A simple question on expression

[Bernard Dautrevaux]

> -----Original Message-----
> From: Jie Zhang [ mailto:jzhang916@sina.com ]
> Sent: Friday, August 31, 2001 11:01 AM
> To: gcc@gcc.gnu.org
> Cc: Bohdan Vlasyuk
> Subject: Re: A simple question on expression
> 
> 
> Thank you for your reply!
> 
> But your "obvious" explanation is not satisfiable.
> 
> If the following statement
> 
> a = (a<4)?0:a++;
> 
> can be expanded to
> 
> if (a<4)
> 	a=0;
> else
> 	a=a++; /* 2 */
> 
> 
> why
> a = 0?0:a++;
> not be expanded to
> 
> if (0)
> 	a=0;
> else
> 	a=a++; /* 2 */
> 
> 
> ps. I have read the K&R yet.
> 

Don't worry; this is perfectly normal. [2] is always undefined, in both your
expressions (that are effectively both equivalent to the if statments).
Undefined means that, depending on th ecompiler's goodwill *at the moment*,
a=a++; may be compiled as "a=a" or "a=a+1". When I says *at the moment* that
means that this may change from instruction to instruction (or even from
sub-expression to sub-expression) due to register allocation, instruction
sequencing, pipeline handling or various other optimizations... 

So when something is undefined you may egt *varying" results...

Anyway, this is quite off-topic for this list. These kinds of question
should be discussed on a general C programming list.

Regards,

	Bernard

--------------------------------------------
Bernard Dautrevaux
Microprocess Ingenierie
97 bis, rue de Colombes
92400 COURBEVOIE
FRANCE
Tel:	+33 (0) 1 47 68 80 80
Fax:	+33 (0) 1 47 88 97 85
e-mail:	dautrevaux@microprocess.com
		b.dautrevaux@usa.net
--------------------------------------------