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Loop Programs in C

Check whether a number is Strong Number (loop)

Introduction

A Strong number (also called a factorial number) is a number that equals the sum of the factorials of its digits.

Examples:

  • 145 = 1! + 4! + 5! = 1 + 24 + 120 = 145 →  Strong number
  • 40585 = 4! + 0! + 5! + 8! + 5! = 40585 →  Strong number
  • 123 = 1! + 2! + 3! = 9 →  Not a Strong number

 

C Program: Check whether a number is Strong Number (loop)

Method 1: Using for loop

C 
#include <stdio.h>


 


int main() {


    int num, original, remainder, 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;


 


    // Calculate sum of factorials of digits using for loop


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


        remainder = temp % 10;


        int fact = 1;


 


        // calculate factorial of the digit


        for (int i = 1; i <= remainder; i++) {


            fact *= i;


        }


 


        sum += fact; // add factorial to sum


    }


 


    // Compare sum with original number


    if (sum == original)


        printf("%d is a Strong number.\n", original);


    else


        printf("%d is not a Strong number.\n", original);


 


    return 0;


}

Output

 
OUTPUT 1 :
Enter a positive integer: 145
145 is a Strong number.

OUTPUT 2 :
Enter a positive integer: 123
123 is not a Strong number.

OUTPUT 3 :
Enter a positive integer: 40585
40585 is a Strong number.

Explanation

  1. Take a positive integer as input.
  2. Store the value in original for comparison.
  3. Use a for loop to extract each digit of the number:
    • Find the last digit with % 10.
    • Calculate its factorial using another for
    • Add the factorial to the running total sum.
  4. Compare sum and original.
  5. If they are equal → Strong number, else not.

 

C Program: Check whether a number is Strong Number (loop)

Method 2: Using while loop

C 
#include <stdio.h>


 


int main() {


    int num, original, remainder, sum = 0, i, fact;


 


    // 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;


 


    // Check Strong number


    while (num != 0) {


        remainder = num % 10;  // extract last digit


        fact = 1;


 


        // calculate factorial of each digit


        for (i = 1; i <= remainder; i++) {


            fact *= i;


        }


 


        sum += fact;   // add factorial to sum


        num /= 10;     // remove last digit


    }


 


    // Compare sum with original number


    if (sum == original)


        printf("%d is a Strong number.\n", original);


    else


        printf("%d is not a Strong number.\n", original);


 


    return 0;


}

Output

 
OUTPUT 1 :
Enter a positive integer: 145
145 is a Strong number.

OUTPUT 2 :
Enter a positive integer: 123
123 is not a Strong number.

OUTPUT 3 :
Enter a positive integer: -145
Invalid input! Please enter a positive number.

Explanation

  1. The user enters a positive integer.
  2. The number is stored in original for later comparison.
  3. Each digit is extracted using % 10.
  4. The factorial of each digit is calculated using a for
  5. The factorial values are summed.
  6. If the sum equals the original number → it’s a Strong number.

 

C Program: Check whether a number is Strong Number (loop)

Method 3: Using do..while loop

C 
#include <stdio.h>


 


int main() {


    int num, original, remainder, sum = 0, fact, i;


 


    // 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;


 


    // Check Strong number using do...while loop


    do {


        remainder = num % 10;   // extract last digit


        fact = 1;


 


        // calculate factorial of the digit


        for (i = 1; i <= remainder; i++) {


            fact *= i;


        }


 


        sum += fact;   // add factorial to sum


        num /= 10;     // remove last digit


 


    } while (num != 0);


 


    // Compare sum with original number


    if (sum == original)


        printf("%d is a Strong number.\n", original);


    else


        printf("%d is not a Strong number.\n", original);


 


    return 0;


}

Output

 
OUTPUT 1 :
Enter a positive integer: 145
145 is a Strong number.

OUTPUT 2 :
Enter a positive integer: 123
123 is not a Strong number.

OUTPUT 3 :
Enter a positive integer: 40585
40585 is a Strong number.

Explanation

  1. Take a positive integer as input.

  2. Store it in original for comparison later.

  3. Use a do...while loop to process digits:

    • Extract the last digit using % 10.

    • Compute its factorial using a nested for loop.

    • Add the factorial to sum.

    • Remove the last digit using /= 10.

  4. Continue until the number becomes 0.

  5. Compare the sum with the original number.

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