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Posted by Anonymous on 04/11/07 22:25
Steve wrote:
>
> someone asked a question in alt.php about a problem they were having with an
> algorytm. it contained something to the effect of:
>
> $i = $i++;
>
> some example code they'd snatched somewhere. in a loop, the expected $i to
> increment. i explained why i thought it would not - as it does not. however,
> i want to make sure i gave a valid answer.
>
> anyone have input?
I don't read alt.php so I don't know what you wrote, but the answer is
pretty obvious.
First, the ++ operators are increment operators. They execute an
incrementation of the variable by 1. $i++ by itself already executes $i
= $i + 1, which the original poster most likely wanted to do. But the
increment operators also return a value for further usage. The
post-increment operator returns the value before the variable was
incremented, the pre-increment operator returns the value after
incrementation.
$i++ is post-increment. That means, first the content of the variable is
evaluated, then it is incremented, then the evaluated (meaning the
former) value is passed to the equation which gets executed and resets
$i to its former value.
In other words, what the line $i=$i++; actually does is:
$evaluated = $i; //first save the current value for later usage
$i = $i + 1; // execute the increment after that because we are doing
post-increment
$i = $evaluated; // execute the actual equation
So I'd be very surprised if $i would change. :-)
Imagine the code:
$a = 1;
$b = $a++;
Then $a would be 2 and $b would be 1. If you wanted $b to get the new
value of $a and not the old one you would have to use pre-increment. The
variable would first be incremented and the new value passed to the
equation. Pre-increment would be ++$a instead of $a++. And if you wanted
$b to be just 1 larger than $a without changing $a at all you would
write $b = $a + 1;.
Pretty simple once you understand the concept of the increment (or
decrement) operators. If you (respectively the original posters) are
still unsure about them you should read them up in chapter 14 und 15 of
the PHP manual. They give several examples for using them. Feel free to
forward my explanation to alt.php, Steve.
Bye!
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