Thread: Need help.. Counting Prime #'s up to 50 w/while loop

Hi everyone,

I'm taking an intro Java course, which I'm quite honestly, not very good at it. Anyways, we have this Prime # program that counts all the Prime #'s up to 50 using a for loop:

public class PrimeNumber {
  public static void main(String[] args) {
    final int NUMBER_OF_PRIMES = 50; // Number of primes to display
    final int NUMBER_OF_PRIMES_PER_LINE = 10; // Display 10 per line
    int count = 0; // Count the number of prime numbers
    int number = 2; // A number to be tested for primeness
 
    System.out.println("The first 50 prime numbers are \n");
 
    // Repeatedly find prime numbers
    while (count < NUMBER_OF_PRIMES) {
      // Assume the number is prime
      boolean isPrime = true; // Is the current number prime?
 
      // Test if number is prime
      for (int divisor = 2; divisor <= number / 2; divisor++) {
        if (number % divisor == 0) { // If true, number is not prime
          isPrime = false; // Set isPrime to false          
          break; // Exit the for loop
        }
      }
 
      // Print the prime number and increase the count
      if (isPrime) {
        count++; // Increase the count
 
        if (count % NUMBER_OF_PRIMES_PER_LINE == 0) {
          // Print the number and advance to the new line
          System.out.println(number);
        }
        else
          System.out.print(number + " ");
      }
 
      // Check if the next number is prime
      number++;
    }
  }
}

The task is to convert the for-loop under the comment //Test if number is prime, into a while-loop, which I can't seem to get right.
This is how I changed the code:

public class Lab3_2 {
  public static void main(String[] args) {
    final int NUMBER_OF_PRIMES = 50; // Number of primes to display
    final int NUMBER_OF_PRIMES_PER_LINE = 10; // Display 10 per line
    int count = 0; // Count the number of prime numbers
    int number = 2; // A number to be tested for primeness
 
 
    System.out.println("The first 50 prime numbers are \n");
 
    // Repeatedly find prime numbers
    while (count < NUMBER_OF_PRIMES) {
      // Assume the number is prime
      isPrime = true; // Is the current number prime?
 
      // Test if number is prime
      int divisor = 2;
      while (divisor <= number / 2; divisor++){
        if (number % divisor == 0) { // If true, number is not prime
           isPrime = false; // Set isPrime to false
          break; // Exit the for loop
        }
      }
 
      // Print the prime number and increase the count
      if (isPrime) {
        count++; // Increase the count
 
        if (count % NUMBER_OF_PRIMES_PER_LINE == 0) {
          // Print the number and advance to the new line
          System.out.println(number);
        }
        else
          System.out.print(number + " ");
      }
 
      // Check if the next number is prime
      number++;
    }
  }
}

Any assistance is greatly appreciated!!

Sam

A few things:

First off, it is SO refreshing to see a first time poster post correctly. It is very appreciated.

Second, does your changed code work?

Third, if it doesn't work, here is basically what you need to do. Essentially, you need to check to see if each number between 2 and half of the number is divisible. Now, I believe you are doing this now, but there is a little something you can do to make it faster. You don't have to check every number.
The very nice thing about even numbers is:
If the number is divisible by 2, it will not be prime.
But if the number is not divisible by 2, it will also not be divisible by any other even number, as all even numbers are divisible by 2. So if you are after making it a little faster, you can actually count by 2s instead of 1s. Even better than that, if I am not mistaking, every number between 2 and 9 serve as a standard set of possible divisibles. Where if the number is not divisible by any numbers below 10, and if it is not prime, then it must be divisible by another prime number. Essentially, after getting to 11, there is no need to check every number but rather only the prime numbers for divisibility. This way of finding prime numbers would require you to keep an array of prime numbers you find, and it is really only practical if you are looking for super high prime numbers. It is an interesting thought though.

Quicker method: Sieve of Eratosthenes

A simple method for converting a for-loop into a while loop:

for(init; condition; increment)
{
 // loop code
}

init;
while(condition)
{
    // loop code
    increment;
}

Why do you increment divisor prior to checking if number is prime or not?