while loops

while (condition) { ... } keeps running as long as the condition is true; use it when the number of rounds is not known in advance. Something in the body must move toward the condition becoming false, or the loop never ends; the lesson runner stops runaway code after a while and tells you.

n / 10 chops the last digit off a whole number and n % 10 reads it, which is how a loop walks through digits. do { ... } while (condition); is the variant that always runs the body at least once.

The library project checks member card numbers with a simple checksum: the digits of a valid card add up to a multiple of ten. This lesson adds the class Checksum that does the adding.

int n = 1, steps = 0;
while (n < 100) {
    n *= 2;
    steps++;
}
System.out.println(n + " " + steps);   // 128 7

Your task

In Main, write static int digits(long n) returning how many digits a non-negative whole number has, by repeatedly dividing by 10 (digits(0) is 1).

In the class Checksum, write static int digitSum(long n) returning the sum of the digits of a non-negative number, so digitSum(1234) is 10.

Part of the final project. Checksum adds up the digits of a card number; a valid card sums to a multiple of ten. It is saved in this browser and loaded into Project: the library when you get there.

public class Main {
    static int digits(long n) {
        if (n == 0) {
            return 1;
        }
        int count = 0;
        while (n > 0) {
            n /= 10;
            count++;
        }
        return count;
    }

    public static void main(String[] args) {
        System.out.println(digits(12345) + " " + Checksum.digitSum(12345));
    }
}

class Checksum {
    static int digitSum(long n) {
        int sum = 0;
        while (n > 0) {
            sum += (int) (n % 10);
            n /= 10;
        }
        return sum;
    }
}