Java program to display prime numbers from 1 to 100 and 1 to n

The number which is only divisible by itself and 1 is known as prime number. For example 2, 3, 5, 7…are prime numbers. Here we will see two programs: 1) First program will print the prime numbers between 1 and 100 2) Second program takes the value of n (entered by user) and prints the prime numbers between 1 and n.

Program to Check Prime Number using a for loop

/*
 * Author: Zameer Ali
 * */
public class Main {

  public static void main(String[] args) {

    int num = 29;
    boolean flag = false;
    for (int i = 2; i <= num / 2; ++i) {
      // condition for nonprime number
      if (num % i == 0) {
        flag = true;
        break;
      }
    }

    if (!flag)
      System.out.println(num + " is a prime number.");
    else
      System.out.println(num + " is not a prime number.");
  }
}

Output:

Explanation:

In the above program, for loop is used to determine if the given number num is prime or not.

Here, note that we are looping from 2 to num/2. It is because a number is not divisible by more than its half.

Inside the for loop, we check if the number is divisible by any number in the given range (2...num/2).

• If num is divisible, flag is set to true and we break out of the loop. This determines num is not a prime number.
• If num isn’t divisible by any number, flag is false and num is a prime number.

Program to Check Prime Number using a while loop

/*
 * Author: Zameer Ali
 * */
public class Main {

  public static void main(String[] args) {

    int num = 33, i = 2;
    boolean flag = false;
    while (i <= num / 2) {
      // condition for nonprime number
      if (num % i == 0) {
        flag = true;
        break;
      }

      ++i;
    }

    if (!flag)
      System.out.println(num + " is a prime number.");
    else
      System.out.println(num + " is not a prime number.");
  }
}

Output:

Program to Check Prime Number using nested for loop

/*
 * Author: Zameer Ali
 * */
public class PrimeNumbers {

 public static void main(String[] args) {

  int num = 20, count;

  for (int i = 1; i <= num; i++) {
   count = 0;
   for (int j = 2; j <= i / 2; j++) {
    if (i % j == 0) {
     count++;
     break;
    }
   }

   if (count == 0) {
    System.out.println(i);
   }

  }
 }
}

Output:

Explanation:

First you have to create a class name PrimeNumbers inside which the main() method is declared. Now the main() method contains two integer type variables name – num and count. Variable num is initialized with the value 20.

Now, to check for all the integer numbers which is less than or equal to 20, you have to iterate the calculations for each value using a for loop.