[Color Computer] [Coco] Bug in FOR NEXT ?

[Arthur Flexser]

I agree, but in the case of the bug I mentioned, the loop counter is not being
used outside a loop, but inside a newly defined loop.  Stuff left over on the
stack from the previous loop in which the same variable was used doesn't get
cleared out when the variable is redefined as an index for a new loop, which is
what causes the bug.

Art

On Thu, 1 Jun 2006, Frank Pittel wrote:

> I always that the loop counter was "undefined" outside the loop. By undefined
> I mean that the value is implementation dependant and not guaranteed to be any
> specific value.
> 
> 
> On Thu, Jun 01, 2006 at 05:31:18PM -0400, Arthur Flexser wrote:
> > There is actually a genuine bug in the way CoCo Basic processes FOR/NEXT.  It
> > was in fact the subject of an article in Byte in the early 80s because the same
> > bug occurs in various other Microsoft Basics as well.  My memory is a bit hazy
> > on the details, but here's what I recall;  perhaps someone here can concoct a
> > Basic program that illustrates the bug.  I believe that the bug produces a
> > spurious ?NF (NEXT without FOR) error.  I did once encounter it in a program I
> > wrote, soon after I bought my CoCo and before seeing the Byte article;  I called
> > Tandy about it (back in those days they actually had phone support), but the
> > techie I spoke with was stumped, not surprisingly.
> > 
> > The bug occurs when you exit an initial FOR/NEXT loop without completing it
> > (which is perfectly legal to do) and then reuse the same loop variable name in a
> > later loop.  It is critical that the NEXT statements contain the variable name
> > (i.e., NEXT Z, not just NEXT) for the bug to occur.  I think maybe the variable
> > that was previously used in an inner loop must be used later in an outer loop,
> > or possibly vice versa.  As I said, my memory is pretty vague on this, even
> > though I did at one time trace the source of the bug in the ROM code.
> > 
> > Anyone want to fiddle around with coming up with some code that produces the
> > spurious ?NF error?
> > 
> > Art 
> > 
> > 
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