Java program that gives prime numbers between two intervals

Question

I need a program that prints all the prime numbers between any two intervals and than prints how many prime numbers there were between the two intervals.

So I have a running code but it won't print the number 2 and I know 2 is a prime number. It's doing everything else correctly. I tried some other codes that would print 2 but it would also give negatives if I put in a negative number.

import java.util.Scanner;

class Main {

  public static void main(String args[]) {

    int first, last, flag = 0, i, j;

    Scanner scanner = new Scanner(System.in);

    System.out.print("\nEnter the lower bound : ");
    first = scanner.nextInt();
    System.out.print("\nEnter the upper bound : ");
    last = scanner.nextInt();
    System.out.println("The prime numbers in between the entered limits are :");

    int x = 0;
    for (i = first; i <= last; i++) {
      for (j = 2; j < i; j++) {
        if (i % j == 0) {
          flag = 0;
          break;
        } else {
          flag = 1;
        }
      }
      if (flag == 1) {
        x++;
        System.out.println(i + " ");
      }
    }
    System.out.println("Total number of prime numbers between " + first + " and " + last + " are " + x);
  }
}

So if I was to put in -5 (upperbound) and 10 (lowerbound) It should print: 2 3 5 7 Total number of prime numbers between -5 and 10 are 4

But instead it prints 3 5 7 Total number of prime numbers between -5 and 10 are 3

Answer 1

inner loop ignore number 2. j < i => 2 < 2 is false

Answer 2

Just add the following lines before your for loop and it will give your the expected output:

int x = 0;
if (first<3) {
    System.out.println(2);
    x++;
}

Your updated program will be:

import java.util.Scanner;

class Main {

  public static void main(String args[]) {

    int first, last, flag = 0, i, j;

    Scanner scanner = new Scanner(System.in);

    System.out.print("\nEnter the lower bound : ");
    first = scanner.nextInt();
    System.out.print("\nEnter the upper bound : ");
    last = scanner.nextInt();
    System.out.println("The prime numbers in between the entered limits are :");

    int x = 0;
    if (first<3) {
        System.out.println(2);
        x++;
    }
    for (i = first; i <= last; i++) {
      for (j = 2; j < i; j++) {
        if (i % j == 0) {
          flag = 0;
          break;
        } else {
          flag = 1;
        }
      }
      if (flag == 1) {
        x++;
        System.out.println(i + " ");
      }
    }
    System.out.println("Total number of prime numbers between " + first + " and " + last + " are " + x);
  }
}

Answer 3

You can check this code (optimized as it will run faster for big ranges. Loop returns before iterating all the numbers)

 public static void main(String args[]) {
        int first;
        int last;
        Scanner scanner = new Scanner(System.in);
        System.out.print("\nEnter the lower bound : ");
        first = scanner.nextInt();
        System.out.print("\nEnter the upper bound : ");
        last = scanner.nextInt();
        System.out.println("The prime numbers in between the entered limits are :");

        int x = 0;
        for (int i = first; i <= last; i++) {
            if (isPrime(i)) {
                x++;
                System.out.println(i + " ");
            }
        }
        System.out.println("Total number of prime numbers between " + first + " and " + last + " are " + x);
    }

    static boolean isPrime(int n)
    {
        if (n <= 1) //less than 2 are not
            return false;
        if (n <=3) // 2 and 3 are prime
            return true;

        if (n % 2 == 0 || n % 3 == 0)
            return false;

        for (int i = 5; i * i <= n; i = i + 6)
            if (n % i == 0 || n % (i + 2) == 0)
                return false;

        return true;
    }

You do not need a flag.

Answer 4

Try this:

import java.util.Scanner;

public class Main {
    public static void main(String args[]) {
        int i, n;
        int num;
        int maxCheck;
        boolean isPrime = true;
        String primeNumbersFound = "";

      Scanner sc=new Scanner(System.in);
      System.out.println("Enter the first number: ");
        num = sc.nextInt();
        System.out.println("Enter the second number: ");
        maxCheck= sc.nextInt();

        for (i = num; i <= maxCheck; i++) {
            isPrime = CheckPrime(i);
            if (isPrime) {
            primeNumbersFound = primeNumbersFound + i + " ";
            }
        }
        System.out.println("Prime numbers from " + num + " to " + maxCheck + " are:");

        System.out.println(primeNumbersFound);

}

public static boolean CheckPrime(int n) {
    int remainder;
    for (int i = 2; i <= n / 2; i++) {
        remainder = n % i;
        if (remainder == 0) {
        return false;
          }
        }
        return true;

        }
}