[Info-vax] decrementing & for loops in C

[Bob Eager]

On Thu, 8 Jan 2009 14:17:02 UTC, billg999 at cs.uofs.edu (Bill Gunshannon) 
wrote:

> In article <176uZD2KcidF-pn2-KPYqms0nkLMf at rikki.tavi.co.uk>,
> 	"Bob Eager" <rde42 at spamcop.net> writes:
> > On Thu, 8 Jan 2009 13:07:17 UTC, FrankS <sapienza at noesys.com> wrote:
> > 
> >> The conditional equivalance operator in C is == not =.
> >> 
> >> Your syntax error occurs in the FOR statement.  You have:
> >> 
> >>    for (i = n; i = 0; i--)
> > 
> > No, it's syntactically correct. Not what is wanted, perhaps, but 
> > syntactically correct.
> > There is a semantic error if a countdown loop is wanted.
> > 
> >> but it should be:
> >> 
> >>    for (i = n; i == 0; i--)
> > 
> > No....it should be:
> > 
> >     for (i = n; i != 0; i--)
> > 
> > (see my other post)
> 
> Or it could be:
>     for (i = n; i == 0; i--)
> 
> which would work but is probably questionable stylisticaly.

No. That has a different meaning. The 'i == 0' means (in this case) 
'continue the loop while i is zero' - which it isn't to start with.

> I think the answer is to explain in more detail what is wrong with the
> first one.
> 
>     for (i = n; i != 0; i--)

That isn't the first one.

> In a C For loop the first part sets up the initial condition.
> In this loop i is set to the value of n which is 10.  Good so far.
> 
> I will skip to the third part.  This sets the action to be performed
> until some defined condition is met.  In this case, decrement i.
> 
> And now the important part, where the logic, not syntax, error occurs.
> 
> The second part specifies the end condition.

NO! It specifies the condition which, when no longer true, causes the 
loop to terminate.

>  When this conditionis met
> the result is a logical "true" and the loop ends.  Here we have:
> 
>      i = 0;
> 
> Note, that is the assignment operator in C and not the comparison operator.
> The result is to set i to 0.  This operation is, of course, successful and
> results in returning a logical "true", thus, the loop is terminated.  the
> for loop did exactly as you asked it to do.  All C compilers will treat it
> exactly the same.

I agree, and understood that. My point was that you have the loop 
termination condition back to front.

-- 
Bob Eager