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From: Darren New <dnew@fv.com>
Subject: Re: Software landmines (loops)
Date: 1998/09/01
Message-ID: <35EC36E9.2FBC9E79@fv.com>#1/1
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Organization: First Virtual Holdings Inc
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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 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

And of course, this becomes much more readable when you use variables
whose names mean what they say.

equal (l,r: LIST): BOOLEAN is
       require
           l /= Void and r /= Void
       local stacks_are_known_different : BOOLEAN
       do
           from
               stacks_are_known_different := (l.count /= r.count)
               l.start; r.start
           until
               stacks_are_known_different or l.off
           loop
               stacks_are_known_different := (l.item /= r.item)
               l.forth; r.forth
           end
           Result := not stacks_are_known_different
       end

(With a minor renaming to get rid of "not". I don't *think* I broke it.)
I think picking the proper names for locals invariably clarifies the
code, even at the expense of declaring another local. The first time you
assign to "Result", you're not assigning the "result".

One of my rules of thumb for clear programming is "have a meaning for
every variable that is appropriate at every line of the algorithm".
Otherwise, you're using the same variable for multiple unrelated values.
Here, it's clear just by looking that you know the stacks are different
if the counts are different or the items are different, you stop the
loop as soon as you know the stacks are different, and the result is
whether you know the stacks are different at the bottom of the loop.

This is a bit more pendantic than I usually am, but I think it's *very*
clear, and certainly more clear than embedded returns.

-- 
Darren New / Senior Software Architect / First Virtual Holdings Inc
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