How to determine whether a number is strong in C

In programming, a strong number is a special number whose sum of the factorial of every digit is equal to the original number. For example:

!1 + !4 + !5 = 145

And nope, the ! is not the negation unary operator of programming, this means a factorial number (read how to know the factorial of a number in C here) in mathematics. In this article, we'll explain you a pretty useful snippet in C that allows you to determine wheter a number is strong or not.


The logic of the code to determine if a number is strong may be tricky for newbies, but once explained it's pretty easy to do. Our code to determine wheter a number is strong is the following:


// Factorial function that returns the factorial of a given integer
// For more information about factorials:
int fact(int n){
	int i,fac=1;


	return fac;

int main(){
	int number, lastDigit, sum, count;
    sum = 0;

	printf("Provide the number to determine:");

	count = number;

	while(count != 0){
        // Obtain the last digit of the number
		lastDigit = count % 10;

        // Keep the count of the sum of the factorial of the last digit
		sum += fact(lastDigit);

		// Divide the count to know when the loop should end
		// e.g 145/10 = 14.5
		// e.g 14.5/10 = 1.45
		// e.g 1.45/10 = 0.145
		// This means the loop of 145, will iterate 3 times (because 0.145/10 = 0.014499999999999999).
		count = count / 10;
    // If the sum is equal to the given number, it's strong !
	if(sum == number){
		printf("The number is Strong");
		printf("The number is NOT strong");

	return 0;

The code works like this: our program will prompt our user for an integer that will be stored in the number variable. Now, for calculation purposes assign the value of the number to another variable that we named count. We defined as well a variable named sum that will contain the sum of the factorial of all the digits of the given number, this variable will have as initial value 0.

In order to find the last digit of a number mathematically, you can do this through the modulo operator and the remaining of the processed number will be the last digit, we'll store this value on the lastDigit variable. Now we will create a loop that will do 2 things until the count variable is equal to 0:

  1. Find the factorial of the last digit and add it to the sum.
  2. Remove the last digit from the number that we don't need (update count value)

Finally, once the loop ends, you will be able to compare whether the number is strong or not, comparing the sum variable with the given number at the beginning of our code, and that's it !

Happy coding !

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