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From: "Dan Higdon" <hdan@charybdis.com>
Subject: Re: Software landmines (loops)
Date: 1998/09/02
Message-ID: <9hdH1.464$08.176350@news.giganews.com>#1/1
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Newsgroups: comp.lang.eiffel,comp.object,comp.software-eng,comp.lang.ada
Date: 1998-09-02T00:00:00+00:00
List-Id: <comp.lang.ada>

Robert Martin wrote in message <6sjk3p$4tc$1@hirame.wwa.com>...
>Consider:
>
>   do {
>      Get(N);
>      if (N)
>        <process N>
>   } while (N);
>
>Structured programming allows the loop condition to be either at the top or
>the bottom.

You just rolled the loop around - you're still testing the condition twice,
you just
made the *test* redundant.  Having a single entry/exit point is good.
Forcing
the exit point to the top or bottom of the construct is artificial, IMHO.
There are
many cases (and this is a particularly good one) where the test to exit does
not naturally occur at either the top or bottom of the loop, but midway
through it.

As an aside, a valid (IMHO) Eiffel extension would be to allow loops a
little more freedom (yeah, I know, "everyone's a language designer" :-):

-- standard
from <init> until <test> loop
    <statements>
end

-- post test
from <init> loop
    <statements>
until <test> end

-- in test
from <init> loop
    <statements>
until <test> else
    <statements>
end

Ignoring {in}variants for simplicity.  (I suppose {in}variants would need to
hold
true at the "until".)

so, our example becomes
from
    <init>
loop
    Get (N)
until
    not N
else
    <process N>
end

----------------------------------------
hdan@charybdis.com
"Throwing fire at the sun"