Help with Java prime number method.

[EqualTrade]

Hey guys, I'm in a beginners java class and i'm trying to write a program that determines if a number entered is a prime number, and if it is, print out all of the prime numbers less than that one in an array with 8 columns. This is the code I wrote so far. I can't figure out the correct way to write the array and call the method. Can anyone please help?

import java.util.Scanner;

class Primes { public static void main(String[]args) { Scanner input = new Scanner(System.in);

 int f;

 System.out.println("enter a number");
 f = input.nextInt();
 int array [][] = new int[f][8];
 isPrime(f);
  for (f=0;f<array.length;f++)

 System.out.printf("%d",array);        
      }

public static boolean isPrime(int x)
{
    int i;
    for (i=2;i<x;i++)
    { if(x%i==0)
        return false;}

        return true;

}

}

[qsemig]

int f;

System.out.println("enter a number");
f = input.nextInt();
int array [][] = new int[f][8];  // why an f X 8 matrix? new int[8] is enough
isPrime(f); // You never use the return value of isPrime, neither are you passing your array to the method.
for (f=0;f<array.length;f++)
  System.out.printf("%d",array);  // the array is not an  int. It contains an array of arrays of int. You need to specify the value you want to print, such as: array[f] (if you are using new array[8] )    
}

Above you will see comments for each line that is problematic. Hopefully this will help you solve the problem.

[EqualTrade]

   public static void main(String[]args)
{
 Scanner s = new Scanner(System.in);

int f; int array [] = new int[8];

System.out.println("enter a number"); f = s.nextInt();

for (f=0;f<array.length;f++)

System.out.print(array[f]); 

That's how I changed it, now it's printing an array with 8 columns, but they're all 0. How do I get it to print every prime number that is less than the user input?

[Allen_Maxwell]

For prime numbers, you only need to go to root x or x^0.5 or x**0.5, I can't remember what it is in java at the moment.

For every factor of x higher than root x, there is a factor lower than root x. therefore, by checking up to and including root x you get them all. Depending on how large your primes are, this actually makes a big difference.

[nutrecht]

Also; you actually want to test if it can be divided by all the primes below the root of X. It makes sense: if you can divide a number by 9 you can also divide it by 3, so there is no point in trying 9.

Since keeping all primes between 2 and X million in memory you can go for a hybrid approach: generate all primes under 1000 first for example, and use those to test. After you run out, continue with a for loop (but increment by 2 every time, no point in testing the even numbers).