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Re: Operator precedence [message #185040 is a reply to message #185038] Tue, 25 February 2014 01:12 Go to previous messageGo to previous message
Christoph Michael Bec is currently offline  Christoph Michael Bec
Messages: 207
Registered: June 2013
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Thomas 'PointedEars' Lahn wrote:

> Christoph Michael Becker wrote:
>
>> Thomas 'PointedEars' Lahn wrote:
>>> Ben Bacarisse wrote:
>>>> Thomas 'PointedEars' Lahn <PointedEars(at)web(dot)de> writes:
>>>> > Ben Bacarisse wrote:
>>>> >> […] Expressions are built from operators, but is ',' an operator in
>>>> >> [PHP?
>>>> >> It appears in the operator precedence table but it isn't an operator
>>>> >> in an sense that would normally be understood by someone familiar with
>>>> >> these terms.
>>>> >
>>>> > ECMAScript has a “Comma Operator”, too.
>>>>
>>>> But my point was the PHP doesn't have one --
>>>
>>> But it does:
>>>
>>> for ($i = 0, $j = 42; $i < $j; ++$i);
>>> ^
>>
>> The comma is used in sevaral places in the PHP grammar[1]. However,
>> IMHO your example (inside a for_expr) is the only case where the comma
>> is used as an operator. The other uses are for declaration and
>> parameter lists (spoken in a broad sense).
>
> I am aware of that.
>
>>>> not in any normal sense of the word.
>>>
>>> Please define what you consider to be an operator in a “normal sense of
>>> the word”.
>>
>> It might be defined as an operator that can be used in arbitrary
>> expressions.
>
> A speculation,

ACK. :)

> one that is almost self-evidently false. Just consider the
> operators (including, but not just in, programming languages) that require
> their operands to be of a certain type.

I have to admit that this "definition" was far to inaccurate. However,
regarding the syntax alone it has some appeal.

>>>> ECMAScript really does have one (like C and C++ do).
>>>
>>> Why do you think C and C++ have a comma operator (like ECMAScript)?
>>
>> According to the C grammar[2]:
>>
>> exp : assignment_exp
>> | exp ',' assignment_exp
>>
>> [1] <http://lxr.php.net/xref/PHP_5_5/Zend/zend_language_parser.y>
>> [2] <http://www.cs.man.ac.uk/~pjj/bnf/c_syntax.bnf>
>
> ACK; I stand corrected. For reasons I cannot explain (probably it is just
> late), I had assumed that the comma operator in ECMAScript also worked with
> assignments, and this is what I tested with C and C++:
>
> $ cat foo.c
> #include <stdio.h>
>
> int main() {
> int x = 42, 23;

According to the C grammar[1], this use of the comma is not part of an
"exp", but rather an "init_declarator_list".

> printf("%i\n", x);
> return 0;
> }
>
> $ gcc -Wall -o foo.out foo.c
> foo.c: In function ‘main’:
> foo.c:4:15: error: expected identifier or ‘(’ before numeric constant
> int x = 42, 23;
> ^
>
> However, in ECMAScript:
>
> | >>> var x = 42, 23;

This use of the comma is not part of ECMAScript's "Expression", but
rather of "VariableDeclarationList".

> | SyntaxError: Unexpected number
>
> What I meant instead was
>
> | >>> 42, 23
> | 23
>
> which does work like that in certain contexts in C and C++, but not in PHP:

That's my point. :)

[1] <http://www.cs.man.ac.uk/~pjj/bnf/c_syntax.bnf>

--
Christoph M. Becker

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