyway, you might as well just change the values of the original array as you go.

A custom function can do the successive subtraction for each subset.

function successive_subtraction(&$array) {
    $subtrahend = 0;               // store the previous value for subtraction
    foreach ($array as &$value) {
        $temp = $value;            // store the current value in a temporary variable
        $value -= $subtrahend;     // subtract the previous value
        $subtrahend = $temp;       // original current value become new previous value
    }
}

Then apply that function to each element of the outer array with array_walk.

array_walk($a, 'successive_subtraction');

This code is actually more complex that the way you're already doing it. The main advantage to modifying the existing array rather than creating a modified copy and then overwriting it would be less peak memory usage, but if the arrays you're dealing with aren't very large, then that shouldn't really be a problem, and I would say just keep it simple.

Thank you very much ! It's really interesting. – user1719210 Apr 10 '17 at 20:14 — user1719210, Apr 10 '17 at 20:14

Updating a value in a multidimensionnal array regarding another value and using array_walk (PHP)

2 Answers2