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Re: Operator precedence [message #184996 is a reply to message #184990]

Sun, 23 February 2014 19:42

GS
Messages: 3
Registered: February 2014

Karma:

Junior Member

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

> 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.
>
> Why does the mentioned statement has the "expected" result? Is it
> reliable to write it that way?
>
> [1] <http://www.php.net/manual/en/language.operators.precedence.php>
>

This shows one of the major problems with PHP: there is no formal language
definition, you have to go by examples in the help file to intuit what is
or is not valid. Where is the BNF definition of the language? There is none
AFAICT.

Under "Control structures" the PHP help file says "Any PHP script is built
out of a series of statements" but offers no definition of what constitutes
a statement. Under "Expressions" the manual says "Expressions are the most
important building stones of PHP. In PHP, almost anything you write is an
expression." How do we square that with the previous claim? In the end I
think you should avoid using PHP for anything critical since it is a
language defined by handwaving, where you can't formally determine the
meaning of a construct, yuo either have to try it out or find an analogous
example in the help file.

In your example, the answer to why the construction works is given in a
footnote to the section "Operator Precedence" which states

"Note: Although = has a lower precedence than most other operators, PHP
will still allow expressions similar to the following: if (!$a = foo()), in
which case the return value of foo() is put into $a. "

The formal meaning of this footnote is "BOOGA BOOGA!"

This kind of thing give me the heeby jeebies and makes me think I should be
using some other language for server side scripting (maybe perl is better
defined?)

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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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