prime numbers

[ashiphire]

I've been trying to make a program that finds prime numbers but all I've been able to do is check if specific numbers are prime numbers not check if all are

can anyone help?

here's my code

Turing:

%if false is output then it is not a prime number var number : int put "please enter a number: " .. get number for decreasing i : number div 2 .. 2     if number mod i > 0 then     else         put"false"     end if end for put "finished"

Edit by Clayton: Code tags.

[Clayton]

You need to keep track of how many factors (or lack thereof) that you have found. Any more than 0 and you know that the number is not prime.

[Tony]

more than 2

1 and number itself are valid factors for a prime

Edit: nevermind, I can see that Clayton was referring to the specific implementation above, that doesn't get to check either of those numbers. Also, the loop doesn't work well for confirming that 2 is prime.

[Vorpike]

That just tells you whether a number is even...the code you're looking for is this

Turing:

% PROGRAM: Prime.t % NAME: Vorpike % DATE: November 16, 2012 % PURPOSE: Determines whether a number is a prime var number : int var count : int := 0 put "Enter number" get number for i : 2 .. number     count := count + 1     exit when number mod i = 0 end for if count < number - 1 then     put number, " is not prime" elsif count = number - 1 then     put number, " is prime" end if

Basically, you check if any number between 2 and the number is divisible, and exit when a match is found, the count is to prevent submitting both messages "is prime" and "not prime".

Sorry for the late reply, just got on this forum...hate the registration, took me 10 tries to get past the confirmation code...how does one get the turing embed

EDIT (by AJ): added code tags

[Panphobia]

Basically make a for loop from 1, half the number you are checking, then check if the num mod counter not= 0 c+=1 and if c is equal to the num/2 -1 then it is a prime

[A.J]

@ashiphire- Based on your question, I am assuming that you are looking for a method to check if some set of numbers are prime without individually checking each number. One well known way of finding all primes less than or equal to some specific number is the Sieve of Eratosthenes (wiki does a pretty good job at explaining what this is: http://en.wikipedia.org/wiki/Sieve_of_Eratosthenes).