for != while

Dan Tilque dant at tekla.UUCP
Sat Sep 6 06:44:41 AEST 1986

From: mikeb at copper.UUCP (Mike Beckerman)
>In article <15525 at ucbvax.BERKELEY.EDU> ballou at brahms.UUCP (Kenneth R. Ballou) writes:
>>In article <86900030 at haddock> karl at 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";

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
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