Question

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:

Answer 1

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. }

Step-by-step 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