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From: Nick Leaton <nickle@calfp.co.uk>
Subject: Re: Software landmines (loops)
Date: 1998/09/02
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Date: 1998-09-02T00:00:00+00:00
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John Volan wrote:
> 
> Loryn Jenkins wrote:
> >
> > equal (l,r: LIST): BOOLEAN is
> >       require
> >           l /= Void and r /= Void
> >       do
> >           from
> >               Result := (l.count = r.count)
> >               l.start; r.start
> >           until
> >               not Result or l.off
> >           loop
> >               Result := (l.item = r.item)
> >               l.forth; r.forth
> >           end
> >       end
> 
> This has a certain elegance, but note that:
> 
> (1) If Result is False after the first assignment, then the 'start'
> calls are executed unnecessarily.
> 
> (2) If Result is False after the second assignment (in the body of the
> loop), then the 'forth' calls and the 'off' test are executed
> unnessarily.

If the until clause is changed to

	not Result or else l.off

Then if Result is false l.off is not evaluated. 'or else' and 'and then'
are semi-strict boolean operators and only evaluate the right hand side
if necessary.



-- 

Nick