Edit report at https://bugs.php.net/bug.php?id=61188&edit=1
ID: 61188
Updated by: ras...@php.net
Reported by: antickon at gmail dot com
Summary: Assignment changes order of evaluation of binop
expression
Status: Not a bug
Type: Bug
Package: Variables related
Operating System: linux
PHP Version: 5.3.10
Block user comment: N
Private report: N
New Comment:
I do see your argument, but you are making assumptions about how PHP handles
sequence points in expressions which is not documented and thus not stricly
defined.
Previous Comments:
------------------------------------------------------------------------
[2012-02-26 19:00:09] antickon at gmail dot com
What C or perl does is not the issue. The PHP documentation on operator
precedence states parentheses force precedence, not evaluation order of
subexpressions. You are saying that arbitrarily changing evaluation order
depending on the type of subexpression is correct behavior. You have not yet
provided any rational justification for this behavior, except that C and perl
also behave this way.
------------------------------------------------------------------------
[2012-02-26 18:38:39] ras...@php.net
Then C/C++/Perl and the other C-like languages are all wrong as well then.
Try this in C:
#include <stdio.h>
int main(char *argv[], int argc) {
int a=3;
printf("%d\n",(a==(a=4)));
}
Or this in Perl:
$a=3;
print $a==($a=4);
------------------------------------------------------------------------
[2012-02-26 18:35:21] antickon at gmail dot com
I'm afraid you're mistaken. The brackets denote a change in expression binding,
not evaluation order. If what you are saying is true
<?php
function a(){echo 'a';}
function b(){echo 'b';}
a() == (b());
would output ba since the bracketed expression would be evaluated first.
However
it (correctly) outputs ab.
In fact, the brackets are completely irrelevant. I added them for clarity.
Consider the equivalent example:
<?php
$a = 3;
var_dump( $a == $a = 3 );
------------------------------------------------------------------------
[2012-02-26 18:16:37] ras...@php.net
You are outsmarting yourself here. Look at your brackets.
$a == ($a=4)
So we do the bracketed expression first: $a=4 which sets $a to 4 obviously and
returns the value 4. So what are we left with?
$a == 4
But what is $a at this point? Well, it is 4 of course, because we just set it.
So we have:
4 == 4
If you can find a language where this expression doesn't return true,
regardless
of the initial value of $a, then you should file a bug against that language.
------------------------------------------------------------------------
[2012-02-26 17:54:18] antickon at gmail dot com
Description:
------------
An assignment expression can change the order of operation for == and != (and
possibly other binops)
Test script:
---------------
<?php
$a = 3;
var_dump( $a == ( $a = 4 ) );
Expected result:
----------------
bool(false)
evaluation of $a == ( $a = 4 ) should be as follows:
left side of the comparison is evaluated (evaluates to 3)
right side of the comparison is evaluated (4 is assigned to $a, evaluates to 4)
3 == 4 finally evaluates to false
Actual result:
--------------
bool(true)
------------------------------------------------------------------------
--
Edit this bug report at https://bugs.php.net/bug.php?id=61188&edit=1
