Issue in Incrementing array values in PHP

Question

In my project I increase values of firstArray +3.After I increase values of secondArray +7 .

   $firstArray = array(
      'a' => 1,
      'b' => 3,
      'c' => 5,
   );

foreach ($firstArray as &$item) {
    $item += 3;
}
print_r($firstArray);

Result: Array ( [a] => 4 [b] => 6 [c] => 8)

  $secondArray = array(
    'a' => 11,
    'b' => 13,
    'c' => 17,
);

foreach ($secondArray as $key=>$item) {
    $secondArray[$key] += 7;
}

print_r($secondArray);

Result: Array ( [a] => 18 [b] => 20 [c] => 24 )

But problem is when I did print_r($firstArray) again; I have this result: Array ( [a] => 4 [b] => 6 [c] => 17 )

Why there is the difference in result fisrt and thisrd?

Where you get Result: Array ( [a] => 12 [b] => 22 [c] => 32 ) from? — Ed Heal
– Ed Heal, Commented Jan 11, 2014 at 10:51
Because you're looping with a reference (&) in the first loop, you need to unset($item) immediately after that loop otherwise it's still a reference to the last entry in $firstArray... looping by reference should always be approached with caution — Mark Baker
– Mark Baker, Commented Jan 11, 2014 at 11:02

sergio · Accepted Answer · 2014-01-11 10:51:58Z

1

The problem is, that you used $item as a link, and after one loop you used it again(I think after reuse $item some of value were rewritten ). You need to unset($item) after first loop, and even alwas unset link after used them.

$firstArray = array(
    'a' => 1,
    'b' => 3,
    'c' => 5,
);

foreach ($firstArray as &$item) {
    $item += 3;
}
print_r($firstArray);

unset($item);

Now result is good

answered Jan 11, 2014 at 10:51

sergio

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Comments

elclanrs · Accepted Answer · 2014-01-11 10:56:23Z

1

You could use array_map instead of loops and pass-by-reference. You avoid mutation this way. For example:

$add = function($x) {
  return function($y) use($x) {
    return $x + $y;
  };
};

$firstArray = array_map($add(3), $firstArray);
$secondArray = array_map($add(7), $secondArray);

answered Jan 11, 2014 at 10:56

elclanrs

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2 Comments

user2984590 Over a year ago

Yes, but it is not the answer on my question

elclanrs Over a year ago

The other answer has the solution to your particular issue; this is an alternative solution.

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