[fpc-pascal]Re: "for" loop questions

[XHajT03 at mbox.vol.cz]

From:           	"Balogh, Karoly (Charlie/iNQ)" <charlie at scenergy.dfmk.hu>
To:             	fpc-pascal at deadlock.et.tudelft.nl
Date sent:      	Sun, 08 Dec 2002 08:50:51 +0100
Subject:        	[fpc-pascal]Re: "for" loop questions
Send reply to:  	fpc-pascal at deadlock.et.tudelft.nl

   Hi,

> > I have a question regarding for loops.  If you execute the following
> > code:
> > 
> > var
> >  L : TList;
> >  P : Pointer;
> >  I : Integer;
> > begin
> >  L := TList.Create;
> > 
> >  for I := 0 to L.Count - 1 do begin
> >    P := L[I];
> >  end;
> > end;
> > 
> > You get an exception telling you that the list index is out of
> > bounds. L.Count returns "0", but the loop still executes (or at
> > least, it tries to) once, which is incorrect.  According to every
> > language I've ever used, if L.Count is 0 the first time, the loop
> > should never execute at all.

 I just tried it. If I just add "{$MODE DELPHI}" and "uses Classes;" 
at the beginning, and change your final semicolon to period, I don't 
get any error message - the loop does _not_ execute. At least this is 
true with my 1.0.7 snapshot compiled November 20. So it works 
correctly.

> > Is this a bug, or am I missing something?  Of course the fix is to
> > just put an "if L.Count > 0" before the "for" loop, but that's not
> > the point - the point is I shouldn't have to.  :)
> 
> But you should. :) I don't know the type of L.Count, but i suppose
> it's unsigned. If L.Count is 0, and you try to count until L.Count-1,
> it it simply underflows at the -1 operation. And i will count to $FF,
> $FFFF or $FFFFFFF, or whatever, depending on the type of L.Count. 

 This isn't really correct either; this would only be true if you 
assign the result of subtraction to an unsigned variable; the 
calculation itself is done after conversion to a signed type.

> And this is why you get an exception.

 Not really, see above. The exception doesn't seem to come from the 
quoted code fragment (unless you've used some other compiler version 
and there's some bug in that particular version).

Tomas