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23 nothing is so as it seems, but why

php.internals

Marco Kaiser

20 years ago

Today during a session i had a strange "magic" feature found in php.

<?php
$a = 10;
echo ++$a + $a++;
?>

this works perfect as expected. it returns 22.
but if i assign $a as a ref. to another variable it will be return 23.

<?
$a = 10;
$b = &$a;
echo ++$a + $a++;
?>

this will gave me to 23
(used the $b to the pre post stuff)

<?
$a = 10;
$b = &$a;
echo ++$b + $b++;
?>

this just happens if i have a ref. count to my var. WHY? :)

http://en.wikipedia.org/wiki/23_%28film%29

--
Marco Kaiser

Hartmut Holzgraefe

20 years ago

Marco Kaiser wrote:

> Today during a session i had a strange "magic" feature found in php.
>
> <?php
> $a = 10;
> echo ++$a + $a++;
> ?>

there is a very simple rule that PHP inherited from C:

never use pre- and postfix increment operators in the same expression

as the results are undefined

--
Hartmut Holzgraefe, Senior Support Engineer                            .
MySQL AB, www.mysql.com

Etienne Kneuss

20 years ago

Notice that this behavior can be reproduced using only prefix increment
operators.

<?php

$a = 10;
$b = &$a;
echo (++$a) + (++$a); // 24

?>

<?php

$a = 10;
//$b = &$a;
echo (++$a) + (++$a); // 23

?>

Is that also expected to give undefined results ?


Best regards

Hartmut Holzgraefe a écrit :

> Marco Kaiser wrote:
>> Today during a session i had a strange "magic" feature found in php.
>>
>> <?php
>> $a = 10;
>> echo ++$a + $a++;
>> ?>
>
> there is a very simple rule that PHP inherited from C:
>
>   never use pre- and postfix increment operators in the same expression
>
> as the results are undefined
>

--
Etienne Kneuss

http://www.colder.ch/
colder@php.net

Christian Schneider

20 years ago

Etienne Kneuss wrote:

> Notice that this behavior can be reproduced using only prefix increment
> operators.
>
> Is that also expected to give undefined results ?

Yes, see
<http://en.wikipedia.org/wiki/C_syntax#Undefined_behavior>

- Chris

Marcus Börger

20 years ago

Hello Marco,

though Hartmut is perfectly correct in his statement here's what happens:

$a = 10; // 10
++$a     // 11
$a + $a  // 22
$a++     // 23

Thursday, January 19, 2006, 5:41:17 PM, you wrote:

> Today during a session i had a strange "magic" feature found in php.

> <?php
> $a = 10;
> echo ++$a + $a++;

?>>

just to verfiy echo $a after your echo line, it will show 12

> this works perfect as expected. it returns 22.
> but if i assign $a as a ref. to another variable it will be return 23.

> <?
> $a = 10;
> $b = &$a;
> echo ++$a + $a++;

?>>

> this will gave me to 23
> (used the $b to the pre post stuff)

> <?
> $a = 10;
> $b = &$a;
> echo ++$b + $b++;

?>>

> this just happens if i have a ref. count to my var. WHY? :)

> http://en.wikipedia.org/wiki/23_%28film%29

> --
> Marco Kaiser

Best regards,
Marcus

Marco Kaiser

20 years ago

Hmm yes i know this,
but its very interesting for me to see how php internaly handles ++$a with a
pointer.
Now i understand it :)

-- Marco

2006/1/19, Marcus Boerger <helly@php.net>:

>
> Hello Marco,
>
>   though Hartmut is perfectly correct in his statement here's what
> happens:
>
> $a = 10; // 10
> ++$a     // 11
> $a + $a  // 22
> $a++     // 23
>
> Thursday, January 19, 2006, 5:41:17 PM, you wrote:
>
> > Today during a session i had a strange "magic" feature found in php.
>
> > <?php
> > $a = 10;
> > echo ++$a + $a++;
> ?>>
>
> just to verfiy echo $a after your echo line, it will show 12
>
> > this works perfect as expected. it returns 22.
> > but if i assign $a as a ref. to another variable it will be return 23.
>
> > <?
> > $a = 10;
> > $b = &$a;
> > echo ++$a + $a++;
> ?>>
>
> > this will gave me to 23
> > (used the $b to the pre post stuff)
>
> > <?
> > $a = 10;
> > $b = &$a;
> > echo ++$b + $b++;
> ?>>
>
> > this just happens if i have a ref. count to my var. WHY? :)
>
> > http://en.wikipedia.org/wiki/23_%28film%29
>
> > --
> > Marco Kaiser
>
>
>
> Best regards,
> Marcus
>
>

--
Marco Kaiser

Jani Taskinen

20 years ago

For the 2nd time: references ARE NOT POINTERS! :-p

--Jani

On Thu, 19 Jan 2006, Marco Kaiser wrote:

