Re: permutation of strings

[Brian Drummond <brian3@shapes.demon.co.uk>]

On Mon, 06 Oct 2014 22:38:25 -0700, mockturtle wrote:

> On Tuesday, October 7, 2014 7:23:53 AM UTC+2, Stribor40 wrote:
>> I have created this code using some online tutorials and some examples
>> that i tried to learn... i havent been able to find whats wrong with my
>> code....i am guessing what my variable j is somehow getting reset
>> during the recursive calls...
>> 
>> i have been trying to retrace recursive calls by writing each call on
>> the on the paper but i get lost trying to track it down......
>> 
>> my code spits out numbers...123 123 123 123 123 123 instead of 123 132
>> 213 231 321 312 so i was hoping if someone can take a look and suggest
>> how to fix this....
>> 
>> i pasted the code here it is ready to be compiled and there are no
>> errors when it is compiled....http://pastebin.com/rDF46RbR
> 
> At the moment I do not have a compiler at hand (I'm having breakfast
> :-), but I see at least one strange thing in your code: in the loop "for
> Index ...." I would expect calls to "change(s, i, Index)" rather than
> "change(s, i, j)".  With the latter there is no difference between
> different iterations and this could give rise to your error.

Nailed it by inspection!

> By the way, there is no need to have j declared "in out" in Change,
> since j is not changed. Moreover, I suspect that with the "in out"
> specification the compiler will complain, since you cannot change a loop
> index.

Having corrected the real bug, it did complain...

If I might add a couple more style changes:
1) Eliminate global variable "tmp" which should be local to procedure 
"change" 
2) Eliminate global variable "Start" and potential for off-by-1 bugs, by 
using the array attributes in the main call to Perms

Works as I would expect anyway.

- Brian
---------------------------------------------

with Ada.Text_IO, Ada.Integer_Text_IO;
use Ada.Text_IO, Ada.Integer_Text_IO;

procedure permutation is

  str : String := "123";

  procedure perms (s :  in out String; i : in Integer; n : in Integer);
  procedure change (s : in out String; i : in Integer; j : in Integer);

  procedure  perms (s : in out String; i : in Integer; n : in Integer) is
    j : Integer := i;
  begin
    if i = n then
      put(s);
      New_Line;
    else
      for Index in j..n loop
        change(s,i,index); 
        perms(s,i+1,n);
        change(s,i,index); 
      end loop;
    end if;
  end perms;
  
 
  procedure  change (s : in out String; i : in  Integer; j : in Integer) 
is
    tmp : Character;
  begin
    tmp  := s(i);
    s(i) := s(j);
    s(j) := tmp;
 end change;

begin
    perms(str,str'first,str'last);
end permutation;