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Factorial of a large number
In computers, variables are stored in memory locations. But the size of the memory location is fixed, so when we try to find the factorial of some greater value like 15! or 20! the factorial value exceeds the memory range and returns wrong results.
For calculation of large numbers, we have to use an array to store results. In each element of the array, is storing different digits of the result. But here we cannot multiply some number with the array directly, we have to perform manual multiplication process for all digits of the result array.
Input and Output
Input: A big number: 50 Output: Factorial of given number is: 30414093201713378043612608166064768844377641568960512000000000000
Algorithm
multiply(x, multiplicand)
Input: The number x, and the large multiplicand as an array.
Output: Result after multiplication.
Begin carry := 0 for all digits i of multiplicand, do prod := i*x+carry i := prod mod 10 carry := prod / 10 done while carry ≠ 0, do insert (carry mod 10) at the end of multiplicand array carry := carry/10 done End
factorial(n)
Input: The number n.
Output: Find factorial of n.
Begin define result array. insert 1 in the result for i := 2 to n, do multiply(i, result) done reverse the result return result End
Example
#include<iostream>
#include<vector>
#include<algorithm>
using namespace std;
void multiply(int x, vector<int>&multiplicand) { //multiply multiplicand with x
int carry = 0; // Initialize carry to 0
vector<int>::iterator i;
for (i=multiplicand.begin(); i!=multiplicand.end(); i++) { //multiply x with all digit of multiplicand
int prod = (*i) * x + carry;
*i = prod % 10; //put only the last digit of product
carry = prod/10; //add remaining part with carry
}
while (carry) { //when carry is present
multiplicand.push_back(carry%10);
carry = carry/10;
}
}
void factorial(int n) {
vector<int> result;
result.push_back(1); //at first store 1 as result
for (int i=2; i<=n; i++)
multiply(i, result); //multiply numbers 1*2*3*......*n
cout << "Factorial of given number is: "<<endl;
reverse(result.begin(), result.end());
vector<int>::iterator it; //reverse the order of result
for(it = result.begin(); it != result.end(); it++)
cout << *it;
}
int main() {
factorial(50);
}Output
Factorial of given number is: 30414093201713378043612608166064768844377641568960512000000000000
Updated on: 17-Jun-2020
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