C programming code

#include <stdio.h>
 
int main()
{
   int n, t, sum = 0, remainder;
 
   printf("Enter an integer\n");
   scanf("%d", &n);
 
   t = n;
 
   while (t != 0)
   {
      remainder = t % 10;
      sum       = sum + remainder;
      t         = t / 10;
   }
 
   printf("Sum of digits of %d = %d\n", n, sum);
 
   return 0;
}

If you wish you can modify input variable (n) and do not use an additional variable (t) but it is not recommended.
Output of program:

For example if the input is 98, sum(variable) is 0 initially
98%10 = 8 (% is modulus operator which gives us remainder when 98 is divided by 10).
sum = sum + remainder
so sum = 8 now.
98/10 = 9 because in c whenever we divide integer by another integer we get an integer.
9%10 = 9
sum = 8(previous value) + 9
sum = 17
9/10 = 0.
So finally n = 0, loop ends we get the required sum.
Download Add digits program.

Add digits using recursion

#include <stdio.h>
 
int add_digits(int);
 
int main() 
{
  int n, result;
 
  scanf("%d", &n);
 
  result = add_digits(n);
 
  printf("%d\n", result);
 
  return 0;
}
 
int add_digits(int n) {
  static int sum = 0;
 
  if (n == 0) {
    return 0;
  }
 
  sum = n%10 + add_digits(n/10);
 
  return sum;
}

Static variable sum is used and is initialized to 0, it' value will persists after function calls i.e. it is initialized only once when a first call to function is made.

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