Sum of digits until single digit

Logic: repeatedly extract each digit with n % 10, add it, and remove it with n / 10. Take the sum as the new number and repeat until the number is below 10 (a single digit).

#include <stdio.h>
int main(void)
{
    long n;
    int sum;
    printf("Enter a number: ");
    scanf("%ld", &n);
    while (n > 9)
    {
        sum = 0;
        while (n > 0)
        {
            sum += n % 10;
            n /= 10;
        }
        printf("Sum of digits = %d\n", sum);
        n = sum;
    }
    printf("Single digit = %ld\n", n);
    return 0;
}

Output for 12345:

Enter a number: Sum of digits = 15
Sum of digits = 6
Single digit = 6

Output for 891:

Enter a number: Sum of digits = 18
Sum of digits = 9
Single digit = 9

Explanation

• The inner while (n > 0) loop adds the digits: 1+2+3+4+5 = 15.
• The outer loop while (n > 9) repeats with n = 15: 1+5 = 6, and 6 is a single digit, so it stops.