> Hmm yes i know this,
> but its very interesting for me to see how php internaly handles ++$a with a
> pointer.
> Now i understand it :)
>
> -- Marco
>
> 2006/1/19, Marcus Boerger <helly@php.net>:
>>
>> Hello Marco,
>>
>>   though Hartmut is perfectly correct in his statement here's what
>> happens:
>>
>> $a = 10; // 10
>> ++$a     // 11
>> $a + $a  // 22
>> $a++     // 23
>>
>> Thursday, January 19, 2006, 5:41:17 PM, you wrote:
>>
>>> Today during a session i had a strange "magic" feature found in php.
>>
>>> <?php
>>> $a = 10;
>>> echo ++$a + $a++;
>> ?>>
>>
>> just to verfiy echo $a after your echo line, it will show 12
>>
>>> this works perfect as expected. it returns 22.
>>> but if i assign $a as a ref. to another variable it will be return 23.
>>
>>> <?
>>> $a = 10;
>>> $b = &$a;
>>> echo ++$a + $a++;
>> ?>>
>>
>>> this will gave me to 23
>>> (used the $b to the pre post stuff)
>>
>>> <?
>>> $a = 10;
>>> $b = &$a;
>>> echo ++$b + $b++;
>> ?>>
>>
>>> this just happens if i have a ref. count to my var. WHY? :)
>>
>>> http://en.wikipedia.org/wiki/23_%28film%29
>>
>>> --
>>> Marco Kaiser
>>
>>
>>
>> Best regards,
>> Marcus
>>
>>
>
>
> --
> Marco Kaiser
>

--
Give me your money at @ <http://pecl.php.net/wishlist.php/sniper>
Donating money may make me happier and friendlier for a limited period!
Death to all 4 letter abbreviations starting with P!

Marco Kaiser

20 years ago

Sorry Jani,

i mean a reference (alias) :)

-- Marco

2006/1/19, Jani Taskinen <sniper@iki.fi>:

>
>
>      For the 2nd time: references ARE NOT POINTERS! :-p
>
>      --Jani
>
>
> On Thu, 19 Jan 2006, Marco Kaiser wrote:
>
> > Hmm yes i know this,
> > but its very interesting for me to see how php internaly handles ++$a
> with a
> > pointer.
> > Now i understand it :)
> >
> > -- Marco
> >
> > 2006/1/19, Marcus Boerger <helly@php.net>:
> >>
> >> Hello Marco,
> >>
> >>   though Hartmut is perfectly correct in his statement here's what
> >> happens:
> >>
> >> $a = 10; // 10
> >> ++$a     // 11
> >> $a + $a  // 22
> >> $a++     // 23
> >>
> >> Thursday, January 19, 2006, 5:41:17 PM, you wrote:
> >>
> >>> Today during a session i had a strange "magic" feature found in php.
> >>
> >>> <?php
> >>> $a = 10;
> >>> echo ++$a + $a++;
> >> ?>>
> >>
> >> just to verfiy echo $a after your echo line, it will show 12
> >>
> >>> this works perfect as expected. it returns 22.
> >>> but if i assign $a as a ref. to another variable it will be return 23.
> >>
> >>> <?
> >>> $a = 10;
> >>> $b = &$a;
> >>> echo ++$a + $a++;
> >> ?>>
> >>
> >>> this will gave me to 23
> >>> (used the $b to the pre post stuff)
> >>
> >>> <?
> >>> $a = 10;
> >>> $b = &$a;
> >>> echo ++$b + $b++;
> >> ?>>
> >>
> >>> this just happens if i have a ref. count to my var. WHY? :)
> >>
> >>> http://en.wikipedia.org/wiki/23_%28film%29
> >>
> >>> --
> >>> Marco Kaiser
> >>
> >>
> >>
> >> Best regards,
> >> Marcus
> >>
> >>
> >
> >
> > --
> > Marco Kaiser
> >
>
> --
> Give me your money at @ <http://pecl.php.net/wishlist.php/sniper>
> Donating money may make me happier and friendlier for a limited period!
> Death to all 4 letter abbreviations starting with P!
>

--
Marco Kaiser

Jon Dowland

20 years ago

On Thu, Jan 19, 2006 at 06:41:17PM +0200, Marco Kaiser wrote:

> Today during a session i had a strange "magic" feature found in php.
>
> <?php
> $a = 10;
> echo ++$a + $a++;
> ?>
>
> this works perfect as expected. it returns 22.

Odd, I expected 21:
echo (11 + 10)
a = a + 1

--
Jon Dowland
http://alcopop.org/

Carl P. Corliss

20 years ago

Jon Dowland wrote:

> On Thu, Jan 19, 2006 at 06:41:17PM +0200, Marco Kaiser wrote:
>
>>Today during a session i had a strange "magic" feature found in php.
>>
>><?php
>>$a = 10;
>>echo ++$a + $a++;
>>?>
>>
>>this works perfect as expected. it returns 22.
>
>
> Odd, I expected 21:
> 	echo (11 + 10)
> 	a = a + 1
>

nope - discounting the 'undefined behavior' of using pre/post increment
operators, 22 would be the correct assumption - remember, broken into
seperate statements, it's effectively:

$a = 10;
$a = $a + 1; // $a == 11
$a + $a;     // $a == 22

echo $a;

$a = $a + 1; // $a == 23


Cheers,

--
Carl

Andi Gutmans

20 years ago

Guys, please take this offlist. Undefined means undefined. There's no
assumed way of it working.

Andi

At 07:14 AM 1/20/2006, Carl P. Corliss wrote: