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Re: Operator precedence [message #185038 is a reply to message #185034] Tue, 25 February 2014 00:37 Go to previous messageGo to previous message
Thomas 'PointedEars' is currently offline  Thomas 'PointedEars'
Messages: 701
Registered: October 2010
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Senior Member
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, 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.

> Consider the following ECMAScript expression:
>
> a = 1, b = 2;
>
> The "equivalent" expression in PHP is a syntax error:
>
> $a = 1, $b = 2;

I am aware of that.

>>> 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;
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;
^

$ cat foo.cpp
#include <iostream>
using namespace std;

int main() {
int x = 42, 23;
cout << x << endl;
return 0;
}

$ g++ -Wall -o foo.out foo.cpp 2>&1| tee /tmp/v6wTKDF/0
foo.cpp: In function ‘int main()’:
foo.cpp:5:15: error: expected unqualified-id before numeric constant
int x = 42, 23;
^

However, in ECMAScript:

| >>> var x = 42, 23;
| 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:

$ cat foo.c
#include <stdio.h>

int main() {
printf("%i\n", (42, 23));
return 0;
}

$ gcc -Wall -o foo.out foo.c 2>&1| tee /tmp/vJ0RqVR/0
foo.c: In function ‘main’:
foo.c:4:21: warning: left-hand operand of comma expression has no effect
[-Wunused-value]
printf("%i\n", (42, 23));
^

$ ./foo.out
23

$ cat foo.cpp
#include <iostream>
using namespace std;

int main() {
cout << (42, 23) << endl;
return 0;
}

$ g++ -Wall -o foo.out foo.cpp
foo.cpp: In function ‘int main()’:
foo.cpp:5:16: warning: left operand of comma operator has no effect
[-Wunused-value]
cout << (42, 23) << endl;
^

$ ./foo.out
23

$ php -r 'echo (42, 23) . "\n";'
PHP Parse error: syntax error, unexpected ',' in Command line code on line
1

[Interesting that gcc says “operand” and “comma *expression*”, while g++
says “operand” and “comma *operator*”.]


PointedEars
--
Sometimes, what you learn is wrong. If those wrong ideas are close to the
root of the knowledge tree you build on a particular subject, pruning the
bad branches can sometimes cause the whole tree to collapse.
-- Mike Duffy in cljs, <news:Xns9FB6521286DB8invalidcom(at)94(dot)75(dot)214(dot)39>

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