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Loop Programs in C

Check whether a number is Armstrong (loop)

Introduction

An Armstrong number (also called a narcissistic number) is a number that is equal to the sum of its own digits each raised to the power of the number of digits.

Example:

  • 153 = 1³ + 5³ + 3³ = 153 → Armstrong
  • 370 = 3³ + 7³ + 0³ = 370 → Armstrong
  • 123 = 1³ + 2³ + 3³ = 36 → Not Armstrong

This program checks whether a given number is an Armstrong number using a for, while and do..while loop.

 

C Program: Check whether a number is Armstrong (loop)

Method 1: Using for loop

C 
#include <stdio.h>


#include <math.h> // for pow() function


 


int main() {


    int num, original, remainder, digits = 0;


    double result = 0.0;


 


    // 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;


 


    // Count digits using for loop


    for (int temp = num; temp != 0; temp /= 10)


        digits++;


 


    // Calculate sum of powers using for loop


    for (int temp = num; temp != 0; temp /= 10) {


        remainder = temp % 10;


        result += pow(remainder, digits);


    }


 


    // Check Armstrong condition


    if ((int)result == original)


        printf("%d is an Armstrong number.\n", original);


    else


        printf("%d is not an Armstrong number.\n", original);


 


    return 0;


}

Output

 
OUTPUT 1 :
Enter a positive integer: 153
153 is an Armstrong number.

OUTPUT 2 :
Enter a positive integer: 370
370 is an Armstrong number.

OUTPUT 3 :
Enter a positive integer: 123
123 is not an Armstrong number.

Explanation

  1. The user enters a positive integer.
  2. The program first counts the number of digits using a for
  3. Another for loop extracts each digit and computes its power using pow(remainder, digits).
  4. The results are summed into result.
  5. If the sum equals the original number → Armstrong number.

 

C Program: Check whether a number is Armstrong (loop)

Method 2: Using while loop

C 
#include <stdio.h>


#include <math.h> // for pow() function


 


int main() {


    int num, original, remainder, digits = 0;


    double result = 0.0;


 


    // 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;


 


    // Count number of digits


    int temp = num;


    while (temp != 0) {


        temp /= 10;


        digits++;


    }


 


    // Calculate sum of powers of digits


    temp = num;


    while (temp != 0) {


        remainder = temp % 10;


        result += pow(remainder, digits);


        temp /= 10;


    }


 


    // Check Armstrong condition


    if ((int)result == original)


        printf("%d is an Armstrong number.\n", original);


    else


        printf("%d is not an Armstrong number.\n", original);


 


    return 0;


}

Output

 
OUTPUT 1 :
Enter a positive integer: 153
153 is an Armstrong number.

OUTPUT 2 :
Enter a positive integer: 371
371 is an Armstrong number.

OUTPUT 3 :
Enter a positive integer: 123
123 is not an Armstrong number.

OUTPUT 4 :
Enter a positive integer: -370
Invalid input! Please enter a positive number.

Explanation

  1. Take a positive integer input.
  2. Store it in original for comparison.
  3. Count the total number of digits using a while
  4. Again, use a while loop to extract each digit and compute
    pow(digit, number_of_digits) and sum them.
  5. Compare the sum with the original number.
  6. If they match → Armstrong number; else → Not Armstrong.

 

Key Points

  • Works for any number of digits.
  • Uses <math.h> for the pow()
  • Uses two while loops: one for digit count, one for power-sum calculation.
  • Checks only positive integers as requested.

 

C Program: Check whether a number is Armstrong (loop)

Method 3: Using do..while loop

C 
#include <stdio.h>


#include <math.h> // for pow() function


 


int main() {


    int num, original, remainder, digits = 0;


    double result = 0.0;


 


    // 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;


 


    // Count digits using do...while loop


    int temp = num;


    do {


        digits++;


        temp /= 10;


    } while (temp != 0);


 


    // Calculate sum of powers of digits using do...while loop


    temp = num;


    do {


        remainder = temp % 10;


        result += pow(remainder, digits);


        temp /= 10;


    } while (temp != 0);


 


    // Check Armstrong condition


    if ((int)result == original)


        printf("%d is an Armstrong number.\n", original);


    else


        printf("%d is not an Armstrong number.\n", original);


 


    return 0;


}

Output

 
OUTPUT 1 :
Enter a positive integer: 153
153 is an Armstrong number.

OUTPUT 2 :
Enter a positive integer: 371
371 is an Armstrong number.

OUTPUT 3 :
Enter a positive integer: 123
123 is not an Armstrong number.

Explanation

  1. The user enters a positive integer.
  2. The program counts how many digits the number has using a do..while loop.
  3. Another do..while loop extracts each digit and calculates the sum of its powers.
  4. If this sum equals the original number, it’s an Armstrong number.

 

Key Points

  • Works for any number of digits.
  • Uses <math.h> for the pow()
  • Uses two do..while loops: one for digit count, one for power-sum calculation.
  • Checks only positive integers as requested.

 

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