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Re: Operator precedence [message #185005 is a reply to message #184990]

Sun, 23 February 2014 20:56

Ben Bacarisse
Messages: 82
Registered: November 2013

Karma:

Member

Christoph Michael Becker <cmbecker69(at)arcor(dot)de> writes:

> I've stumbled upon the following code:
>
> !$a && $a = 42;
>
> Apparently, this is meant instead of
>
> if (!$a) $a = 42;
>
> or
>
> !$a AND $a = 42;
>
> However, I am quite confused that it works because the = operator has
> lower precendence than the && operator[1]. Actually that should mean
> the statement is equivalent to
>
> (!$a && $a) = 42;
>
> But this obviously won't work.

The precedence table is wrong -- or at least something of a
simplification. The grammar for PHP (for which I had to resort to the
source code) does not reflect that table for the = operator. The
grammar has explicit rules for = that result in the precedence being
ignored in this particular case.

The precedence comes into play when the YACC (or Bison or whatever)
source says something like

exp: ...
| exp '=' exp
| ...

but instead is says:

exp: ...
| variable = exp

(this is a somewhat abbreviated explanation) so there can never be a
full expression on the left hand side. Note that this works:

$a + $a = 42;

and parses like

$a + ($a = 42);

There is nothing special about && in this respect. The precedence does
count in some situations. For example, compare

$a = true && $b = false; // $a = (true && ($b = false));

and

$a = true and $b = false; // ($a = true) and ($b = false);

> Why does the mentioned statement has the "expected" result? Is it
> reliable to write it that way?

Interesting question. I don't know where the language is defined.
Maybe one day the source code will reflect the documentation of vice
versa. Maybe one day both will match some other, external,
specification.

> [1] <http://www.php.net/manual/en/language.operators.precedence.php>

--
Ben.

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[Message index]

