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Java program to print Twin Prime Numbers within a range

Question:

Write a Program in Java to print all the Twin Prime numbers within a given range.

Note: Twin Prime numbers are a pair of numbers which are both prime and their difference is 2.

Example: Twin Prime numbers in the range 1 to 100 are :
(3,5) (5,7) (11,13) (17,19) (29,31) (41,43) (59,61) (71,73)

Programming Code:

/**
* The class TwinPrimeRange inputs 2 numbers and prints all the 
* twin prime numbers within that range
* @author : www.javaforschool.com
* @Program Type : BlueJ Program - Java
*/
 
import java.io.*;
class TwinPrimeRange
{        
     boolean isPrime(int n) //funton for checking prime
        {
            int count=0;
            for(int i=1; i<=n; i++)
                {
                    if(n%i == 0)
                        count++;
                }
            if(count == 2)
                return true;
             else
                return false;
        }
 
    public static void main(String args[]) throws IOException
        {
            TwinPrimeRange ob = new TwinPrimeRange();
            BufferedReader br=new BufferedReader(new InputStreamReader(System.in));

            System.out.print("Enter the lower range : ");
            int p = Integer.parseInt(br.readLine());
            System.out.print("Enter the upper range : ");
            int q = Integer.parseInt(br.readLine());
            
            if(p>q)
                System.out.println("Invalid Range !");
            else
            {
                System.out.println("nThe Twin Prime Numbers within the given range are : ");
                for(int i=p; i<=(q-2); i++)
                {
                    if(ob.isPrime(i) == true && ob.isPrime(i+2) == true)
                    {
                        System.out.print("("+i+","+(i+2)+") ");
                    }
                }
            }                 
        }
}

Output:

Enter the lower range : 1
Enter the upper range : 200

The Twin Prime Numbers within the given range are :
(3,5) (5,7) (11,13) (17,19) (29,31) (41,43) (59,61) (71,73) (101,103) (107,109) (137,139) (149,151) (179,181) (191,193) (197,199)

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5 comments

  1. What is the difference between = and ==?

    • = is assignment operator which assigns the value on the right hand side to the variable on the left hand side.
      Example: a = 5 will assign variable ‘a’ with a value 5.

      == is relational operator which checks whether the value on the left hand side is equal to the value on the right hand side or not.
      Example: a == 5 checks if a is equal to 5 or not.

  2. why d loop is till q-2… logic…..??

    • because if you are told to find twin primes from 1 to 100 and if you run the loop till hundred, you will be checking 2 numbers (99, 101) and (100, 102) which are out of range.

      So, if you go till, 98, you will be checking (98, 100) and then stop.

  3. nice program….

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