Posted by Tyno Gendo on 04/11/07 22:42

Anonymous wrote:
> 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!

Ah, right, I see.. so from my previous post...

$i = 2;
echo $i++ + $i; // expect 5, gives 5, applies ++ increment

i complete messed up on that one...

but seeing your explanation i can see this gives

2 (return value of $i++) + 3 (now incremented value) = 5

it all makes sense, in a non-sensical fashion, avoid this syntax :)

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