Re: Prime number program?

["Robert I. Eachus" <eachus@mitre.org>]

Oly wrote:
  
> I have to build an Ada program which tells the user if the number which they
> have entered is a prime number or not.
> I am still a newbie at Ada (and crap at maths)..and have spent many wasted
> and frustring hours trying to solve the problem.

  I have always liked this method for testing primality.  But I suggest
you study both Ada and Number Theory before handing this example in as
your own work...

with Ada.Text_IO, Ada.Integer_Text_IO;
procedure Prime is
-- a program using Wilson's Theorem to test for primality.  May fail for
-- numbers greater than 2**(Natural'Size/2).
-- Enter zero or a negative value to exit.
  P: Integer;
  Temp: Integer;
  use Ada.Text_IO, Ada.Integer_Text_IO;
begin
  loop
    Put_Line("Enter candidate:");
    Get(P);
    Skip_Line;
    exit when P <= 0;
    Temp := 1;
    for I in 2..P-1 loop
      Temp := (Temp*I) mod P;
    end loop;
    if Temp = P-1
    then Put(P,12); Put_Line(" is prime.");
    else Put(P,12); Put_Line(" is not a prime.");
    end if;
  end loop;
  Put_Line("  All done.");
exception
  when others => Put_Line("  Ooops!");
end Prime;

-- 

                                        Robert I. Eachus

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