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From: dant@tekla.UUCP (Dan Tilque)
Newsgroups: net.lang.c
Subject: Re: for != while
Message-ID: <749@tekla.UUCP>
Date: Fri, 5-Sep-86 16:44:41 EDT
Article-I.D.: tekla.749
Posted: Fri Sep  5 16:44:41 1986
Date-Received: Sun, 7-Sep-86 05:56:42 EDT
Organization: Tektronix, Inc., Beaverton, OR
Lines: 63


From: mikeb@copper.UUCP (Mike Beckerman)
>In article <15525@ucbvax.BERKELEY.EDU> ballou@brahms.UUCP (Kenneth R. Ballou) writes:
>>In article <86900030@haddock> karl@haddock writes:
>>>
>>>It's well known that the equivalence between for and while breaks down if
>>>there's a "continue" statement.  Here's another case I just discovered:
>>>
>>>main() {
>>>	char *foo = "outer";
>>>	for (;; printf(foo),exit(0)) {
>>>		char *foo = "inner";
>>>	}
>>>}
>>>
>>>This prints "outer" (vax SVR2 compiler), though the for-while equivalence
>>>might lead one to expect "inner".
>>
>>I don't think the issue here is equivalence of for and while statements.
>>The point is that the scope of the inner 'foo' is the compound statement
>>which is the body of the for statement.  So, quite rightly, the 'foo'
>>given as the argument to printf in the third expression of the for statement
>>refers to the most closely nested declaration of 'foo' -- the body of the
>>for statement is one block level higher and is not visible at this point.
>
>That was my first thought as well, but both K&R and the proposed ANSI C standard
>define the "for" loop as follows:
>
>	for (expression-1 ; expression-2 ; expression-3) statement
>
>		is equivalent to
>
>	expression-1;
>	while (expression-2) {
>		statement
>		expression-3;
>	}
>
>which to me says that the example should have printed "inner".
>

Ken Ballou is correct, and the equivalence above is correct.  The
equivalence of the first example is this:

main() {
	char *foo = "outer";
	while () { 
		{
			char *foo = "inner";
		}
	printf(foo),exit(0));
	}
}

The *foo = "inner" applies only within the inner braces (which 
delineate the statement).  The printf(foo) goes outside those braces.


 Dan Tilque		UUCP:		tektronix!dadla!dant
			CSnet:		dant%dadla@tektronix
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 Mass extinction; it's not just for dinosaurs anymore.