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Find the Factorial of a large number

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  • Difficulty Level : Medium
  • Last Updated : 17 Oct, 2022
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Find the factorial of a large number. 

What is Factorial of a number?

Factorial of a non-negative integer, is the multiplication of all integers smaller than or equal to n. 

Factorial of a number

Factorial of a number

Examples: 

Input: 100
Output: 933262154439441526816992388562667004-
         907159682643816214685929638952175999-
         932299156089414639761565182862536979-
         208272237582511852109168640000000000-
         00000000000000

Input: 50
Output: 3041409320171337804361260816606476884-
         4377641568960512000000000000

We have discussed a simple program for factorial.

Why conventional way of computing factorial fails for large numbers?

A factorial of 100 has 158 digits. It is not possible to store these many digits even if we use long int. 

The idea is to use basic mathematics for multiplication.

Illustration:

Example to show working of multiply(res[], x)

  • A number 5189 is stored in res[] as following: res[] = {9, 8, 1, 5}
  • let x = 10
    Initialize carry = 0

 

  • At i = 0, prod = res[0]*x + carry = 9*10 + 0 = 90.
    res[0] = 0, carry = 9

 

 

  • At i = 1, prod = res[1]*x + carry = 8*10 + 9 = 89
    res[1] = 9, carry = 8

 

 

  • At i = 2, prod = res[2]*x + carry = 1*10 + 8 = 18
    res[2] = 8, carry = 1

 

 

  • At i = 3, prod = res[3]*x + carry = 5*10 + 1 = 51
    res[3] = 1, carry = 5

 

 

  • res[4] = carry = 5
    res[] = {0, 9, 8, 1, 5} 

 

Follow the steps below to solve the given problem:

  • Create an array res[] of MAX size where MAX is a number of maximum digits in output. 
  • Initialize value stored in res[] as 1 and initialize res_size (size of ‘res[]’) as 1. 
  • Multiply x with res[] and update res[] and res_size to store the multiplication result for all the numbers from x = 2 to n.
  • To multiply a number x with the number stored in res[], one by one multiply x with every digit of res[].
  • To implement multiply function perform the following steps:
    • Initialize carry as 0. 
    • Do following for i = 0 to res_size – 1 
      • Find value of res[i] * x + carry. Let this value be prod. 
      • Update res[i] by storing the last digit of prod in it. 
      • Update carry by storing the remaining digits in carry. 
    • Put all digits of carry in res[] and increase res_size by the number of digits in carry.

Below is the implementation of the above algorithm. 

NOTE: In the below implementation, the maximum digits in the output are assumed as 500. To find a factorial of a much larger number ( > 254), increase the size of an array or increase the value of MAX. This can also be solved using Linked List instead of using res[] array which will not waste extra space.

C++




// C++ program to compute factorial of big numbers
#include <iostream>
using namespace std;
 
// Maximum number of digits in output
#define MAX 500
 
int multiply(int x, int res[], int res_size);
 
// This function finds factorial of large numbers
// and prints them
void factorial(int n)
{
    int res[MAX];
 
    // Initialize result
    res[0] = 1;
    int res_size = 1;
 
    // Apply simple factorial formula n! = 1 * 2 * 3
    // * 4...*n
    for (int x = 2; x <= n; x++)
        res_size = multiply(x, res, res_size);
 
    cout << "Factorial of given number is \n";
    for (int i = res_size - 1; i >= 0; i--)
        cout << res[i];
}
 
// 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 returns the
// new value of res_size
int multiply(int x, int res[], int res_size)
{
    int carry = 0; // Initialize carry
 
    // One by one multiply n with individual digits of res[]
    for (int i = 0; i < res_size; i++) {
        int prod = res[i] * x + carry;
 
        // Store last digit of 'prod' in res[]
        res[i] = prod % 10;
 
        // Put rest in carry
        carry = prod / 10;
    }
 
    // Put carry in res and increase result size
    while (carry) {
        res[res_size] = carry % 10;
        carry = carry / 10;
        res_size++;
    }
    return res_size;
}
 
// Driver program
int main()
{
    factorial(100);
    return 0;
}

Java




// JAVA program to compute factorial
// of big numbers
class GFG {
 
    // This function finds factorial of
    // large numbers and prints them
    static void factorial(int n)
    {
        int res[] = new int[500];
 
        // Initialize result
        res[0] = 1;
        int res_size = 1;
 
