Factorial program in C using a for loop, using recursion and by creating a function. Factorial is represented by '!', so five factorial is written as (5!), n factorial as (n!). Also, n! = n*(n-1)*(n-2)*(n-3)...3.2.1 and zero factorial is defined as one, i.e., 0! = 1.

## Factorial in C using a for loop

int main()

{

int c, n, f = 1;

printf("Enter a number to calculate its factorial\n");

scanf("%d", &n);

for (c = 1; c <= n; c++)

f = f * c;

printf("Factorial of %d = %d\n", n, f);

return 0;

}

Output of C factorial program:

Download Factorial program.

As n! grows at a faster rate than exponential function 2^{n}, overflow occurs even for two-digit numbers if we use built-in data type. To calculate factorials of such numbers, we need to use data structures such as array or strings.

## Factorial program in C using recursion

long factorial(int);

int main()

{

int n;

long f;

printf("Enter an integer to find its factorial\n");

scanf("%d", &n);

if (n < 0)

printf("Factorial of negative integers isn't defined.\n");

else

{

f = factorial(n);

printf("%d! = %ld\n", n, f);

}

return 0;

}

long factorial(int n)

{

if (n == 0) // Base case

return 1;

else

return (n*factorial(n-1));

}

In recursion, a function calls itself. In the above program, the factorial function is calling itself. To solve a problem using recursion, you must first express its solution in recursive form.

## C program to find factorial of a number

long factorial(int);

int main()

{

int n;

printf("Enter a number to calculate its factorial\n");

scanf("%d", &n);

printf("%d! = %ld\n", n, factorial(n));

return 0;

}

long factorial(int n)

{

int c;

long r = 1;

for (c = 1; c <= n; c++)

r = r * c;

return r;

}

To calculate the number of permutations, combinations, and to compute the probability, we use factorials.