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From: bart@reed.UUCP (Bart Massey)
Newsgroups: net.micro.mac
Subject: Re: Megamax bug?
Message-ID: <3687@reed.UUCP>
Date: Wed, 18-Jun-86 12:08:13 EDT
Article-I.D.: reed.3687
Posted: Wed Jun 18 12:08:13 1986
Date-Received: Sat, 21-Jun-86 10:06:36 EDT
References: <1023@k.cs.cmu.edu> <219@analog.UUCP>
Reply-To: bart@reed.UUCP (Bart Massey)
Distribution: net
Organization: Reed College, Portland, Oregon
Lines: 66

In article <219@analog.UUCP> kim@analog.UUCP (Kim Helliwell) writes:
>Greg Stein at CMU said...
> > I'm working in Megamax C, and I noticed a problem while I was in the
> > debugger.  It comes from these statements:
> > 
> > 	for (i = ntables; i--; ) <statement>
> > 	if (i == -1) <more statements>
> > 
> > I would expect this to get the value of i, decrement the stored value,
> > and then exit or execute the statement.  Rather, it tests the value
> > before it decrements, and the loop exits with i = 0.  Is this a bug
> > in the code generation, or is C supposed to do this?
> 
> It's not a MegaMax bug, it's a Greg bug (:-).  You have the i-- in the
> slot that is supposed to be the termination condition--so your description
> sounds like exactly what I would expect to happen.  If you want the i--
> to decrement i without testing it for equality to 0 first, it has to
> be after the second semicolon:
> 
> 	for (i = ntables; ; i--) <statement>
> 	if (i = -1) <more statements>

It may be that you have to do this, (BTW, you almost certainly meant
"for(i = ntables; i; i--) <statement>") but it's not a "Greg Bug", it's
a Megamax bug.  Here's a copy of what I wrote Greg, which he probably
didn't receive, because of mailer problems...  (Sorry, Greg).

K&R specify (page 202) that

	for( i = ntables; i--; )
		<statement>

should be equivalent to

	i = ntables;
	while( i-- )
		<statement>

They also specify that

	"When postfix -- is applied to an lvalue the result is the value of
the object referred to by the lvalue.  After the result is noted, the object
is incremented in the same manner as for the prefix -- operator." (page 187)

Putting this all together...

Imagine that ntables is 37.  The while statement will clearly decrement "i"
down to one.  Then what will happen?  Answer:  one is boolean true, so "i"
will be decremented to zero.  The statement will be executed.  zero is
boolean false.  So the statement won't be executed.  BUT "I" WILL STILL BE
DECREMENTED, as a result of evaluating "i--".  Thus, the <statement> will
be executed 37 times, (with i = 37 downto 1), and "i" will end up with the
value -1.

As a cross check, our 4.2BSD C compiler (reasonably close to K&R), when
fed the original "for" statement, does indeed set "i" to -1.

The Megamax C compiler is probably putting the branch on the wrong side
of the decrement.  Understandable (perhaps especially so to Greg, who
co-authored the original Rascal compiler), but wrong.

Sorry for the length of this posting, but it would have been hard for me
to say it shorter...

				Bart Massey
				..tektronix!reed!bart