1// C++ program to compute factorial of big numbers
2#include<iostream>
3using namespace std;
4
5// Maximum number of digits in output
6#define MAX 500
7
8int multiply(int x, int res[], int res_size);
9
10// This function finds factorial of large numbers
11// and prints them
12void factorial(int n)
13{
14 int res[MAX];
15
16 // Initialize result
17 res[0] = 1;
18 int res_size = 1;
19
20 // Apply simple factorial formula n! = 1 * 2 * 3 * 4...*n
21 for (int x=2; x<=n; x++)
22 res_size = multiply(x, res, res_size);
23
24 cout << "Factorial of given number is
25";
26 for (int i=res_size-1; i>=0; i--)
27 cout << res[i];
28}
29
30// This function multiplies x with the number
31// represented by res[].
32// res_size is size of res[] or number of digits in the
33// number represented by res[]. This function uses simple
34// school mathematics for multiplication.
35// This function may value of res_size and returns the
36// new value of res_size
37int multiply(int x, int res[], int res_size)
38{
39 int carry = 0; // Initialize carry
40
41 // One by one multiply n with individual digits of res[]
42 for (int i=0; i<res_size; i++)
43 {
44 int prod = res[i] * x + carry;
45
46 // Store last digit of 'prod' in res[]
47 res[i] = prod % 10;
48
49 // Put rest in carry
50 carry = prod/10;
51 }
52
53 // Put carry in res and increase result size
54 while (carry)
55 {
56 res[res_size] = carry%10;
57 carry = carry/10;
58 res_size++;
59 }
60 return res_size;
61}
62
63// Driver program
64int main()
65{
66 factorial(100);
67 return 0;
68}
69