Posted by Jerry Stuckle on 07/31/07 00:38

Jerry Stuckle wrote:
> google2006@sensation.net.au wrote:
>> The following code never sees the end of the array, and generates an
>> out of memory error under PHP5 (both CLI and Apache module) :
>>
>> while (isset($rank[$i])) {
>> $rank[$i] = trim($rank[$i++]);
>> }
>>
>> Moving the post increment to a separate line fixes this issue:
>>
>> while (isset($rank[$i])) {
>> $rank[$i] = trim($rank[$i]);
>> $i++;
>> }
>>
>> For some reason PHP5 appears to be initializing $rank[($i+1)] each
>> time, perhaps because it's passing the value to trim(). The subsequent
>> isset() test on that element succeeds so the loop continues
>> indefinitely as each new element is initialized.
>>
>> It works as expected under PHP4, stopping once it hits the end of the
>> array. I can't find any documentation which hints at any change to the
>> handling of uninitialized variables which caused this code to break.
>>
>> Thanks in advance for any help...
>>
>
> I did a quick look and don't see where this was behavior was explicitly
> changed, but it looks more like a bug to me.
>
> $i++ returns the old value of $i, so $rank[$i + 1] should never be
> evaluated, IMHO.
>
> I'd suggest putting it up as a bug on php.net and see what they say
> about it.
>

I take this back - the result of the operation is unpredictable. That's
because:

$rank[$i] = trim($rank[$i++]);

$rank[$i++]

is evaluated as $rank[$i]. However, when

$rank[$i]

is evaluated, does it use the old or the new version of $i? Is it the
old version or the new version? There's no indication in PHP, and in
both C and C++ it is documented that the result is unpredictable.

So the effective action might be:

while (isset($rank[$i])) {
$tmp = trim($rank[$i]);
$i = $i + 1;
$rank[$i] = $tmp;
}

The bottom line: don't change a value and use it in the same statement.
Results are unpredictable.

--
==================
Remove the "x" from my email address
Jerry Stuckle
JDS Computer Training Corp.
jstucklex@attglobal.net
==================

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