PrimeFinder.java

import java.util.Scanner;

public class PrimeFinder
{

  /** Returns the number of prime numbers between lower and upper inclusive
   *  Precondition: 0 < lower <= upper
   */
  public static int primesBetween(int lower, int upper)
  {
	  /* to be implemented in part (a) */
    int count = 0;

    for (int i = lower; i <= upper; i++) {
      if (isPrime(i))
        count++;
    }

    return count;
  }

  /** Returns the difference between num and the closest prime number which
   *  is greater than or equal to num
   *  Precondition: num is positive
   */
  public static int gapToNextPrime(int num)
  {
    int count = 0;
    for (int i = num; !isPrime(i); i++) {
      count++;
    }

	  /* to be implemented in part (b) */
    return count;
  }

  /** Returns true if the integer n is a prime number
   *  Precondition: n is positive
   */
  // private static boolean isPrime(int n)
  // {
	//   /* implementation not shown */
  //   if (n <= 1) {
  //     return false;
  //   }
    
  //   // if n % (n - 1) is equal to 0, n is not a prime number. if not, check whether n % (n - 2) is equal to 0. continue until n - (n - 1) 
  //   for (int i = n - 1; i > 1; i--) {
  //     System.out.println("DEBUG: n = " + n + ", i = " + i + ", n % i = " + (n % i));
  //     if (n % i == 0) {
  //       return false;
  //     }
  //   }

  //   // if n is not divisible by a number that is between 2 to n - 1, n is a prime number
  //   return true;
  // }

  private static boolean isPrime(int n)
  {
    /* COMPLETE WORKING METHOD PROVIDED */
    /* DO NOT MODIFY THE IMPLEMENTATION OF THIS METHOD*/
    if(n <= 1) return false;
    if(n==2) return true;
    for(int i = 2; i < Math.sqrt(n)+1; i++) if(n%i == 0) return false;
    return true;
  }

  public static void main(String[] args) {
  
    int x = 47;
    System.out.println("Is " + x + " a prime number? : " + isPrime(x));
    
    int lower = 48;
    int upper = 100;
    System.out.println("Number of prime numbers between " + lower + " and " + upper + " inclusive: " + primesBetween(lower, upper));

    int num = 90;
    System.out.println("The difference between " + num + " and the closest prime number: " + gapToNextPrime(num));
    

  }

}