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Re: Last element in an array? [message #169475 is a reply to message #169471]

Tue, 14 September 2010 20:10

matt[1]
Messages: 40
Registered: September 2010

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On Sep 14, 2:46 pm, Thomas Mlynarczyk <tho...@mlynarczyk-webdesign.de>
wrote:
> matt schrieb:
>
>> $a = array(str_repeat("A", 10000));
>> echo memory_get_usage() . "\n";
>> $b = $a; // a full copy of a has NOT been made yet
>> echo memory_get_usage() . "\n";
>> $b[] = "B"; // now it has, I believe
>> echo memory_get_usage() . "\n";
>
> [outputs]
>
>> 328436
>> 328504
>> 328752
>> I would have expected the third number to be almost double the first
>> based on my understanding. If PHP were making a copy right at the
>> assignment operator, then I'd expect to see the 2nd and 3rd be double
>> the first.
>
> No. First, you should prepend a $before = memory_get_usage() to your
> above code and then subtract $before from the three echoed values to
> measure only what you intend to measure. On my system, that gives:
>
> 10248
> 10296
> 10488
>
> As Marious explained, PHP doesn't actually create a copy unless it
> really has to. So when you do $b = $a, no copy is made and both
> variables point to the same value stored in memory. Only when you change
> $b, the value is actually copied and the change performed on the copy.
> But in your example above, you are not changing the 10000 chars string!
> When you create the array $a, its first element will simply point to a
> location in memory where the long string is stored. Now, when you do the
> $b[] = "B", you are simply appending another string to the array, but
> you leave its first element -- the long string -- unchanged. If you did
> $b[0][0] = "B", then you would observe the expected memory consumption,
> because you would change the long string's first character (and thereby
> the whole string), so it would have to be copied:
>
> 10248
> 10296
> 20456
>

Aha! I didn't realize that PHP was actually managing memory down to
individual array elements. That makes my results make a lot more
sense, thanks!

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[Message index]

		Last element in an array? By: MikeB on Sat, 11 September 2010 19:08
		Re: Last element in an array? By: Thomas Mlynarczyk on Sat, 11 September 2010 19:32
		Re: Last element in an array? By: MikeB on Sat, 11 September 2010 21:35
		Re: Last element in an array? By: Piyush Gupta on Mon, 13 September 2010 10:44
		Re: Last element in an array? By: MikeB on Mon, 13 September 2010 15:54
		Re: Last element in an array? By: Jerry Stuckle on Mon, 13 September 2010 16:46
		Re: Last element in an array? By: matt[1] on Mon, 13 September 2010 18:14
		Re: Last element in an array? By: Marious Barrier on Mon, 13 September 2010 18:56
		Re: Last element in an array? By: matt[1] on Mon, 13 September 2010 21:34
		Re: Last element in an array? By: Marious Barrier on Mon, 13 September 2010 21:53
		Re: Last element in an array? By: Thomas Mlynarczyk on Tue, 14 September 2010 18:46
		Re: Last element in an array? By: matt[1] on Tue, 14 September 2010 20:10

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