C++ , cout line in function effects the result, function does not work without cout line

Question

I'm trying to solve Codewars task and facing issue that looks strange to me.

Codewars task is to write function digital_root(n) that sums digits of n until the end result has only 1 digit in it. Example: 942 --> 9 + 4 + 2 = 15 --> 1 + 5 = 6 (the function returns 6).

I wrote some bulky code with supporting functions, please see code with notes below.

The problem - digital_root function works only if I put cout line in while loop. The function returns nonsense without this cout line (please see notes in the code of the function).

My questions are:

Why isn't digital_root working without cout line?
How cout line can effect the result of the function?
Why does cout line fix the code?

Thanks a lot in advance! I'm a beginner, spent several days trying to solve the issue.

#include <iostream>
#include <cmath>
#include <string>
using namespace std;

int getDigit (int, int);
int sumDigits (int);
int digital_root (int);

int main() 
{
    cout << digital_root (942); // expected output result is 6 because 9 + 4 + 2 = 15 -> 1 + 5 = 6
}

int getDigit (int inputNum, int position) // returns digit of inputNum that sits on a particular position (works)
{
    int empoweredTen = pow(10, position-1);
    return inputNum / empoweredTen % 10;
}

int sumDigits (int inputNum) // returns sum of digits of inputNum (works)
{
    int sum;
    int inLen = to_string(inputNum).length();
    int i = inLen;
    while (inLen --)
    {
        sum += getDigit(inputNum, i);
        i --;
    }
    return sum;
}

int digital_root (int inputNum) // supposed to calculate sum of digits until number has 1 digit in it (abnormal behavior)
{
    int n = inputNum;
    while (n > 9)
    {
        n = sumDigits(n);
        cout << "The current n is: " << n << endl; // !!! function doesn't work without this line !!!
    }
    return n;
}

I've tried to rewrite the code from scratch several times with Google to find a mistake but I can't see it. I expect digital_root() to work without any cout lines in it. Currently, if I delete cout line from while loop in digital_root(), the function returns -2147483647 after 13 seconds of calculations. Sad.

This is why c++ must be learnt using a [good c++ book](https://stackoverflow.com/questions/388242/the-definitive-c-book-guide-and-list) instead of by solving random online puzzles. — Jason, Nov 22 '22 at 05:54
[What is a debugger and how can it help me diagnose problems?](https://stackoverflow.com/questions/25385173/what-is-a-debugger-and-how-can-it-help-me-diagnose-problems) — Jesper Juhl, Nov 22 '22 at 06:03
`sum` is uninitialised when read as part of the addition (with `+=`). The program has [undefined behaviour](https://en.cppreference.com/w/cpp/language/ub). — Jesper Juhl, Nov 22 '22 at 07:05
A better implementation imo would be to NOT use `to_string()` and `pow()` at all. You could instead implement it using just integers: you need two variables, a "running remainder" and a "running sum", and simply use the (integer) `/` and `%` operators. A side-note, `using` namespaces like `std` is typically not recommended. — Constantine Georgiou, Nov 22 '22 at 07:24

Sandil Ranasinghe · Answer 1 · 2022-11-22T06:30:02.040

0

int sumDigits (int inputNum) // returns sum of digits of inputNum (works)
{
    int sum = 0; // MAKE SURE YOU INITIALIZE THIS TO 0 BEFORE ADDING VALUES TO IT!
    int inLen = to_string(inputNum).length();
    int i = inLen;
    while (inLen --)
    {
        sum += getDigit(inputNum, i);
        i --;
    }
    return sum;
}

Initialize your variables before adding values to them, otherwise you could run into undefined behaviour. Also for the record, adding the cout line printed out something, but it wasn't the correct answer.

edited Nov 22 '22 at 06:30

answered Nov 22 '22 at 06:25

Sandil Ranasinghe

582
5
18

You should add comments in the lines where you have made changes compared to the original code. – Rohan Bari Nov 22 '22 at 06:26
I did put a comment, but yeah on retrospect it was not very apparent. I edited it to just show the function with the line I changed. – Sandil Ranasinghe Nov 22 '22 at 06:32

Constantine Georgiou · Accepted Answer · 2022-11-22T08:21:25.150

Here is an implementation using integer operators instead of calling std::to_string() and std::pow() functions - this actually works with floating-point numbers. It uses two integer variables, nSum and nRem, holding the running sum and remainder of the input number.

// calculates sum of digits until number has 1 digit in it
int digital_root(int inputNum)
{
    while (inputNum > 9)
    {
        int nRem = inputNum, nSum = 0;
        do // checking nRem after the loop avoids one comparison operation (1st check would always evaluate to true)
        {
            nSum += nRem % 10;
            nRem /= 10;
        } while (nRem > 9);
        inputNum = nSum + nRem;
        std::cout << "The current Sum is: " << inputNum << endl; // DEBUG - Please remove this
    }
    return inputNum;
}

As for the original code, the problem was the uninitialized sum variable, as already pointed out by other members - it even generates a compiler error.

C++ , cout line in function effects the result, function does not work without cout line

2 Answers2