For loop madness

[Nagi21]

So I recently came upon a problem that required me to add the digits of an integer together and output the sum (ie 111 would equal 1+1+1 = 3). Originally I had attempted a for loop to try and solve this, but could not get it to output anything except 0. I also had an issue with another for loop previously in a similar way. I've already solved the actual problem with a while loop but I can't understand why this didn't work.

int digitSum(int n) {
    
    int lengthCount = 0;
    int num = n;
    int sum = 0;
    
    for (; num != 0; num /= 10, lengthCount++);
    for (int i = 1; i <= lengthCount; i++){
        sum = sum + num % 10;
        num = num / 10;
    }

    return sum;
}

[48T7M4Gy]

What is line 7 doing?

[Nagi21]

Line 7 is keeping count of how many digits are in the number to tell the second loop how many times to iterate.

[Chervil]

After the first loop has completed, num will be zero, so there is nothing for the second loop to work upon. Why not do the whole thing in a single for loop (and get rid of lengthCount, it isn't needed).

[48T7M4Gy]

OK

By the look of it lines 9 and 10 are using a value of num = 0. Better use n instead.

[48T7M4Gy]

#include <iostream>

int main()
{
    int n = 1234;
    int sum = 0;
  
  while(n != 0)
  {
      sum += n % 10;
      n /= 10;
  }
  
  std::cout << sum << '\n';
  return 0;
}