PHP consecutive subtraction two arrays-CodePudding

I have 2 arrays like this

a = Array([101] => 5,[109] =>100,[220] => 50,[231] => 45,[245] => 90)
b = Array(['PRO'] => 12,['LOG'] => 15,['DEV'] => 100)

I want to consecutive subtract 2 array and result like this

c = Array([101] => 0,[109] => 0, [220] => 28, [231] => 45,  [245] => 90)

Explain: I assumed used foreach to get value of a and b

1st: 5-12=0(move 7 to next time) <=> c[101] => 0;
2nd: 100-7 = 93 (7 at 1st)
3rd : 93-15 = 78
4th: 78-100 = 0 (move 22 to 5th time) <=> c[109] => 0
5th: 50-22 = 28 <=> c[220] => 28

But I don't know how to express in PHP

CodePudding user response：

The following should work:

$rest = 0;
foreach ($a as &$value) {
    if ($value < $rest) {
        $rest -= $value;
        $value = 0;
        continue;
    } else {
        $value -= $rest;
        $rest = 0;
    }

    while ($value > 0 && key($b) !== null) {
        if ($value < current($b)) {
            $rest = current($b) - $value;
            $value = 0;
        } else {
            $value -= current($b);
            $rest = 0;
        }
    
        next($b);
    }
}

Basically, this loops over the first array and in each loop:

it first checks whether there was a rest value from the previous iteration, and lowers the value accordingly
next, it'll loop over the second array from where it left off the previous iteration of the outer loop and lower the value until it reaches 0 or when the second array is 'used up'.

A working example can be found on 3v4l.

CodePudding user response：

I'll try to rephrase your requirements, because I had to read your question about 10 times before I understood what you needed.

You want to reduce the values in $a (using subtraction) by consuming the values in $b in the order by which the associative elements occur. When a value in $b is large enough to reduce the value in $a to zero or lower, you want to set the $a value to zero and carry the remaining value (difference) to the subtraction of the next value in $a. This carried value must be consumed by one or more values in $a until it is depleted. When a carried value is depleted, then the next occurring value in $b should be used to reduce the current value in $a.

12($b[PRO]) is used to reduce 5($a[101]) to 0(new $a[101]) then 7 becomes the carried value and the next value in $a will be the new focus.
100($a[109]) is first reduce by 7(carried) and becomes 93(new $a[109).
93(new [109]) is then reduced again by 15($b[LOG]) and becomes 78(new [109]).
78(new [109]) is then reduced again by 100($b[DEV]) flooring the value to 0, making a new carried value of 22, and moving focus to $a[220].
50($a[220]) is reduced by 22(carried) making 28(new $a[220]).
At this point, the carried value is depleted AND there are no more values in $b to reduce $a values by.
$a[231] and $a[245] remain unchanged because there are no remaining values to subtract with.

My snippet uses text book arithmetic jargon: minuend - suprahend = difference.
The $difference is what is carried.
array_shift() is appropriate because it consumes the values in $b as they are accessed.

Code: (Demo)

$a = [
    101 => 5,
    109 => 100,
    220 => 50,
    231 => 45,
    245 => 90
];
$b = [
    'PRO' => 12,
    'LOG' => 15,
    'DEV' => 100
];

$difference = 0;
foreach ($a as &$minuend) {
    while ($minuend && ($b || $difference < 0)) {
        $suprahend = $difference < 0 ? -$difference : array_shift($b);
        $difference = $minuend - $suprahend;
        $minuend = max(0, $difference);
    }
}
var_export($a);

Output:

array (
  101 => 0,
  109 => 0,
  220 => 28,
  231 => 45,
  245 => 90,
)