## C, , factorial_large_number.c

``````/**
* @file
* \brief Compute factorial of arbitrarily large numbers by
* storing individual digits in a byte.
* \author [Krishna Vedala](https://github.com/kvedala)
*/
#include <stdio.h>
#include <stdlib.h>
#include <time.h>

/**
* dynamically large number
*/
typedef struct _large_num
{
char *digits;            /**< array to store individual digits */
unsigned int num_digits; /**< number of digits in the number */
} large_num;

/**
* create a new large number
* \returns pointer to a large number
*/
large_num *new_number(void)
{
large_num *new_num = (large_num *)malloc(sizeof(large_num));
new_num->num_digits = 1;
new_num->digits = (char *)malloc(1 * sizeof(char));
new_num->digits[0] = 1;
return new_num;
}

/**
* delete all memory allocated for large number
* \param[in] num pointer to large_num to delete
*/
void delete_number(large_num *num)
{
free(num->digits);
free(num);
}

/**
* add a digit to the large number
* \param[in,out] num
* \param[in] value value of the digit to insert
*/
void add_digit(large_num *num, unsigned int value)
{
if (value > 9)
{
fprintf(stderr, "digit > 9!!\n");
delete_number(num);
exit(EXIT_FAILURE);
}

num->num_digits++;
num->digits = (char *)realloc(num->digits, num->num_digits * sizeof(char));
num->digits[num->num_digits - 1] = value;
}

/**
* multiply large number with another integer and
* store the result in the same large number
*/
void multiply(large_num *num, unsigned long n)
{
int i;
unsigned long carry = 0, temp;
for (i = 0; i < num->num_digits; i++)
{
temp = num->digits[i] * n;
temp += carry;
if (temp < 10)
carry = 0;
else
{
carry = temp / 10;
temp = temp % 10;
}
num->digits[i] = temp;
}

while (carry != 0)
{
carry /= 10;
}
}

/**
* main function
*/
int main(int argc, char *argv[])
{
int number, i;

// Asks for the number/position of term in Fibonnacci sequence
if (argc == 2)
number = atoi(argv[1]);
else
{
printf("Enter the value of n(n starts from 0 ): ");
scanf("%d", &number);
}

large_num *result = new_number();

clock_t start_time = clock();
for (i = 2; i <= number; i++) /* Multiply every number from 2 thru N */
multiply(result, i);
double time_taken = (clock() - start_time) * (double)1e3 / CLOCKS_PER_SEC;
// time_taken = (clock() - start_time) / (double) CLOCKS_PER_SEC;

printf("%d! = ", number);
for (i = result->num_digits; i > 0; i--)
putchar(result->digits[i - 1] + '0');
printf("\nTime taken: %.4g ms\n", time_taken);

delete_number(result);
return 0;
}
``````