//
// Example 6.8. Program to add up the digits of a
// nonnegative input integer
//

#include <iostream>

using namespace std;

int main()
{
   int readNum();
   int findDigitSum(int num);


   int num;		// nonnegative input integer


   num = readNum();
   cout << "The sum of the digits of "
	 << num << " is " << findDigitSum(num) << "." << endl;


   return 0;
}

//
// Function to print instructions and repeatedly read
// integers until the user enters a nonnegative integer,
// which the function returns
// Pre:  none
// Post: A nonnegative integer has been read and returned.
//

int readNum()
{
   int num; 		    // nonnegative input integer

   cout << "This program returns the sum of the digits" << endl;
   cout << "of a nonnegative integer." << endl << endl;

   do
   {
	cout << "Enter a nonnegative integer: ";
	cin  >> num;
   }
   while (num < 0);

   return num;
}

//
// Function to return the sum of the digits of a nonnegative
// integer
// Pre:  num is a nonnegative integer.
// Post: The function has returned the sum of the
//       digits of num.
//

int findDigitSum(int num)
{
   int sum = 0; 	   // sum of digits
   int rightmostDigit; // rightmost digit of num

   do
   {
	rightmostDigit = num % 10; // extract rightmost digit
	sum += rightmostDigit;	// add digit to ongoing sum
	num /= 10;	// move next digit into rightmost position
   }
   while (num > 0); // when num is 0, no more digits to extract

   return sum;
}