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From: eachus@spectre.mitre.org (Robert I. Eachus)
Subject: Re: Software landmines (loops)
Date: 1998/09/03
Message-ID: <EACHUS.98Sep3191740@spectre.mitre.org>#1/1
X-Deja-AN: 387654166
References: <slrn6sopmg.4fg.jdege@jdege.visi.com>
 <m31zpxqutn.fsf@mheaney.ni.net> <6sf4gl$hb6@flatland.dimensional.com>
 <m3af4kpyr9.fsf@mheaney.ni.net> <6siqo0$hiv@flatland.dimensional.com>
Organization: The Mitre Corp., Bedford, MA.
Newsgroups: comp.lang.eiffel,comp.object,comp.software-eng,comp.lang.ada
Date: 1998-09-03T00:00:00+00:00
List-Id: <comp.lang.ada>

In article <6siqo0$hiv@flatland.dimensional.com> jtc@dimensional.com (Jim Cochrane) writes:

 > Looking at your routine, you need to check 3 different places to check that
 > the postcondition is upheld, the 3 return statements:  In the first if
 > statement, in the second if statement in the middle of the loop, and at the
 > end of the routine.  This is a pretty straightforward process, but I think
 > it makes it a little more difficult in that you need to check 3 separate
 > locations, including in the middle of the loop.

     But this is irrelevant to the way the code was written, and to
the way it should be understood.  The code flows from this problem
statement:  "Two lists are equal if they are the same length and each
element in the first list is equal to the corresponding element in the
second list."

     Now the postconditions corresponding to the three exits are:

     Lengths not equal, lists not equal.
     Element L.Items(Index) /= R.Items(Index), lists not equal.
     Lengths and all elements equal, lists are equal.

     Before you jump all over me, realize that this is a DIFFERENT way
of treating postconditions, factoring the full statement of the
postcondition into separate cases, and having an exit/return for each
case.  There are many cases where it results in the best code, there
are others where it is not the method of choice, but if you are
willing to accept multiple returns, then you should add this approach
to your bag of tricks.

     (Notice that it works best when you can linearize the cases as
you can here.)
--

					Robert I. Eachus

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