How to check if an int contains specific numbers?

Question

I'm currently working on a college Java project, and I'm stuck. Here's the assignment details for context:

Write a function that accepts integer num and displays all lower numbers composed only with digits 1 and/or 3. (This program can accept any integer value as an input and provide a proper output)

Test your function in a Java application program.

Sample run 1:

Enter an integer: 10

All the numbers lower than 10 and composed only with digits 1 and/or 3: 3, 1

Sample run 2:

Enter an integer: 20

All the numbers lower than 20 and composed only with digits 1 and/or 3: 13, 11, 3, 1

Note 1: This program should only accept positive integer values.
Note 2: All the outputs should be in the same line, separated with a comma. You should not consider a comma after the last output.

So far, this is what I have made:

import java.util.Scanner;

public class IntManipulator
{
    public static void main (String[]args)
    {
        //initialize new system.in scanner object named input
        Scanner input = new Scanner(System.in);
        
        //prompt user to input an integer and use scanner object to store the integer in myInt
        System.out.print("Enter an integer: ");
        int myInt = input.nextInt();
        
        //function call
        intMethod(myInt);
                
        //close input scanner object
        input.close();
    }
    
    static void intMethod(int myInt)
    {
        System.out.print("All the numbers lower than " + myInt + " and composed only with digits 1 and/or 3: ");
        
        for(int i=myInt; i>0; i--)
        {
            if()
            {
                System.out.print(i+", ");
            }
        }
    }
}

Where I'm stuck right now, is as to what to put in my if() statement in my intMethod function's for loop, so that I can only print the values of i that contain 1 and/or 3.

Answer 1

I would use an iterative approach here, starting with 1 up until, but not including, the input number. Then, loop over each number and ensure that every digit be 1 or 3, using the modulus:

public static void intMethod(int myInt) {
    for (int i=1; i < myInt; ++i) {
        int num = i;
        while (num > 0) {
            if (num % 10 != 1 && num % 10 != 3)
                break;
            num /= 10;
        }
        if (num == 0) {
            System.out.println("MATCH: " + i);
        }
    }
}

For an input of intMethod(50) , the following output was generated:

MATCH: 1
MATCH: 3
MATCH: 11
MATCH: 13
MATCH: 31
MATCH: 33