Exercise07_06.java

package ch_07;

import java.util.*;

/**
 * *7.6 (Revise Listing 5.15, PrimeNumber.java)
 * Listing 5.15 determines whether a number n is prime by checking whether 2, 3, 4, 5, 6, ..., n/2 is a divisor.
 * If a divisor is found, n is not prime. A more efficient approach is to check whether any of the
 * prime numbers less than or equal to 2n can divide n evenly. If not, n is prime.
 * Rewrite Listing 5.15 to display the first 50 prime numbers using this approach.
 * You need to use an array to store the prime numbers and later use them to check
 * whether they are possible divisors for n.
 *
 * @author Harry Dulaney
 */
public class Exercise07_06 {
    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?
            // Exercise25_01 whether 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
                }
            }
            // Display the prime number and increase the count
            if (isPrime) {
                count++; // Increase the count
                if (count % NUMBER_OF_PRIMES_PER_LINE == 0) {
                    // Display 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++;
        }
    }
}