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From: Richard Melvin <rmelvin@radm.demon.co.uk>
Subject: Re: Software landmines (loops)
Date: 1998/09/04
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Date: 1998-09-04T00:00:00+00:00
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In article <35EFD468.BDD7CB0A@irisa.fr>, Jean-Marc Jezequel <Jean-
Marc.Jezequel@irisa.fr> writes
>
>        Result := l.first /= r.first
>        from l.start; r.start
>       invariant equal_so_far: --foreach i such as 0<i<l.current_index 
>                                       -- r.item(i) = l.item(i)
>        variant l.count-l.current_index+1
>        until not Result or l.off --r.off omitted since l.off=>r.off
>        loop
>              Result := l.item /= r.item
>              l.forth; r.forth
>        end

Nice proof.

Unfortunately your code contains at least one, and arguably two, bugs:

- will throw an exception or return wrong result on unequal 
  length lists.

- will interfere with iteration over lists in the calling function 
 (unless the language you are using passes lists by value, 
  not identity).

I've got a theory as to the people writing strict single/entry single
code are having such a hard time with this (defect rate > 20%):

- iteration through a list is actually quite hard, through two 
  lists multiply so - there are quite a lot of opportunities for error.
  Of course, it doesn't seem that hard to most people, as they have
  memorised the correct way to loop through a list, given their 
  language and library.

- With tree structured code, you can use this memorised pattern
  of iteration every time, and put the custom code within the loop 
  body, including early exit.

- with strict structured code, you can't do this, as you have to 
  adjust the loop condition, at least introducing new terms, 
  and possibly adjusting existing ones.

Anyway, it's just a theory - no doubt someone will now post a batch 
of tree-structured code with bugs.
-- 
Richard Melvin