C Program to Find Sum of Digits of a Number

A C program to find the sum of digits uses a loop to extract each digit using num % 10, adds it to a sum, and reduces the number by num / 10 until the number is zero.

Examples

Input1234

OutputSum of digits = 10

Input0

OutputSum of digits = 0

Input99999

OutputSum of digits = 45

How to Think About It

To find the sum of digits, repeatedly take the last digit of the number by using num % 10, add it to a running total, then remove that digit by dividing the number by 10 using num / 10. Repeat until the number becomes zero.

Algorithm

1

Get input number from the user

2

Initialize sum to 0

3

While number is greater than 0:

4

Extract last digit using modulo 10

5

Add digit to sum

6

Remove last digit by dividing number by 10

7

Print the sum

Code

c

#include <stdio.h>

int main() {
    int num, sum = 0, digit;
    printf("Enter a number: ");
    scanf("%d", &num);
    if (num == 0) {
        printf("Sum of digits = 0\n");
        return 0;
    }
    while (num > 0) {
        digit = num % 10;
        sum += digit;
        num /= 10;
    }
    printf("Sum of digits = %d\n", sum);
    return 0;
}

Output

Enter a number: 1234
Sum of digits = 10

Dry Run

Let's trace the input 1234 through the code

1

Initial input

num = 1234, sum = 0

2

First iteration

digit = 1234 % 10 = 4, sum = 0 + 4 = 4, num = 1234 / 10 = 123

3

Second iteration

digit = 123 % 10 = 3, sum = 4 + 3 = 7, num = 123 / 10 = 12

4

Third iteration

digit = 12 % 10 = 2, sum = 7 + 2 = 9, num = 12 / 10 = 1

5

Fourth iteration

digit = 1 % 10 = 1, sum = 9 + 1 = 10, num = 1 / 10 = 0

6

Loop ends

num is 0, exit loop, sum = 10

Iteration	num	digit	sum
1	1234	4	4
2	123	3	7
3	12	2	9
4	1	1	10

Why This Works

Step 1: Extract last digit

Using num % 10 gets the last digit of the number, which we add to the sum.

Step 2: Remove last digit

Dividing the number by 10 with num /= 10 removes the last digit, preparing for the next iteration.

Step 3: Repeat until done

We repeat this process until the number becomes zero, ensuring all digits are added.

Alternative Approaches

Using recursion

c

#include <stdio.h>

int sumDigits(int n) {
    if (n == 0) return 0;
    return (n % 10) + sumDigits(n / 10);
}

int main() {
    int num;
    printf("Enter a number: ");
    scanf("%d", &num);
    printf("Sum of digits = %d\n", sumDigits(num));
    return 0;
}

Recursion is elegant but uses more memory due to function calls.

Using string conversion

c

#include <stdio.h>
#include <string.h>

int main() {
    char numStr[20];
    int sum = 0;
    printf("Enter a number: ");
    scanf("%s", numStr);
    for (int i = 0; i < strlen(numStr); i++) {
        sum += numStr[i] - '0';
    }
    printf("Sum of digits = %d\n", sum);
    return 0;
}

Converting to string simplifies digit access but uses extra space.

Complexity: O(d) time, O(1) space

Time Complexity

The loop runs once for each digit in the number, so time depends on the number of digits d.

Space Complexity

Only a few variables are used, so space is constant O(1).

Which Approach is Fastest?

The iterative method is fastest and uses least memory; recursion adds overhead, and string conversion uses extra space.

Approach	Time	Space	Best For
Iterative loop	O(d)	O(1)	Simple and efficient
Recursion	O(d)	O(d)	Elegant but uses more memory
String conversion	O(d)	O(d)	Easy digit access but extra space