Add the number digits with recursion

Question

I tried to add all the digits in a number but it only adds the first and the last digit. For example 1111 -> 1+1+1+1 = 4 but I got 2, 47879 -> 35 but I got 13, and so on...

Here's the code:

#include <iostream>
using namespace std;
int input(int &n)
{
    cin >> n;
    return 0;
}
int sum(int n, int &s)
{
    int left = n % 10;
    
    if (n < 10)
    {
        return left;
    }
    else
    { 
        s =s+ left + sum( n / 10, s);
    }
}
int main() {
    int n, s = 0;
    input(n);
    sum(n, s);
    cout << s;
}

Also, I can only use recursion; no string, no array, no loop, etc.

Answer 1

You can use this much simpler function:

constexpr int sum(int n) {
  return n ? n%10 + sum(n/10) : 0;
}

Or if you want it to be tail-recursive:

int sum(int n, int s = 0) {
  return n ? sum(n/10, s + n%10) : s;
}

In your implementation you never return on the else branch.

Answer 2

The function sum have return type of int. But in the recursion path some function calls does not return anything. (if else execute rather than if )

you can simply use this function:

void sum(int n, int &s)
{
    int left = n % 10;

    s += left;

    if (n < 10)
        return;
    else
        sum(n / 10, s);
}