		Operator precedence By: Christoph Michael Bec on Sun, 23 February 2014 17:03
		Re: Operator precedence By: GS on Sun, 23 February 2014 19:42
		Re: Operator precedence By: Christoph Michael Bec on Sun, 23 February 2014 20:23
		Re: Operator precedence By: Jerry Stuckle on Sun, 23 February 2014 21:34
		Re: Operator precedence By: Richard Damon on Sun, 23 February 2014 23:36
		Re: Operator precedence By: Jerry Stuckle on Mon, 24 February 2014 02:09
		Re: Operator precedence By: Ben Bacarisse on Mon, 24 February 2014 04:49
		Re: Operator precedence By: Thomas 'PointedEars' on Mon, 24 February 2014 11:56
		Re: Operator precedence By: Christoph Michael Bec on Mon, 24 February 2014 16:38
		Re: Operator precedence By: Ben Bacarisse on Mon, 24 February 2014 21:25
		Re: Operator precedence By: Thomas 'PointedEars' on Mon, 24 February 2014 23:38
		Re: Operator precedence By: Christoph Michael Bec on Tue, 25 February 2014 00:07
		Re: Operator precedence By: Thomas 'PointedEars' on Tue, 25 February 2014 00:37
		Re: Operator precedence By: Christoph Michael Bec on Tue, 25 February 2014 01:12
		Re: Operator precedence By: Thomas 'PointedEars' on Tue, 25 February 2014 02:29
		Re: Operator precedence By: Christoph Michael Bec on Tue, 25 February 2014 18:41
		Re: Operator precedence By: Thomas 'PointedEars' on Tue, 25 February 2014 18:55
		Re: Operator precedence By: Christoph Michael Bec on Tue, 25 February 2014 20:36
		Re: Operator precedence By: Ben Bacarisse on Tue, 25 February 2014 03:08
		Re: Operator precedence By: Thomas 'PointedEars' on Tue, 25 February 2014 03:17
		Re: Operator precedence By: Ben Bacarisse on Tue, 25 February 2014 04:45
		Re: Operator precedence By: GS on Tue, 25 February 2014 21:45
		Re: Operator precedence By: Jerry Stuckle on Mon, 24 February 2014 12:00
		Re: Operator precedence By: Christoph Michael Bec on Mon, 24 February 2014 17:11
		Re: Operator precedence By: Jerry Stuckle on Mon, 24 February 2014 19:27
		Re: Operator precedence By: Christoph Michael Bec on Tue, 25 February 2014 00:25
		Re: Operator precedence By: Jerry Stuckle on Tue, 25 February 2014 01:40
		Re: Operator precedence By: Ben Bacarisse on Mon, 24 February 2014 21:23
		Re: Operator precedence By: Jerry Stuckle on Mon, 24 February 2014 21:45
		Re: Operator precedence By: Ben Bacarisse on Tue, 25 February 2014 04:32
		Re: Operator precedence By: Jerry Stuckle on Tue, 25 February 2014 04:47
		Re: Operator precedence By: Ben Bacarisse on Tue, 25 February 2014 16:56
		Re: Operator precedence By: Jerry Stuckle on Tue, 25 February 2014 18:59
		Re: Operator precedence By: Christoph Michael Bec on Tue, 25 February 2014 19:19
		Re: Operator precedence By: Thomas 'PointedEars' on Tue, 25 February 2014 19:43
		Re: Operator precedence By: Jerry Stuckle on Tue, 25 February 2014 21:02
		Re: Operator precedence By: Jerry Stuckle on Tue, 25 February 2014 21:01
		Object constructors/destructors (was: Operator precedence) By: Christoph Michael Bec on Tue, 25 February 2014 21:19
		Re: Object constructors/destructors By: Jerry Stuckle on Tue, 25 February 2014 21:32
		Re: Object constructors/destructors By: Christoph Michael Bec on Tue, 25 February 2014 21:55
		Re: Object constructors/destructors By: Jerry Stuckle on Tue, 25 February 2014 22:01
		Re: Object constructors/destructors By: Adam Harvey on Tue, 25 February 2014 23:22
		Re: Object constructors/destructors By: Jerry Stuckle on Wed, 26 February 2014 02:10
		Re: Object constructors/destructors By: Christoph Michael Bec on Wed, 26 February 2014 20:10
		Re: Object constructors/destructors By: Jerry Stuckle on Wed, 26 February 2014 21:01
		Re: Object constructors/destructors By: Christoph Michael Bec on Wed, 26 February 2014 21:59
		Re: Object constructors/destructors By: Jerry Stuckle on Wed, 26 February 2014 22:19
		Re: Object constructors/destructors By: Christoph Michael Bec on Wed, 26 February 2014 22:39
		Re: Object constructors/destructors By: Jerry Stuckle on Thu, 27 February 2014 03:30
		Re: Object constructors/destructors By: Christoph Michael Bec on Thu, 27 February 2014 17:28
		Re: Object constructors/destructors By: Jerry Stuckle on Thu, 27 February 2014 21:09
		Re: Object constructors/destructors By: Christoph Michael Bec on Fri, 28 February 2014 13:52
		Re: Object constructors/destructors By: Jerry Stuckle on Fri, 28 February 2014 14:08
		Re: Object constructors/destructors By: Christoph Michael Bec on Fri, 28 February 2014 15:52
		Re: Object constructors/destructors By: Jerry Stuckle on Fri, 28 February 2014 16:03
		Re: Object constructors/destructors By: Thomas 'PointedEars' on Thu, 27 February 2014 00:00
		Re: Object constructors/destructors By: Jerry Stuckle on Thu, 27 February 2014 03:39
		Re: Object constructors/destructors By: Thomas 'PointedEars' on Thu, 27 February 2014 17:29
		Re: Object constructors/destructors By: Jerry Stuckle on Thu, 27 February 2014 21:14
		Re: Object constructors/destructors By: Christoph Michael Bec on Tue, 25 February 2014 23:18
		Re: Object constructors/destructors By: Jerry Stuckle on Wed, 26 February 2014 02:08
		Re: Object constructors/destructors By: Richard Damon on Wed, 26 February 2014 12:50
		Re: Object constructors/destructors By: Jerry Stuckle on Wed, 26 February 2014 13:44
		Re: Object constructors/destructors By: Richard Damon on Thu, 27 February 2014 04:49
		Re: Object constructors/destructors By: Jerry Stuckle on Thu, 27 February 2014 13:25
		Re: Object constructors/destructors By: Richard Damon on Sat, 01 March 2014 17:56
		Re: Object constructors/destructors By: Jerry Stuckle on Sat, 01 March 2014 19:22
		Re: Object constructors/destructors By: Richard Damon on Sun, 02 March 2014 19:28
		Re: Object constructors/destructors By: Jerry Stuckle on Sun, 02 March 2014 20:42
		Re: Object constructors/destructors By: Richard Damon on Sun, 02 March 2014 22:14
		Re: Object constructors/destructors By: Jerry Stuckle on Sun, 02 March 2014 22:48
		Re: Object constructors/destructors By: Tim Streater on Sun, 02 March 2014 23:05
		Re: Operator precedence By: Ben Bacarisse on Tue, 25 February 2014 21:08
		Re: Operator precedence By: Christoph Michael Bec on Tue, 25 February 2014 21:29
		Re: Operator precedence By: Ben Bacarisse on Tue, 25 February 2014 23:15
		Re: Operator precedence By: Richard Damon on Tue, 25 February 2014 03:41
		Re: Operator precedence By: Jerry Stuckle on Tue, 25 February 2014 04:59
		Re: Operator precedence By: Richard Damon on Sat, 01 March 2014 18:39
		Re: Operator precedence By: Jerry Stuckle on Sat, 01 March 2014 19:29
		Re: Operator precedence By: Richard Damon on Sun, 02 March 2014 20:22
		Re: Operator precedence By: Jerry Stuckle on Sun, 02 March 2014 20:49
		Re: Operator precedence By: Christoph Michael Bec on Tue, 25 February 2014 18:15
		Re: Operator precedence By: Ben Bacarisse on Sun, 23 February 2014 20:56
		Re: Operator precedence By: Christoph Michael Bec on Mon, 24 February 2014 13:28

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