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From: Matthew J Heaney <mheaney@on2.com>
Subject: Re: How to make like Fortran "do i = 1,20,2"
Date: 2000/07/27
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Newsgroups: comp.lang.ada
Date: 2000-07-27T00:00:00+00:00
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Gary Scott <Gary.L.Scott@lmtas.lmco.com> writes:

> Hmmm, these and similar examples posted do not make Ada look very
> elegant...it makes a very simple concept seem somewhat convoluted.

This "very simple concept" is a major source of bugs -- that's why it's
not in the language.

If you want to increment by 2 through the loop, it's simple enough:

declare
   I : Positive := A'First;
begin
   while I <= A'Last loop
      ... A(I) ...
      I := I + 2;
   end loop;
end;

QED


If you want to do odd-ball things, then sometimes that requires extra
work.  The common things (like incrementing the loop index by 1) are
easy to do.

I can't even remember the last time I had to iterate over an array by an
increment other than 1.

Consider this analogy.  Suppose you want to iterate over a linked list:

   list<int> intList;
...
   list<int>::iterator iter = intList.begin();

In the normal case, when you increment by 1, it's simple:

   while (iter != intList.end())
   {
     ...
     iter++;
   }


Asking how to iterate in increments of 2 is like asking, How do I visit
every other list element?  That's an odd-ball thing to do, so you have
to do extra work.  (Try it and you'll see.)