How to convert a while loop to recursion?

Question

C++ / C#

I am a 1st year student. The programming teacher gave an interesting task: to create a calculator for complex numbers (namely, their sum) using Karatsuba's recursive algorithm.
I was able to cope with this task only with the help of cycles. So far, I have little idea how you can use recursion instead of a while loop.

Calculator code below:

#include <math.h>
#include <iostream>
using namespace std;

// подсчет количества разрядов
int getLength(long long value) {
    int counter = 0;
    while (value != 0) {
        counter++;
        value /= 10;
    }
    return counter;
}

long long multiply(long long x, long long y) {
    int xLength = getLength(x);
    int yLength = getLength(y);

    // наибольший из двоих значений
    int N = (int)(fmax(xLength, yLength));

    // если максимальная длина достаточно маленькая, быстрее просто перемножить два числа обычным методом
    if (N < 10)
        return x * y;

    
    N = (N / 2) + (N % 2);
    // смещение для десятичной системы счисления
    long long multiplier = pow(10, N);

    long long b = x / multiplier;
    long long a = x - (b * multiplier);
    long long d = y / multiplier;
    long long c = y - (d * multiplier);

    long long d0 = multiply(a, c);
    long long d1 = multiply(a + b, c + d);
    long long d2 = multiply(b, d);

    // формула с(t)
    return d0 + ((d1 - d0 - d2) * multiplier) + (d2 * (long long)(pow(10, 2 * N)));

}
int main()
{
    
    long long a, b, c, d;
    cout << "a: "; cin >> a;
    cout << "b: "; cin >> b;
    cout << "c: "; cin >> c;
    cout << "d: "; cin >> d;
    cout << "r = " << (a * c - b * d) << endl;
    cout << "s = " << (a * d + b * c) << "i" << endl;
}

Answer 1

how to use recursion instead of while loop

int getLength(long long value) {
    return (value == 0) ? 0 : getLength(value / 10) + 1;
}