[RESOLVED] foreach can't modify an array?

banzaimonkey

I've come across this problem twice, lately, and I'm not sure if it's a problem with my methodology or a limitation of PHP.

I have code something like this:

$array = array(1, 2, 3, 4);

foreach($array as $value);
{
  $value = $value++;
}

This doesn't work, though. Instead, I have to do this:

foreach($array as $key => $value);
{
  $array[$key] = $value++;
}

Can someone explain why this is the case? Is there a way to tweak the first example to make it work?

ahundiak

It's because by default foreach returns a copy of $value.
http://us.php.net/manual/en/control-structures.foreach.php
For PHP5 you can use:
foreach($array as &$value);

bradgrafelman

banzaimonkey wrote:
Can someone explain why this is the case?

The [man]foreach/man is accessing each item of the array and copying that item's value into the $value variable. Two separate variables ($array[0] and $value) with no relation between them.

bonzaimonkey wrote:
Is there a way to tweak the first example to make it work?

Certainly. Something like this:

foreach($array as $key => $value); 
{ 
  $array[$key] = $value++; 
}

laserlight

Unfortunately, banzaimonkey, your original code has a devious typographical error, as well as another possible problem. The typographical error is the semi-colon at the end of the foreach loop's conditional. The other problem is that you are attempting to use the post-increment operator, so your loop (even when fixed) is actually a no-op.

If your intention is to add 1 to each element of the array using a foreach loop, then you should write:

$array = array(1, 2, 3, 4);

foreach ($array as $key => $value)
{
    $array[$key] = $value + 1;
}

There is another interesting option: use [man]array_walk/man with [man]create_function/man

array_walk($array, create_function('&$value', '$value++;'));

banzaimonkey

laserlight wrote:
Unfortunately, banzaimonkey, your original code has a devious typographical error.

Doh. >.<

Thanks very much for the other suggestions, such as &$value and array_walk(). I'll experiment with those next time this comes up. 🙂

The foreach() is accessing each item of the array and copying that item's value into the $value variable. Two separate variables ($array[0] and $value) with no relation between them.

And thanks much for that explanation, bradgrafelman. That clarifies the issue for me. I guess next time I should read the PHP manual a bit more thoroughly. 😉

Thanks again to each of you. Your responses were very helpful.

bradgrafelman

Don't forget to mark this thread resolved (if it is).