Thanks for taking time to provide the examples, Clancy, I'll know what
potential pitfalls to wary of now :)
On Fri, Feb 26, 2010 at 11:01 PM, <clancy_1@xxxxxxxxxxxx> wrote:
> A week ago Dasn asked a question about converting arrays, and I quoted one
> possible way of
> achieving his task, using the operation:
>
> $i = 0; while ($i < $k) { $b[$a[$i++]] = $a[$i++]; }
>
> I added the comment that "I have always been wary of using statements like
> this because I
> was unsure when the incrementing would occur, so I tried it."
>
> I received several CC e-mails replying to this post, including one rather
> critical comment
> to the effect that pre-and post-increment were all quite simple, and I
> really ought to
> learn the fundamentals before I started trying to do anything elaborate.
>
> I posted a reply to these e-mails, but as neither they, nor my reply, or
> any follow-up
> discussion ever appeared in the discussion group I will repost this reply.
> (I did have a
> power failure at this time, so it is conceivable that any follow-up was
> lost as a result
> of a glitch in my mailer, but I think it is more likely that there was a
> glitch in the
> discussion group server.)
>
> Unfortunately things aren't nearly as simple as this writer believes. The
> rule I have
> always used is that if you use the same variable as an index on both sides
> of an assign
> statement it is not safe to change the value of the index within the
> statement. While I
> have achieved the result I wanted in the example above (using PHP 5.1.6 --
> there is no
> guarantee it would work with other implementations of PHP) the results of
> doing this in
> the general case can be quite inexplicable.
>
> The particular case which prompted my comment was the one where you want to
> copy part of
> one array into the corresponding elements of another array. In accordance
> with my rule, I
> normally write:
>
> $i = 0; $j=count($a); while ($i < $j) { $b[$i] = $a[$i]; ++$i; }
>
> It is tempting to try to put the increment into the assignment statement.
> Clearly the
> value of $a[$i] has to be read before it can be written to $b[$i], so the
> logical
> expression would be:
>
> while ($i < $j) { $b[$i++] = $a[$i]; } A.
>
> However if you try this, you get $b[1] = $a[0], and so on. But if you try
> the alternative:
>
> while ($i < $j) { $b[$i] = $a[$i++]; } B.
>
> You get $b[0] = $a[1], and so on (as you would expect).
>
> Out of curiosity, I then tried:
>
> $i = -1; $j=count($a) - 1; while ($i < $j) { $b[$i] = $a[++$i]; }
> C
>
> This gave the desired result, and seemed moderately logical. However when I
> tried:
>
> $i = -1; $j=count($a) - 1; while ($i < $j) { $b[++$i] = $a[$i]; }
> D
>
> This gave exactly the same result. It is quite impossible to explain the
> results in cases
> A and D from the definitions of the pre-and post-increment operator, so I
> think I will
> stick to my safe rule!
>
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