Subtract row values in Array2 from specific row values in Array1

Question

I would like to subtract the quantity of $array2 from the stocks of $array1.

$array1= ([product_id]=>4, [stocks]=>20)

$array2= ([product_id]=>4, [quantity]=>3)

So that would be:
$array1= ([0]=> 4, [1] => 20);
$array2= ([0]=> 4, [1] => 3);

And then the output should be:

$array1= ([0]=> 4, [1] => 17);

Answer 1

Your array structure looks slightly different with multiple records, the code works out like this in an ugly manner. I'm assuming you're talking about something like this:

$array1 = array(
    0=>array('product_id'=>4, 'stocks'=>20), 
    1=>array('product_id'=>5, 'stocks'=>60));
$array2 = array(
    0=>array('product_id'=>4, 'quantity'=>3)
    1=>array('product_id'=>5, 'quantity'=>30));

...It's a multi-dimensional array (typical for records pulled from a database).

foreach($array1 as $key=>$value){
    foreach($array2 as $key2=>$value2) {
        if($value['product_id']==$value2['product_id']){
            $value['stocks'] -= $value2['quantity'];
            //optimization to avoid searching this again.
            unset($array2[$key]);
        }
    }}

Answer 2

With what you have given the following will do what you are asking for:

if($array1['product_id'] == $array2['product_id']) {
    $array1['stocks'] -= $array2['quantity'];
}

If you need to loop through a bigger array then what I have given is only part of the larger puzzle.

Answer 3

Jesse's answer wasn't tested and will not provide the desired output because the "stocks" array wasn't being modified -- a copy of the array was being modified in the loop -- so if you try to print the result to screen, there would be no change.

To modify by reference, use & just before the value variable in the first loop.

Also the unset() key must come from the inner loop to be accurate.

Additionally, if the "sales" "product_id"s are unique, then breaking the inner loop upon matching will improve performance. (This is how array_search() works.)

Code: (Demo)

$stocks = [
    ['product_id'=>2, 'stocks'=>50], 
    ['product_id'=>3, 'stocks'=>100],
    ['product_id'=>4, 'stocks'=>20], 
    ['product_id'=>5, 'stocks'=>60]
];
$sales = [
    ['product_id'=>4, 'quantity'=>3],
    ['product_id'=>5, 'quantity'=>30]
];

foreach ($stocks as &$row) {                             // modify by reference
    foreach ($sales as $k => $row2) {                    // search for product_id match
        if ($row['product_id'] == $row2['product_id']) {
            $row['stocks'] -= $row2['quantity'];         // subtract
            unset($sales[$k]);                           // eliminate match from lookup array
            break;                                       // assuming $sales['product_id'] values are unique
        }
    }
}

var_export($stocks);

Output:

array (
  0 => 
  array (
    'product_id' => 2,
    'stocks' => 50,
  ),
  1 => 
  array (
    'product_id' => 3,
    'stocks' => 100,
  ),
  2 => 
  array (
    'product_id' => 4,
    'stocks' => 17,
  ),
  3 => 
  array (
    'product_id' => 5,
    'stocks' => 30,
  ),
)

---

Alternatively, you can converted the sales array into a flattened, product_id-keyed array to serve as a lookup.

Code: (Demo)

$keyed = array_column($sales, 'quantity', 'product_id');
var_export($keyed);
echo "\n---\n";

foreach ($stocks as &$row) {                          // modify by reference
    if (isset($keyed[$row['product_id']])) {          // search for product_id match
        $row['stocks'] -= $keyed[$row['product_id']]; // subtract
    }
}

var_export($stocks);

Output:

array (
  4 => 3,
  5 => 30,
)
---
array (
  0 => 
  array (
    'product_id' => 2,
    'stocks' => 50,
  ),
  1 => 
  array (
    'product_id' => 3,
    'stocks' => 100,
  ),
  2 => 
  array (
    'product_id' => 4,
    'stocks' => 17,
  ),
  3 => 
  array (
    'product_id' => 5,
    'stocks' => 30,
  ),
)