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From: Gene <gene.ressler@gmail.com>
Newsgroups: comp.lang.ada
Subject: Re: Processing array subsections, a newbie question.
Date: Sat, 12 Jun 2010 18:20:26 -0700 (PDT)
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Date: 2010-06-12T18:20:26-07:00
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On Jun 12, 4:54=A0pm, Ludovic Brenta <ludo...@ludovic-brenta.org> wrote:
> "Peter C. Chapin" <pcc482...@gmail.com> writes:
>
> > Ok =A0 =A0 =A0 =A0: Boolean :=3D True;
> > Index_Fst : Natural :=3D Buffer'First;
> > Index_Lst : Natural;
> > ...
>
> > while Ok and Index_Fst <=3D Buffer'Last loop
> > =A0 Do_Something(Buffer, Index_Fst, Index_Lst, Ok);
> > =A0 Index_Fst :=3D Index_Lst + 1;
> > end loop;
>
> > The problem with this code is that the assignment to Index_Fst inside t=
he loop
> > might raise Constraint_Error if Index_Lst =3D Buffer'Last after Do_Some=
thing
> > finishes. I can work around this problem but my solutions tend to be ra=
ther
> > ungainly looking. Surely there must be an easy way to handle this.
>
> How about:
>
> Ok =A0 =A0 =A0 =A0: Boolean :=3D True;
> Index_Fst : Natural :=3D Buffer'First;
> Index_Lst : Natural;
> ...
> loop
> =A0 =A0Do_Something(Buffer, Index_Fst, Index_Lst, Ok);
> =A0 =A0exit when not Ok;
> =A0 =A0exit when Index_Fst =3D Buffer'Last;
> =A0 =A0Index_Fst :=3D Index_Lst + 1;
> end loop;

Yes!  Or save a line with

loop
=A0 =A0Do_Something(Buffer, Index_Fst, Index_Lst, Ok);
=A0 =A0exit when not Ok or Index_Fst =3D Buffer'Last;
=A0 =A0Index_Fst :=3D Index_Lst + 1;
end loop;

Loop exit (with optional naming, which obviates the problems of
'break' in C and its successors) is one of the small beauties of Ada
and an example of the way things should have been - with benefit of
hindsight - in all ALGOL-like languages.  Rather than while ... and
do ... while ... , which are inherently limited in expressive power,
every language ought to offer loop ... exit ...  end; Lord knows it
would make teaching beginning programming easier.  My favorite example
is

loop
  Put(prompt to user);
  Get(user input);
  exit when user input is okay;
  Put(the input was not correct because...);
end loop;

rather than the more complex, ugly, and potentially less efficient

declare
  Boolean Input_Okay =3D False;
begin
  while not Input_Okay loop
    Put(prompt to user);
    Get(user input);
    Input_Okay :=3D test for user input is okay;
    if not Input_Okay then
      Put(the input was not correct because...);
    end if;
  end while;
end;

And my experience is that a language construct that is good for
beginning programmers is always good for general practice.