ISC Practicals and Theory - Prime Number (Using a Class)

A prime number is a number that has exactly two factors. This version is written as a class Prime with a data member and separate methods to read the number, test it and display the result. The main method only creates an object and calls the methods.

Definition

A natural number $N$ is a prime number if it has exactly two factors: $1$ and $N$ itself.

• $7$ is prime: its only factors are $1$ and $7$.
• $12$ is not prime: its factors are $1, 2, 3, 4, 6, 12$.
• $1$ is not prime: it has only one factor.

Algorithm

1. In main, create the object ob of class Prime. The constructor Prime() sets num = 0.
2. Call ob.input(), which reads the number into the data member num.
3. Call ob.display():
   • If num < 1, display "Invalid input".
   • Otherwise call isPrime() and display whether num is a prime number.
4. The method isPrime() works as follows:
   • Set count = 0.
   • Repeat for i from $1$ to num: if num % i == 0, increase count by $1$.
   • Return true if count == 2, otherwise return false.

Program

The constructor Prime() initialises num, input() reads the number, isPrime() returns whether it has exactly two factors, and display() prints the message.

import java.util.Scanner;
public class Prime
{
    int num;

    Prime()
    {
        num = 0;
    }

    void input()
    {
        Scanner sc = new Scanner(System.in);
        System.out.print("Enter a number: ");
        num = sc.nextInt();
    }

    boolean isPrime()
    {
        int count = 0;
        for (int i = 1; i <= num; i++)
        {
            if (num % i == 0)
                count++;
        }
        return count == 2;
    }

    void display()
    {
        if (num < 1)
            System.out.println("Invalid input");
        else if (isPrime())
            System.out.println(num + " is a prime number");
        else
            System.out.println(num + " is not a prime number");
    }

    public static void main(String args[])
    {
        Prime ob = new Prime();
        ob.input();
        ob.display();
    }
}

Variable description table

Sample input and output

Enter a number: 7
7 is a prime number

Enter a number: 12
12 is not a prime number

Enter a number: 1
1 is not a prime number

Enter a number: -4
Invalid input

Dry run

Input: num = 12

isPrime() returns false because count = 6, so display() shows that $12$ is not a prime number, as in the second sample run.

Variations

1. Range: "Display all prime numbers between m and n."
2. Twin primes: "Display all twin prime pairs in a given range."
3. Emirp number: "Check whether a number is an emirp number: a prime whose reverse is also prime."