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From: Robert Dewar <robert_dewar@my-deja.com>
Subject: Re: Eight Queens problem (was Re: Kindness)
Date: 1999/09/08
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In article <7r2fc6$1sc$1@nnrp1.deja.com>,
  bourguet@my-deja.com wrote:
> Do you want to say it is the canonical example of use or it is
an as
> good example: there is another way to solve the problem which
usually
> come first to mind?

Nothing so complicated, just that, as factorial is typically
used in a text book to illustrate recursion, 8Q is used to
illustrate backtracking.

I agree that simple tail recursion is a horrible example for
motivating recursion. Here is the example I always use, with
a little driver:

with Text_IO; use Text_IO;
procedure p is
   procedure Print_Int (I : Natural) is
   begin
      if I >= 10 then
         Print_Int (I / 10);
      end if;
      Put (Character'Val (Character'Pos ('0') + I mod 10));
   end Print_Int;
begin
   Print_Int (12435);
end;

The nice thing about this is that it does simply something
which is annoyingly fiddly to do iteratively. In particular,
you have to worry about the maximum number of digits which
simply does not arise. The point is made even more strongly
in a language which does not have nasty artificial limits on
the size of integers as in C or Ada or Java :-)

Robert Dewar


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