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From: dennison@telepath.com
Subject: Re: Software landmines (loops)
Date: 1998/09/02
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Date: 1998-09-02T00:00:00+00:00
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In article <6sjk3p$4tc$1@hirame.wwa.com>,
  "Robert Martin" <rmartin@oma.com> wrote:
>
> Matthew Heaney wrote in message ...
>
> >Without being able to exit from the middle of a loop, I'd have to do
> >this to terminate a read sequence:
> >
> >  Get (N);
> >
> >  while N /= 0 loop
> >
> >      <process N>
> >
> >      Get (N);
> >
> >  end loop;
> >
> >So it's the structured approach which causes the redundancy, becuase Get
> >must be called twice.
>
> 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.


In Ada (which is what it looks like Matthew was writing in) there is no
"do-while" loop construct. It has to be simulated with the "exit" statement as
follows:

(sorry if the nesting isn't there. DejaNews' posting software has problems
with leading spaces)

   loop
      Get(N);
      if N then
        <process N>
      end if;
      exit when not N;
   end loop;

which looks pretty awkward (and performs twice as many comparisons) compared
to:

   loop
      Get(N);
      exit when not N;
      <process N>
   end loop;

Anyway, if you think middle-exits are bad from a maintenence perspective, then
in Ada you should probably stick to while and for. In your example there is
nothing stopping a later maintainence person from inserting a line between the
"exit" and the "end loop".

--
T.E.D.

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