optimized sum of digits in c code example

Example 1: Determine the sum of al digits of n

def sum_of_digits(n): 
	sum = 0
	while (n != 0): 
		sum = sum + int(n % 10) 
		n = int(n/10) 
	return sum

Example 2: Given a long number, return all the possible sum of two digits of it. For example, 12345: all possible sum of two digits from that number are:

function digits(num){
  let numArray = num.toString().split('');
  let sumArray = [];
  
  for (let i = 0; i < numArray.length; i++) {
    for (let j = i+1; j < numArray.length; j++) {
      let sum;
      sum = Number(numArray[i]) + Number(numArray[j]);
      sumArray.push(sum);
    }
  } 
  return sumArray;  
}