Java: read an int and check its primality. If it's not prime, repeat, but max x times

Question

Sorry for such a question, but I just can't get through to it why the following code is not working as it is supposed to.

As written in the title, a user should enter an integer for the code to check if it's prime. If it is not, the user gets to enter an other int, but max 5 times.

I think the primality check is working, but what bothers me is the recursion, because it prints out too much when first given a not-prime number and then a prime.

I hope somebody could explain to me why it is that way... :)

Here is the code:

import java.util.Scanner;

public class Task6v5 {

    public static void main(String[] args) {
        Scanner scan = new Scanner(System.in);
        
        System.out.println("Enter an int: ");
        int a = scan.nextInt();
        int counter = 0;
        
        isPrime(a, counter, scan);
        
    }
    
    public static void isPrime(int a, int counter, Scanner scan) {
        
        boolean prime;
        
        do {
        
            if (a <= 1) {
                counter = counter + 1;
                System.out.println("Int to small. Enter a bigger one." 
                        + " Counter: " + counter);
                a = scan.nextInt();
                isPrime(a, counter, scan);
            } else if (a == 2) {
                prime = true;
            } else if (a % 2 == 0) {
                counter = counter + 1;
                System.out.println("This int is even, "
                        + "but not a 2. "
                        + "Enter an other int."
                        + " Counter: " + counter);
                a = scan.nextInt();
                isPrime(a, counter, scan);
            }
            
            for (int i = 2; i <= Math.sqrt(a); i++) {
                if (a % i == 0) {
                    //System.out.println("Are we there?");
                    //isPrime = false;
                    counter = counter + 1;
                    System.out.println("This int is not a prime. "
                            + "Enter an other int. Counter: " + counter);
                    a = scan.nextInt();
                    isPrime(a, counter, scan);
                }
            }
            prime = true;
            System.out.println("This is a prime.");
            
        } while (counter <= 5 && prime == false);
   
    }
}

I get this output for example:

Enter an int: 
1
Int to small. Enter a bigger one. Counter: 1
4
This int is even, but not a 2. Enter an other int. Counter: 2
5
This is a prime.
This is a prime.
This int is not a prime. Enter an other int. Counter: 2

score 0 · Answer 1 · answered Dec 29 '20 at 17:07

Every time your function returns, it is going to end up here:

        prime = true;
        System.out.println("This is a prime.");

Number is a 1? Ends up printing "This is a prime".
Number is a 2? Ends up printing "This is a prime".
Number is a 4? Ends up printing "This is a prime".

Number is 42? Well, that ends up printing "This is a prime" lots of times, once for every function call you make.

In order to get the recursive aspect of this correct, you have to return when you want the function to stop.

That being said, this approach won't work for what you're trying to accomplish. Focus on the algorithm first, and separate the part where you're counting the number of integers entered from the part where you're determining whether the integer is a prime.

score 0 · Answer 2 · answered Dec 29 '20 at 17:39

Personally I would not use recursion here, start with prime = true, set it to false when necessary, and ask for a new number at the end of the outer loop:

public static void isPrime(int a, int counter, Scanner scan) {
    boolean prime;
    do {
        prime = true; // <---
        if (a <= 1) {
            counter = counter + 1;
            System.out.println("Int to small. Enter a bigger one." 
                    + " Counter: " + counter);
            //a = scan.nextInt();
            //isPrime(a, counter, scan);
            prime = false; // <---
        } else if (a == 2) {
            prime = true; // well, it was true already
        } else if (a % 2 == 0) {
            counter = counter + 1;
            System.out.println("This int is even, "
                    + "but not a 2. "
                    + "Enter an other int."
                    + " Counter: " + counter);
            //a = scan.nextInt();
            //isPrime(a, counter, scan);
            prime = false; // <---
        } else { // now it's necessary, so 2 won't be tested
            for (int i = 2; i <= Math.sqrt(a); i++) {
                if (a % i == 0) {
                    counter = counter + 1;
                    System.out.println("This int is not a prime. "
                            + "Enter an other int. Counter: " + counter);
                    //a = scan.nextInt();
                    //isPrime(a, counter, scan);
                    prime = false; // <---
                    break; // so the message doesn't get repeated many times
                }
            }
        }
        if(prime) {
            System.out.println("This is a prime.");
        } else {
            a = scan.nextInt();
        }
    } while (counter <= 5 && prime == false);
}

Java: read an int and check its primality. If it's not prime, repeat, but max x times

2 Answers2