Edit report at https://bugs.php.net/bug.php?id=61188&edit=1
ID: 61188
Comment by: avp200681 at gmail dot com
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:
Well, I understand why it works that way in PHP (and in C).
But
"Can you point to any place on the documentation where it's stated that the
first
operand is evaluated first?"
I've found in PHP documentation:
"Operators on the same line have equal precedence, in which case associativity
decides the order of evaluation."
So, I think it would be good to have a note about this behaviour in PHP manual.
Just to make things clear. Because many people think that "associativity
decides the order of evaluation" also in the examples like antickon posted.
Previous Comments:
------------------------------------------------------------------------
[2012-02-26 19:43:41] antickon at gmail dot com
I assumed subexpression evaluation should be strictly left-to-right. But you
are
right, it is actually not guaranteed either way. Thanks for pointing that out.
I
believe it should be guaranteed strictly left-to-right, but that's just my
opinion. Proper programming practice (PrPrPr (TM)) would be not to compare
expressions with side-effects anyway.
PS. I put that comment under the page on expressions rather than the one on
operator precedence
http://www.php.net/manual/en/language.expressions.php
------------------------------------------------------------------------
[2012-02-26 19:22:56] cataphr...@php.net
I think the point here is that we make no guarantee about the order of
evaluation (or apparent order, as the implementation may just be checking
whether the same variable is both sides; I haven't checked), just as C doesn't.
Can you point to any place on the documentation where it's stated that the
first
operand is evaluated first?
Not that this is particularly relevant, but the only language I tried where the
behavior is what you expect is Mathematica (a = 3; (a = 4) == a is True and a =
3; a == (a = 4) is False), but it documents this behavior in
http://reference.wolfram.com/mathematica/tutorial/TheStandardEvaluationProcedure
.html
------------------------------------------------------------------------
[2012-02-26 19:18:48] antickon at gmail dot com
I suppose it is up to you to implement undocumented behavior as you see fit,
however peculiar it is. I'll post a comment on the operator precedence page to
document it though.
Fyi, I tested this in Javascript and Java and they both evaluate strictly left-
to-right.
------------------------------------------------------------------------
[2012-02-26 19:04:44] ras...@php.net
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.
------------------------------------------------------------------------
[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.
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