How to print an int with comma divisors using a recursive method (no strings)?

Question

In my computer science class, we were assigned a lab on recursion in which we have to print out a number with commas separating groups of 3 digits. Here is the text directly from the assignment (the method has to be recursive):

Write a method called printWithCommas that takes a single nonnegative primitive int argument and displays it with commas inserted properly. No use of String.

For example printWithCommas(12045670); Displays 12,045,670

printWithCommas(1); Displays 1

I am really stumped on this. Here is my code so far:

    public static void printWithCommas(int num) {
    //Find length
    if (num < 0) return;
    int length = 1;
    if (num != 0) {
        length = (int)(Math.log10(num)+1);
    }
    //Print out leading digits
    int numOfDigits = 1;
    if (length % 3 == 0) {
        numOfDigits = 3;
    }
    else if ((length+1) % 3 == 0) {
        numOfDigits = 2;
    }
    System.out.print(num / power(10,length-numOfDigits));
    //Print out comma
    if (length > 3) {
        System.out.print(',');
    }
    printWithCommas(num % power(10,length-numOfDigits));
}

It gets a stack overflow (which I can fix later), but it fails to print out some of the zeros, specifically the ones that are supposed to be after each comma.

I feel like I am taking this on with a completely wrong approach, but can't think of a good one. Any help would be appreciated.

Thanks in advance!

Note: power is a function I made that calculates power. First argument is the base, second is the exponent.

Answer 1

Here is the code I came up with, for anyone else that might be stuck on this:

public static void printWithCommas(int num) {
    if (num > 999) {
        printWithCommas(num/1000);
        System.out.print(',');
        if (num % 1000 < 100) System.out.print('0');
        if (num % 1000 < 10) System.out.print('0');
        System.out.print(num%1000);
    }
    else {
        System.out.print(num);
    }
}