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Posted by "Richard Lynch" on 08/25/05 10:44
On Wed, August 24, 2005 6:10 pm, Jordan Miller wrote:
> Is there a technical reason why PHP does not allow comparison
> operator expressions like the following:
>
> if (2 < $x <= 4) {}
2 < $x <= 4 is a valid expression already.
2 < $x is evaluated first, and returns true/false
true/false is compared with 4 using <= and will always return TRUE,
since true/false will get typecast to 0/1 in order for the comparison
to be made.
At least, I THINK that's what will happen... Not sure on the
type-casting of something that I'd never code in the first place.
At any rate, it's already valid under current syntax, so you'd have a
HUGE backwards incompatibility issue to change it.
> From what I can tell, this expression can currently only be written
> as:
>
> if ( $x > 2 && $x <= 4) {}
I personally would use:
((2 < $x) && ($x <= 4))
Yes, the extra parens are kinda bogus -- But they visually separate
the two tests.
Putting the 2, $x, and 4 in order is similar to the 2 < x <= 4
Algebraic notation.
It wouldn't bug me to see 2 < $x && $x <= 4, mind you.
In code I've read/written, it's seldom been a real problem.
In fact, if the author is CONSISTENT in their ordering, and SYSTEMATIC
in their tests/conditions/expressions, it doesn't even really matter
to me if they use 2 < $x && $x <= 4 or $x > 2 && $x <= 4, so long as
it's the same way in all their code.
> Would adding this syntax to PHP be incredibly difficult or lead to
> performance slowdowns?
Probably not incredibly difficult, per se, and performance would not
suffer in any measurable way...
You'd just break a few zillion existing scripts that, whether the
authors know it or not, rely on the current behaviour.
I suspect it would also unearth a large number of previously
un-noticed bugs :-)
> I think I remember reading that PHP always evaluates expressions from
> right to left,
If you did read that, it was a REALLY BAD thing to have been written
by anybody anywhere. :-)
[Maybe that goof that decided PHP wasn't ready for Enterprise use we
were discussing a couple days ago. He was great at writing FACTUALLY
INCORRECT statements to shore up his pathetic arguments. :-)]
PHP has a very precise order of operations with Operater Precedence:
http://php.net/manual/en/language.operators.php#language.operators.precedence
In fact, some of those are VERY common pitfalls for newbies.
&& || versus AND OR often trips up beginning scripters.
Not only does PHP not always evaluate from left-to-right due to
precedence, even within the precedence ordering, some operators are
right-to-left associative:
$x = 2;
$y = 5;
$x = $y = 3;
Here we have a "tie" in order of operations between two assignment
operators.
Since = is RIGHT associative, the tie-breaker is to read from RIGHT to
LEFT and do the operations in this order:
$y = 3;
$x = $y;
So both $x and $y are 3.
[WRONG]
If = were LEFT associative, which it's NOT, you'd end up with $x being
5 and $y would still be 3:
$x = $y;
$y = 3;
[/WRONG]
'Course, $x = $y = 3; is probably Bad Practice anyway...
Though I guess I've seen:
$x = $y = $z = 0;
at the top of enough functions to understand and accept that it's a
convenient way to initalize all three variables to 0...
I don't LIKE it, but I can live with it.
> so I guess there may be a considerable codebase change
> required. Maybe there could be a default function workaround for this
> or some other way to automagically process these more concise
> expressions without too much of a slowdown?? Just curious.
You could maybe check with PHP-Internals to see if they believe that
nobody's relying on the current behaviour with < > <= >=, and then
there's no problem adding it, but I wouldn't get yer hopes up.
PS Is (2 < $x >= 0) a syntax error, or would you allow that silly
expression? :-)
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