Calculate primes help

TommyFiz · #1 (**permalink**) 27-10-2009, 10:31 PM

I'm getting the following error. The idea is to have the user input a number, and then print out all the primes less than the number.

Java Code

Enter a value: 
9
Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: 0
	at Primes.main(Primes.java:18)

I've tried several numbers, most numbers don't give me the error, but 7, 9, 11 are a few that do. The ones that don't give me the error, what is wrong with my print statement that makes it so it never prints?

Here is my code

Java Code

import java.util.*;
public class Primes {
	public static void main(String[] args){
		Scanner stdin = new Scanner(System.in);
		int x = 0;
		boolean isPrime = true;
		int ARRAY = 0;
		int primes[] = new int[ARRAY];
		
		System.out.println("Welcome to Prime Calculator!");
		System.out.println("Enter a value: ");
		x = stdin.nextInt();
		
		for(int i = 2; i < (x/2) && isPrime == true; i++){
			if (x % i == 0){
				isPrime = false;
			} else {
				primes[ARRAY] = i;
				ARRAY++;
			}
		}
		for(int i = 0; i < ARRAY; i++){
			System.out.println(primes[0]);
		}
	}
}

helloworld922 · #2 (**permalink**) 28-10-2009, 12:56 AM

I'm not sure what algorithm you have there, but the most efficient method to do this is by the Sieve of Eratosthenes.

You're getting the ArrayIndexOutOfBounds exception because you initialized primes[] to size 0, instead of to size x

Java Code

System.out.println("Welcome to Prime Calculator!");
System.out.println("Enter a value: ");
x = stdin.nextInt();
primes = new int[x];

TommyFiz · #3 (**permalink**) 28-10-2009, 02:57 AM

I read a lot about that, and I feel like since we haven't learned about it, that's not what is expected in the program. My algorithm may not be the most efficient, but it seems like it should make sense? If the remainder of something divided by 2 = 0 then its not prime, if it is, try 3, if that is 0, then it's not prime, if it is, try 4, and so on..

Substituting x into the array got rid of the error message, but now onlys prints the number 2--sometimes once, sometimes twice, sometimes three times

helloworld922 · #4 (**permalink**) 28-10-2009, 03:41 AM

That's part of the fun of computer science: learning about new (well, in this case really old) algorithms that perform a given task.

The reason why you're algorithm isn't working is because you're checking the primality of x. If you want to go about it that way, I'd recommend writing a helper function that will check to see if a number is prime, then everytime you find a prime number, print it out.

Java Code

for (int i = 2; i < x; i++)
{
     if(isPrime(i))
     {
          System.out.println(i + " is prime");
     }
}

A simple primality checker:

Java Code

for every integer i from 2 to the number:
if the number is divisible by i, return not prime
end
return true

There are several optimizations that you can make, the biggest being that you only have to check numbers up to the square root of the number. The second is only to check to see if the number is divisible by 2, then return false. Then, only check to see if your number is divisible by odd numbers from 3 to sqrt(number)

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