php nested foreach unexpected result

Question

I dont quite understand what is happening. copy the following code and run it you should see what I am seeing.

$stores = array(
        (object)[
            "store_id" => 1,
        ],
        (object)[
            "store_id" => 2,
        ],
        (object)[
            "store_id" => 3,
        ]
    );

    $currentYear = date('Y');
    $monthes = array();
    for($i = 1; $i <= 4; $i++){
        $temp = new stdClass();
        $temp->month = $i;
        $temp->sales = 0;
        array_push($monthes, $temp);
    }
    foreach($stores as $store){
        $store->sales = array(
            "currentYear" => (object)[
                "year" => $currentYear,
                "monthes" => $monthes,
            ],
        );
    }

    foreach($stores as $store){
        foreach($store->sales as $year){
           foreach($year->monthes as $month){
               $month->sales += 1;
           }
        }
    }

    print_r("<pre>"); 
    print_r($stores);
    print_r("</pre>");

the result it produces look like the following:

   Array
    (
        [0] => stdClass Object
            (
                [store_id] => 1
                [sales] => Array
                    (
                        [currentYear] => stdClass Object
                            (
                                [year] => 2018
                                [monthes] => Array
                                    (
                                        [0] => stdClass Object
                                            (
                                                [month] => 1
                                                [sales] => 3
                                            )

                                        [1] => stdClass Object
                                            (
                                                [month] => 2
                                                [sales] => 3
                                            )

but I am expecting the sales to be 1. instead of 3. because it looks like it would visit each month just 1 time and sales' inital value is 0. so 0 += 1 should just be 1. Looks as if, it looped over itself 3 times.

I cant wrap my head around on what I did wrong here.

Barmar · Accepted Answer · 2018-06-08 23:45:54Z

You're storing the same $monthes array into each of the currentYear objects. While the array is copied when you assign it, the objects that it contains are not; all these arrays contain references to the same four objects. So when you increment the sales in store 1 month 1, it also increments store 2 month 1, store 3 month 1, and store 4 month 1.

You need to put the loop that creates the $monthes array inside the loop that fills in each of the stores.

<?php
$stores = array(
    (object)[
        "store_id" => 1,
        ],
    (object)[
        "store_id" => 2,
        ],
    (object)[
        "store_id" => 3,
        ]
    );

$currentYear = date('Y');
foreach($stores as $store){
    $monthes = array();
    for($i = 1; $i <= 4; $i++){
        $temp = new stdClass();
        $temp->month = $i;
        $temp->sales = 0;
        array_push($monthes, $temp);
    }
    $store->sales = array(
        "currentYear" => (object)[
            "year" => $currentYear,
            "monthes" => $monthes,
            ],
        );
}

foreach($stores as $store){
    foreach($store->sales as $year){
        foreach($year->monthes as $month){
            $month->sales += 1;
        }
    }
}

echo "<pre>";
print_r($stores);
echo "</pre>";

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