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A prime number is a number that is divisible only by itself and 1. Write a program that asks a user for an integer value and then displays all prime numbers less than or equal to that number. For example, if the user enters 17, the program should display: Prime numbers less than or equal to 17:

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teobguan2019

Answer:

The Solution Code is written in Java.

  1. public class Main {
  3.    public static void main(String[] args) {
  5.        System.out.print("Please enter an integer: ");
  6.        Scanner input = new Scanner(System.in);
  7.        int number = input.nextInt();
  9.        System.out.print("Prime numbers less than or equal to " + number + " : ");
  11.        for(int i=2; i <= number; i++){
  12.            if(checkPrime(i)){
  13.                System.out.print(i + " ");
  14.            }
  15.        }
  16.    }
  18.    public static Boolean checkPrime(int num){
  19.        for(int i=2; i < num; i++)
  20.        {
  21.            if(num % i == 0){
  22.                return false;
  23.            }
  24.        }
  25.        return true;
  26.    }
  27. }

Explanation:

Firstly, we create a function to check if a number is prime (Line 18 - 27).

  • This function will take one input value which is an integer, num.
  • To check if the input num is a prime, we can use modulus operator, %, to confirm if the num is divisible by any number starting from 2 to num - 1 (Line 19 - 24).
  • If the num is divisible by any number expect 1 and itself, it should equal to zero and return false to indicate it is not a prime number.
  • If all the num (except 1 and itself) is not divisible, the function will return true (Line 25).

Next, in our main program part (Line 3 - 16)

  • Prompt the user to input a number (Line 5 - 7)
  • Using a for-loop, we can keep calling the checkPrime() function by passing a current number (starting from 2 to input number) as argument (Line 12). The checkPrime() function will run and return true it the current number is prime, return false if it is not prime.
  • If the checkPrime() function return true, it will print the current number before proceed to the iteration of the for-loop to repeat the same check prime procedure (Line 13)
  • At the end all the prime numbers less than or equal to the input number will be printed out in the terminal

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