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From: Matthew Heaney <matthew_heaney@acm.org>
Subject: Re: Software landmines (loops)
Date: 1998/09/01
Message-ID: <m3zpck79xp.fsf@mheaney.ni.net>#1/1
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NNTP-Posting-Date: Tue, 01 Sep 1998 04:01:48 PDT
Newsgroups: comp.lang.eiffel,comp.object,comp.software-eng,comp.lang.ada
Date: 1998-09-01T00:00:00+00:00
List-Id: <comp.lang.ada>

Loryn Jenkins <loryn@s054.aone.net.au> writes:

> 
>  equal (l,r: LIST): BOOLEAN is
>        require
>                l /= Void and r /= Void
>        do
>                Result := l.count /= r.count
>                if Result then
>                        from
>                                l.start; r.start
>                        until
>                                not Result or l.off
>                        loop
>                                Result := l.item /= r.item
>                                l.forth; r.forth
>                        end
>                end
>        end
> 
> 
> > The way I look at this problem, is that it's like searching for a
> > specific house on an unfamiliar block.  As you're driving, when you find
> > the house, you stop immediately and declare success.  You don't have to
> > drive to the end of the block, and then say where the house was.
> 
> Yeah, yeah. Same here, given the amended code.

No, no, not the same here.

If the test l.count /= r.count returns False, then you know immediately
what the return value of the function is, so why not deliver that
information right away?  That way you can remove the test of result
(reducing the level of nesting), and simplify the predicate,

(By way of analogy, if you've found the house, then you can park in the
driveway right way, instead of driving to the end of the block to
announce that you found the house.)

Let's once again compare the decision tables.  If we re-write the code,
to put it into Matt-like (because Matt likes it) syntax:


equal (l,r: LIST): BOOLEAN is
      require
              l /= Void and r /= Void
      do
              if l.count /= r.count then
                 return False
              end
              
              from
                 l.start; r.start
              until
                 l.off
              loop
                 if l.item /= r.item then
                    return False
                 end

                 l.forth; r.forth
              end

              return True
      end


This version has only two rules in the decision table for the loop predicate:

       1  2
l.off  T  F


The uncorrected version has four rules:

            1  2  3  4
not Result  T  T  F  F
l.off       T  F  T  F


The difference in styles is really a debate about the number of edges in
the flow graph vs the number of rules in the decision table.