(using if, if-else, nested if, switch-case)

(using for, while, do-while loops)

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Sum of digits of a number

Introduction

The sum of digits of a positive number is the sum of all its individual digits.

Example:

Input: 12345 → Output: 15 (1 + 2 + 3 + 4 + 5 = 15)
Input: 0 → Output: 0

We’ll use a for loop to extract digits and compute their sum.

C Program: Sum of digits of a number

NOTE: The Program is written using for, while and do..while loop.
Method 1 is for loop, Method 2 is while loop and Method 3 is do..while loop

Method 1: Using for loop

#include <stdio.h>
 
int main() {
    int num, original, digit, sum = 0;
 
    // Input
    printf("Enter a positive integer: ");
    scanf("%d", &num);
 
    // Validate input
    if (num < 0) {
        printf("Invalid input! Please enter a positive number.\n");
        return 1;
    }
 
    original = num; // store original number
 
    // Sum of digits using for loop
    for (; num != 0; num /= 10) {
        digit = num % 10;   // extract last digit
        sum += digit;       // add to sum
    }
 
    // Display result
    printf("Sum of digits of %d = %d\n", original, sum);
 
    return 0;
}

Output

 
OUTPUT 1 :
Enter a positive integer: 12345
Sum of digits of 12345 = 15

OUTPUT 2 :
Enter a positive integer: 0
Sum of digits of 0 = 0

OUTPUT 3 :
Enter a positive integer: -987
Invalid input! Please enter a positive number.

Explanation

Read the positive number num and store it in original.
Validate that the number is not negative.
Use a for loop:
- Extract the last digit with digit = num % 10.
- Add it to sum.
- Remove the last digit using num /= 10.
Print the total sum of digits.

C Program: Sum of digits of a number

Method 2: Using while loop

Introduction

The sum of digits of a positive number is obtained by adding all its digits.

Example:

Input: 12345 → Output: 15 (1 + 2 + 3 + 4 + 5 = 15)
Input: 0 → Output: 0

We’ll use a while loop to extract digits and compute their sum.

#include <stdio.h>
 
int main() {
    int num, original, digit, sum = 0;
 
    // Input
    printf("Enter a positive integer: ");
    scanf("%d", &num);
 
    // Validate input
    if (num < 0) {
        printf("Invalid input! Please enter a positive number.\n");
        return 1;
    }
 
    original = num; // store original number
 
    // Sum of digits using while loop
    while (num != 0) {
        digit = num % 10;   // extract last digit
        sum += digit;       // add to sum
        num /= 10;          // remove last digit
    }
 
    // Display result
    printf("Sum of digits of %d = %d\n", original, sum);
 
    return 0;
}

Output

 
OUTPUT 1 :
Enter a positive integer: 12345
Sum of digits of 12345 = 15

OUTPUT 2 :
Enter a positive integer: 0
Sum of digits of 0 = 0

OUTPUT 3 :
Enter a positive integer: -987
Invalid input! Please enter a positive number.

Explanation

Read the positive number num and store it in original.
Validate that the input is not negative.
Use a while loop:
- Extract the last digit: digit = num % 10.
- Add it to sum.
- Remove the last digit: num /= 10.
Print the total sum of digits.

C Program: Sum of digits of a number

Method 3: Using do..while loop

Introduction

The sum of digits of a positive number is obtained by adding all its digits.

Example:

Input: 12345 → Output: 15 (1 + 2 + 3 + 4 + 5 = 15)
Input: 0 → Output: 0

We’ll use a do-while loop, which ensures that the loop executes at least once, even if the number is 0.

#include <stdio.h>
 
int main() {
    int num, original, digit, sum = 0;
 
    // Input
    printf("Enter a positive integer: ");
    scanf("%d", &num);
 
    // Validate input
    if (num < 0) {
        printf("Invalid input! Please enter a positive number.\n");
        return 1;
    }
 
    original = num; // store original number
 
    // Sum of digits using do-while loop
    if (num == 0) {
        sum = 0;
    } else {
        do {
            digit = num % 10;   // extract last digit
            sum += digit;       // add to sum
            num /= 10;          // remove last digit
        } while (num != 0);
    }
 
    // Display result
    printf("Sum of digits of %d = %d\n", original, sum);
 
    return 0;
}

Output

 
OUTPUT 1 :
Enter a positive integer: 12345
Sum of digits of 12345 = 15

OUTPUT 2 :
Enter a positive integer: 0
Sum of digits of 0 = 0

OUTPUT 3 :
Enter a positive integer: -987
Invalid input! Please enter a positive number.

Explanation

Read the positive number num and store it in original.
Validate that the input is non-negative.
Use a do-while loop:
- Extract the last digit: digit = num % 10.
- Add it to sum.
- Remove the last digit: num /= 10.
Loop continues until num becomes 0.
Print the sum of digits.

Basics of C Programming (Introductory Programs)

Control Statements & Loops in C

Functions

Arrays

Strings

Pointers

Structures and Unions

Dynamic Memory Allocation

File Handling

Data Structures in C

Advanced C Programs

User-Defined Number Programs in C

User-Defined String Programs in C

C Programs

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Loop Programs in C

Sum of digits of a number

Introduction

C Program: Sum of digits of a number

Method 1: Using for loop

Output

Explanation

C Program: Sum of digits of a number

Method 2: Using while loop

Introduction

Output

Explanation

C Program: Sum of digits of a number

Method 3: Using do..while loop

Introduction

Output

Explanation