PrimeTime.java

package tenprojecteulersolutionsinjava;

// Problem 7 primes at index 1001

import java.util.Scanner;

public class PrimeTime

{
  public static void main(String[] args)
  {
     
        final int NUMS_PER_LINE    = 12; // Display 12 Numbers before indenting to next line
        int count                  = 0;  // Count the number of prime numbers
        int number                 = 2;  // A number to be tested for primeness
        int squareRoot             = 1;  // To check whether number less than or equal to  squareRoot
      
       
    Scanner inputN = new Scanner(System.in);
    System.out.print("Find all prime numbers <= n, enter n: ");
    int n = inputN.nextInt();

    // A list to hold prime numbers
    java.util.List<Integer> primeList =
      new java.util.ArrayList<Integer>();
    
    System.out.println("The prime numbers are: \n");
   
    // Repeatedly find prime numbers
    while (number <= n)
   {
      // Assume the number is prime
      boolean isPrime = true; // Is the current number prime?

      if (squareRoot * squareRoot < number) squareRoot++;

      // ClosestPair if number is prime
      for (int i = 0; i < primeList.size()
                        && primeList.get(i) <= squareRoot; i++)
      {
        if (number % primeList.get(i) == 0)
        { // If true, not prime
          isPrime = false; // Set isPrime to false         
          break; // Exit the for loop
        }
      } // close for

      // Print the prime number and increase the count
      if (isPrime)
      {
        count++; // Increase the count
        primeList.add(number); // Add a new prime to the list
        if (count % NUMS_PER_LINE == 0)
        {
          // Print the number and advance to the new line
          System.out.println(number);
        }
        else
          System.out.print(number + " ");
      } // close outer if

      // Check if the next number is prime
      number++;
    } // close while

    System.out.println("\n" + count +
      " prime(s) less than or equal to " + n);
    
    Scanner inputPrimeIndex = new Scanner(System.in);
    System.out.print("Enter an index to check the value at that index (e.g. entering '0' checks the first prime in the list) ");
    int primeIndex = inputPrimeIndex.nextInt();
   
    System.out.println("The prime at index" + primeIndex + " is " + primeList.get(primeIndex) + " \n");
    
    inputPrimeIndex.close();
    inputN.close();
    
  } // close main()
} // close class