fact.py

# Python code to compute factorial of big numbers

import sys

# This function finds factorial of large
# numbers and prints them
def factorial( n) :
	res = [None]*500
	# Initialize result
	res[0] = 1
	res_size = 1

	# Apply simple factorial formula
	# n! = 1 * 2 * 3 * 4...*n
	x = 2
	while x <= n :
		res_size = multiply(x, res, res_size)
		x = x + 1
	
	print ("Factorial of given number is")
	i = res_size-1
	while i >= 0 :
		sys.stdout.write(str(res[i]))
		sys.stdout.flush()
		i = i - 1
		

# This function multiplies x with the number
# represented by res[]. res_size is size of res[]
# or number of digits in the number represented
# by res[]. This function uses simple school
# mathematics for multiplication. This function
# may value of res_size and returns the new value
# of res_size
def multiply(x, res,res_size) :
	
	carry = 0 # Initialize carry

	# One by one multiply n with individual
	# digits of res[]
	i = 0
	while i < res_size :
		prod = res[i] *x + carry
		res[i] = prod % 10; # Store last digit of
							# 'prod' in res[]
		# make sure floor division is used
		carry = prod//10; # Put rest in carry
		i = i + 1

	# Put carry in res and increase result size
	while (carry) :
		res[res_size] = carry % 10
		# make sure floor division is used
		# to avoid floating value
		carry = carry // 10
		res_size = res_size + 1
		
	return res_size

# Driver program
factorial(100)