C++ program to find out the sum of factorial series 1! + 2! + 3! + 4!...

C++ program to find out the sum of factorial series 1! + 2! + 3! + 4!…

In this tutorial, we will learn how to find the sum of factorial of a series up to a specific length. Our program will take the value of n from the user and find out the sum. For example, if the value of n is 2, it will find 1! + 2!, it it is 5, it will find 1! + 2! +… 5! etc.

With this post, you will learn how to get user inputs and how to find factorial in C++.

C++ program :

#include <iostream>

using namespace std;

int findFact(int n)
{
    return n == 1 ? 1 : n * findFact(n - 1);
}

int main()
{
    int n, sum = 0;

    cout << "Enter the value of n : " << endl;

    cin >> n;

    for (int i = 1; i <= n; i++)
    {
        sum += findFact(i);
    }

    cout << "Sum : " << sum << endl;
}

Explanation:

  • findFact method is used to find out the factorial of a number.
  • In the main function, we have two int variables n and sum.
  • Value of n is taken as user input. Using a for loop, we are finding the factorial of all numbers from 1 to n and adding all values to calculate the final result sum.

Sample output:

Enter the value of n : 
4
Sum : 33

Enter the value of n : 
10
Sum : 4037913

Method 2: By using the current factorial value:

We can also keep the current factorial value in a variable and multiply that with the next number to get the next value.

factorial of 1: 1
factorial of 2: factorial of 1 * 2
factorial of 3: factorial of 2 * 3
factorial of 4: factorial of 3 * 4

Below is the C++ program :

#include <iostream>

using namespace std;

int main()
{
    int n, sum = 0, currentFact = 1;

    cout << "Enter the value of n : " << endl;

    cin >> n;

    for (int i = 1; i <= n; i++)
    {
        currentFact *= i;
        sum += currentFact;
    }

    cout << "Sum : " << sum << endl;
}

Here, we are storing the factorial in currentFact variable. In each iteration of the loop, we are multiplying it with the current value of i to get the factorial for i.

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