Calculate Factorial Of a Number

/*

 * C Program to calculate factorial of a number.

 * A factorial of a number n is calculated and represented as follows : 

 * n! = n*(n-1)*(n-2)* ...... 1

 * 0! is considered as 1.

 * factorial of negative number is undefined.

 * Example : 4! = 4*3*2*1 = 24

 *

 * Limitation : Can calculate factorial upto 65 only. 

 * We can increase this number by using unsigned long long, 

 * but it may not be supported on all the compilers.

 */

// Include

#include<stdio.h>

// Declarations

/* factorialSimple

 * Description : Simple factorial function. 

 *               Calculates factorial of a number by using loop.

 * Parameters  : ui_number is the number of which factorial is to be calculated.

 * Return      : returns calculated factorial as an unsigned int.

 */

unsigned long factorialSimple(unsigned long ui_number);

/* factorialRecursive

 * Description : Recursive factorial function. 

 *               Calculates factorial of a number by using recursive function call.

 * Parameters  : ui_number is the number of which factorial is to be calculated.

 * Return      : returns calculated factorial as an unsigned int.

 */

unsigned long factorialRecursive(unsigned long ui_number);

// Definitions

int main()

{

    int i_number;

    unsigned long ui_result;

    printf("Enter number of which factorial needs to be calculated : ");

    scanf("%d",&i_number);

    

    if (i_number >=0 )

    {

        // Let us calculate factorial by using simple method of looping

        ui_result = factorialSimple(i_number);

        printf("Factorial using simple method    : %lu\n",ui_result);

        

        // Now let's try using recursive function call

        ui_result = factorialRecursive(i_number);

        printf("Factorial using recursive method : %lu\n",ui_result);

    }

    else

    {

        printf("Factorial of negative number is undefined.\n");

    }

}

unsigned long factorialSimple(unsigned long ui_number)

{

    unsigned long result = 1;

    

    // If number is 0 or 1 - return 1

    if ((0 == ui_number) || (1 == ui_number))

        return 1;

    // Calculate by multiplying each number starting with ui_number till 2

    for (; ui_number > 1; ui_number--)

        result *= ui_number;

    

    return result;

}

unsigned long factorialRecursive(unsigned long ui_number)

{

    // If number is 0 or 1 - return 1

    if ((0 == ui_number) || (1 == ui_number))

        return 1;

    

    // Calculate factorial by giving recursive call

    return ui_number*factorialRecursive(ui_number-1);

}

Sample Output :
Enter number of which factorial needs to be calculated : 4
Factorial using simple method    : 24
Factorial using recursive method : 24

Enter number of which factorial needs to be calculated : 65

Factorial using simple method    : 9223372036854775808

Factorial using recursive method : 9223372036854775808

Enter number of which factorial needs to be calculated : 0

Factorial using simple method    : 1

Factorial using recursive method : 1

Enter number of which factorial needs to be calculated : 1

Factorial using simple method    : 1

Factorial using recursive method : 1

Enter number of which factorial needs to be calculated : -8

Factorial of negative number is undefined.