        // Apply simple factorial formula
        // n! = 1 * 2 * 3 * 4...*n
        for (int x = 2; x <= n; x++)
            res_size = multiply(x, res, res_size);
 
        System.out.println("Factorial of given number is ");
        for (int i = res_size - 1; i >= 0; i--)
            System.out.print(res[i]);
    }
 
    // 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
    static int multiply(int x, int res[], int res_size)
    {
        int carry = 0; // Initialize carry
 
        // One by one multiply n with individual
        // digits of res[]
        for (int i = 0; i < res_size; i++) {
            int prod = res[i] * x + carry;
            res[i] = prod % 10; // Store last digit of
                                // 'prod' in res[]
            carry = prod / 10; // Put rest in carry
        }
 
        // Put carry in res and increase result size
        while (carry != 0) {
            res[res_size] = carry % 10;
            carry = carry / 10;
            res_size++;
        }
        return res_size;
    }
 
    // Driver program
    public static void main(String args[])
    {
        factorial(100);
    }
}
// This code is contributed by Nikita Tiwari

Python3




# Python program 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)
 
# This code is contributed by Nikita Tiwari.

C#




// C# program to compute
// factorial of big numbers
using System;
 
class GFG {
 
    // This function finds factorial
    // of large numbers and prints them
    static void factorial(int n)
    {
        int[] res = new int[500];
 
        // Initialize result
        res[0] = 1;
        int res_size = 1;
 
        // Apply simple factorial formula
        // n! = 1 * 2 * 3 * 4...*n
        for (int x = 2; x <= n; x++)
            res_size = multiply(x, res, res_size);
 
        Console.WriteLine("Factorial of "
                          + "given number is ");
        for (int i = res_size - 1; i >= 0; i--)
            Console.Write(res[i]);
    }
 
    // 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
    static int multiply(int x, int[] res, int res_size)
    {
        int carry = 0; // Initialize carry
 
        // One by one multiply n with
        // individual digits of res[]
        for (int i = 0; i < res_size; i++) {
            int prod = res[i] * x + carry;
            res[i] = prod % 10; // Store last digit of
                                // 'prod' in res[]
            carry = prod / 10; // Put rest in carry
        }
 
        // Put carry in res and
        // increase result size
        while (carry != 0) {
            res[res_size] = carry % 10;
            carry = carry / 10;
            res_size++;
        }
        return res_size;
    }
 
    // Driver Code
    static public void Main() { factorial(100); }
}
 
// This code is contributed by ajit

PHP




<?php
// PHP program to compute factorial
// of big numbers
 
// Maximum number of digits in output
$MAX = 500;
 
// This function finds factorial of
// large numbers and prints them
function factorial($n)
{
    global $MAX;
    $res = array_fill(0, $MAX, 0);
 
    // Initialize result
    $res[0] = 1;
    $res_size = 1;
 
    // Apply simple factorial formula
    // n! = 1 * 2 * 3 * 4...*n
    for ($x = 2; $x <= $n; $x++)
        $res_size = multiply($x, $res, $res_size);
 
    echo "Factorial of given number is \n";
    for ($i = $res_size - 1; $i >= 0; $i--)
        echo $res[$i];
}
 
// 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
function multiply($x, &$res, $res_size)
{
    $carry = 0; // Initialize carry
 
    // One by one multiply n with individual
    // digits of res[]
    for ($i = 0; $i < $res_size; $i++)
    {
        $prod = $res[$i] * $x + $carry;
 
        // Store last digit of 'prod' in res[]
        $res[$i] = $prod % 10;
 
        // Put rest in carry
        $carry = (int)($prod / 10);
    }
 
    // Put carry in res and increase
    // result size
    while ($carry)
    {
        $res[$res_size] = $carry % 10;
        $carry = (int)($carry / 10);
        $res_size++;
    }
    return $res_size;
}
 
// Driver Code
factorial(100);
     
// This code is contributed by chandan_jnu
?>

Javascript




<script>
 
// Javascript program to compute factorial of big numbers
 
// This function finds factorial of large numbers
// and prints them
function factorial(n)
{
    let res = new Array(500);
 
    // Initialize result
    res[0] = 1;
    let res_size = 1;
 
    // Apply simple factorial formula n! = 1 * 2 * 3 * 4...*n
    for (let x=2; x<=n; x++)
        res_size = multiply(x, res, res_size);
 
    document.write("Factorial of given number is " + "<br>");
    for (let i=res_size-1; i>=0; i--)
        document.write(res[i]);
}
 
