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From: Niklas Holsti <niklas.holsti@tidorum.invalid>
Newsgroups: comp.lang.ada
Subject: Re: Permutation generator in ada library
Date: Fri, 03 Oct 2014 09:28:35 +0300
Organization: Tidorum Ltd
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Date: 2014-10-03T09:28:35+03:00
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On 14-10-03 07:51 , Stribor40 wrote:
> Is there Ada library for this .....
> 
> Code from that link gives wrong output...lets say for 3 gives..
> 
>  1 2 3
>  1 3 2
>  2 1 3
>  2 3 1
>  3 1 2
>  3 2 1
>  3 2 1 <- this shouldnt be there??

Right. The author of this program has confused herself with the
"Is_Last" output parameters of the procedures, and has not documented
them properly. They work differently for the two procedures:

- Generic_Perm.Set_First returns Is_Last = True when the permutation it
PRODUCES is the last one (that is, when N = 1).

- Generic_Perm.Go_To_Next returns Is_Last = True when the permutation it
is GIVEN is the last one, and it does not produce a new permutation in
that case.

The loop in Print_Perms consequently should have a more complex form,
for example like this:

      Perms.Set_To_First(P, Done);
      Print (P);
      if not Done then
         -- There is more than one permutation, print them:
         loop
            Perms.Go_To_Next(P, Done);
            exit when Done;
            -- There was a next permutation, print it:
            Print(P);
         end loop;
      end if;

-- 
Niklas Holsti
Tidorum Ltd
niklas holsti tidorum fi
      .      @       .