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Prime Number program not working

post #1 of 4
2/13/12 at 7:31pm
Thread Starter 
Well I am using nested loops in ADA to find the 10,001 prime number. and here is my current code.
Code:
with Ada.Integer_Text_IO;

procedure test is

   P : Integer;
   C : Integer;
   S : Integer;

begin

   P := 1; -- Prime Number
   C := 0; -- Counter
   S := 1; -- What I am dividing by

   O_Loop :
   loop
      if (P rem S) = 0 then
         P := P + 1;
         S := 2;
      else
         I_Loop :
   loop
      if (P rem S) /= 0 then
         S := S + 1;
         C := C + 1;
            exit I_Loop when (P rem S) = 0;
            exit O_Loop when C = 10_001;
      end if;
   end loop I_Loop;
      end if;
   end loop O_Loop;
   Ada.Integer_Text_IO.Put (Item => P);
end test;

And while it should take 10,001 counter add ups to reach the prime number 104,743 it is taking 497,000,000 counter add ups.

I need to find a better way for finding the prime numbers than my current setup. I know the counter most likely isn't in the right place either thus taking so many counter add ups to reach the prime number.

Any help would be greatly appricated.

Thanks,
Bob
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post #2 of 4
2/13/12 at 8:35pm
Well, let me try to understand and feel to correct me, your code is set up like this:
Code:
P,C,S variable declaration
While loop ( c < 10001 )
     if ( P rem S = 0 )
     else
          While loop ( P rem S = 0 )
               if ( P rem S /= 0 )
               end if
          end loop
     end if
end loop

Now, one small thing:
"1" isn't considered a primer number, so you should just ignore it and begin P at "2" instead of "1"

Second, your counter is adding up every time ( P rem S ) is different from "0", from my understanding, it only should add up every time it finds a prime numbers. IE: Since 3%2 is /=0 then it adds up a counter, that's wrong since that doesn't mean it has found a prime number.

I have a code in mind, but it's totally different from yours, and pretty basic I think, if you want I can post the idea here.
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post #3 of 4
2/13/12 at 8:44pm
Thread Starter 
Please any help would be greatly appricated.

Also should I be using square roots to find it?
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post #4 of 4
2/13/12 at 9:05pm
Well, square root helps alot, my code without square roots takes around 17 secs while using square root takes 0 secs according to netbeans.

This is how I pretty much did it, will write the idea since I have no idea about ADA:

<- will be my assignation
= will be equal to
sqrt(x) will refer to the square root of x
Code:
Integer Num <- 2 // This will be the number that I will be increasing
Integer Counter <- 0 // Primer counter
Integer Divisors // The ammount of divisors a number has
Integer I // The numbers I'll be dividing by
While Loop ( Counter < 10001 ) // This will be the loop until it finds the 10,001 prime number
  Divisors <- 2 // If it's a prime number then it it divisible by 2 numbers
  I <- 2; // It's always divisible by 1 so there is no need to check 1
  While Loop ( I <= sqrt(Num) && Divisors = 2) // If it finds already another number below or equal to its square root, then it isn't a prime. Also, if it finds another divisor, there is no need to keep looking
    If ( Num rem I = 0 ) // Checks if I find a divisor
      Divisors <- Divisors + 1 // If it does, add it
    End If
    I <- I + 1 // Adds to I to to check the next number
  End While Loop
  If ( Divisors = 2 ) // If the divisors stayed at "2" then it's a prime number
    Counter <- Counter + 1 // Therefore I add "1" to my counters
  End If
  Num <- Num + 1 // Adds to number to check the next one
End While Loop
Write Num - 1 // Write Num-1 since I add +1 at the end of my loop

That's how I did it, if you have any questions feel free to ask
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