int sum_to_digit(int a)

sum_to_digit.cpp

#include <iostream>
#include <complex>

using namespace std;

int HowMuchBit(int elem){
    int bit = 1;
    int p = 10;
    while(elem >= p){
        p *= 10;
        bit++;
    }
    return bit;
}

int sum_to_digit(int a){
    int bit = HowMuchBit(a);
    int sum = 0;
    for (int i = 1; i <= bit; ++i) {
        //double pow(double x, double y);
        sum += (a % (int)pow(10,i)) / (int)pow(10,i-1);
    }

    if(sum >= 10){
        return sum_to_digit(sum);
    }
    else{
        return sum;
    }
}


int main() {

    cout << HowMuchBit(12333) << endl;
    cout << sum_to_digit(123456);
    return 0;
}