// 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
function multiply(x, res, res_size)
{
    let carry = 0; // Initialize carry
 
    // One by one multiply n with individual digits of res[]
    for (let i=0; i<res_size; i++)
    {
        let prod = res[i] * x + carry;
 
        // Store last digit of 'prod' in res[]
        res[i] = prod % 10;
 
        // Put rest in carry
        carry = Math.floor(prod/10);
    }
 
    // Put carry in res and increase result size
    while (carry)
    {
        res[res_size] = carry%10;
        carry = Math.floor(carry/10);
        res_size++;
    }
    return res_size;
}
 
// Driver program
    factorial(100);
 
// This  code is contributed by Mayank Tyagi
 
</script>

Output

Factorial of given number is 
93326215443944152681699238856266700490715968264381621468592963895217599993229915608941463976156518286253697920827223758251185210916864000000000000000000000000

Time Complexity: O(N log (N!)), where O(N) is for loop and O(log N!) is for nested while loop
Auxiliary Space: O(max(digits in factorial))

Find the Factorial of a large number using Basic BigInteger

This problem can be solved using the below idea:

Big Integer can also be used to calculate the factorial of large numbers.

Illustration:

N = 5

ans = 1

At i = 2: ans = ans x i = 1 x 2 = 2
At i = 3: ans = ans x i = 2 x 3 = 6
At i = 4: ans = ans x i = 6 x 4 = 24
At i = 5: ans = ans x i = 24 x 5 = 120

Hence factorial of N is 120

Follow the steps below to solve the given problem: 

  • Declare a BigInteger f with 1 and perform the conventional way of calculating factorial
  • Traverse a loop from x = 2 to N and multiply x with f and store the resultant value in f

Below is the implementation of the above idea : 

C++




// C++ program to find large
// factorials using BigInteger
#include <bits/stdc++.h>
using namespace std;
 
#define ull unsigned long long
 
// Returns Factorial of N
ull factorial(int N)
{
 
    // Initialize result
    ull f = 1; // Or BigInt 1
 
    // Multiply f with 2, 3, ...N
    for (ull i = 2; i <= N; i++)
        f *= i;
 
    return f;
}
 
// Driver method
int main()
{
    int N = 20;
    cout << factorial(N) << endl;
}
 
// This code is contributed by phasing17

Java




// Java program to find large
// factorials using BigInteger
import java.math.BigInteger;
import java.util.Scanner;
 
public class Example {
 
    // Returns Factorial of N
    static BigInteger factorial(int N)
    {
        // Initialize result
        BigInteger f
            = new BigInteger("1"); // Or BigInteger.ONE
 
        // Multiply f with 2, 3, ...N
        for (int i = 2; i <= N; i++)
            f = f.multiply(BigInteger.valueOf(i));
 
        return f;
    }
 
    // Driver method
    public static void main(String args[]) throws Exception
    {
        int N = 20;
        System.out.println(factorial(N));
    }
}

Python3




# Python3 program to find large
# factorials
 
# Returns Factorial of N
def factorial(N):
 
    # Initialize result
    f =  1 
     
    # Multiply f with 2, 3, ...N
    for i in range(2, N + 1):
        f *= i
     
    return f;
 
 
# Driver method
N = 20;
print(factorial(N));
 
 
# This code is contributed by phasing17

C#




// C# program to find large
// factorials using BigInteger
using System;
using System.Collections.Generic;
using System.Numerics;
 
public class Example {
 
  // Returns Factorial of N
  static BigInteger factorial(int N)
  {
 
    // Initialize result
    BigInteger f
      = new BigInteger(1); // Or BigInteger.ONE
 
    // Multiply f with 2, 3, ...N
    for (int i = 2; i <= N; i++)
      f = BigInteger.Multiply(f, new BigInteger(i));
 
    return f;
  }
 
  // Driver method
  public static void Main(string[] args)
  {
    int N = 20;
    Console.WriteLine(factorial(N));
  }
}
 
// This code is contributed by phasing17

Javascript




// JavaScript program to find large
// factorials using BigInteger
 
// Returns Factorial of N
function factorial(N)
{
 
// Initialize result
let f =  BigInt(1); // Or BigInt 1
 
// Multiply f with 2, 3, ...N
for (var i = 2; i <= N; i++)
    f *= BigInt(i);
 
return f;
}
 
// Driver method
let N = 20;
console.log(factorial(N));
 
// This code is contributed by phasing17

Output

2432902008176640000

Time Complexity: O(n)
Auxiliary Space: O(1)


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