C Exercises: Add repeatedly all digits of a given non-negative number until the result has only one digit

Last update on August 07 2025 12:49:19 (UTC/GMT +8 hours)

12. Add Digits to One Digit Variants

Write a C program to add repeatedly all digits of a given non-negative number until the result has only one digit.

Example:
Input: 48
Output: 2
Explanation: The formula is like: 4 + 8 = 12, 1 + 2 = 3.

Visual Presentation:

Sample Solution:

C Code:

#include <stdio.h>

// Function to calculate the digital root of a number (single-digit sum)
static int addDigits(int num) {
    // Formula to calculate the digital root of a number using a mathematical property
    // The digital root of a positive integer num is equivalent to num - (num - 1) / 9 * 9
    return num - (num - 1) / 9 * 9;
}

// Main function to test the addDigits function with different numbers
int main(void) {
    int n = 12;
    printf("\nInitial number is %d, Single digit number is %d.", n, addDigits(n));
    n = 47;
    printf("\n\nInitial number is %d, Single digit number is %d.", n, addDigits(n));
    return 0;
}

Sample Output:

Initial number is 12, Single digit number is 3.

Initial number is 47, Single digit number is 2.

Flowchart:

For more Practice: Solve these Related Problems:

• Write a C program to repeatedly add digits of a number until a single-digit result is obtained using recursion.
• Write a C program to compute the digital root of a number in a single expression without loops.
• Write a C program to perform digit addition iteratively and print each intermediate sum.
• Write a C program to optimize the digit sum process using mathematical properties to compute the digital root.

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