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From: Charles Hixson <charleshixsn@earthlink.net>
Subject: Re: Software landmines (loops)
Date: 1998/09/06
Message-ID: <35F246E3.4424EF94@earthlink.net>#1/1
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X-ELN-Date: Sun Sep  6 01:22:07 1998
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Newsgroups: comp.lang.eiffel,comp.object,comp.software-eng,comp.lang.ada
Date: 1998-09-06T00:00:00+00:00
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I find myself not understanding the basis of this discussion.

To me it seems reasonable to ask, "Do reasonable experts in the field
consider that loops that exit in the middle are a part of structured
programming".

The answer, obviously, is:  This is not a matter of total agreement.

There have been agruments that say:  If you have to repeat blocks of
code, then you are violating the principles of structured programming.

There have been statements made that:  "Loops should be single
entry/single exit" (so that program correctness can be proved?).

These are both valid points.  But if the same code can be topologically
transformed from one form to another without altering it's deep flow of
control (e.g., pull all of the code within isolatable blocks into a
function call, and then call it, and accept a returned value.  Then we
can say
while (the)
  ...
  if (the) gostack := distimms (the, doshes,".");
loop

testing a flag each time throught the loop.  When is it true?  That's
hard to say without examining the code in detail (which I haven't
invented).

Is this really any different from:
while (the)
  ...
  exit when (\the);
  gostack := distimms (the, doshes,".");
loop

But this identity is only obvious because the huge chunk of code has
been factored out into the distimms routine.

In fact when we look at this, we can see that it is also equivalent to:

if (the)
{ beginloop
    ...
    exit when (\the);
    gostack := distimms (the, doshes,".");
  endloop
}
If these are all equivalent, does it really make sense to say that one
of them is structured and another isn't.  Consider a further pair of
transforms:
if (the)
{  when (firstPartOfTheLoop (the) )
   {   if (the) gostack := distimms (the, doshes, ".");
   }
}

and
if (the)
{  when (the)
   { flag := firstPartOfTheLoop (the);
     if (the) gostack := distimms (the, doshes, ".");
   }
   flag := firstPartOfTheLoop (the);
}

Now, I obviously haven't checked these with a compiler, but baring typos
I think that is is obvious that these pieces of code are all isomorphic,
so it seems TO ME unreasonable to claim that some of them are structured
and others of them aren't.

I would like to hear about why others would find it reasonable.