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Efficient Program to Compute Sum of Series 1/1! + 1/2! + 1/3! + 1/4! + .. + 1/n!
  • Difficulty Level : Easy
  • Last Updated : 07 May, 2018

Given a positive integer n, write a function to compute sum of the series 1/1! + 1/2! + .. + 1/n!

A Simple Solution solution is to initialize sum as 0, then run a loop and call factorial function inside the loop.

Following is the implementation of simple solution.

C++




// A simple C++ program to compute sum of series 1/1! + 1/2! + .. + 1/n!
#include <iostream>
using namespace std;
  
//  Utility function to find
int factorial(int n)
{
    int res = 1;
    for (int i=2; i<=n; i++)
       res *= i;
    return res;
}
  
// A Simple Function to return value of 1/1! + 1/2! + .. + 1/n!
double sum(int n)
{
    double sum = 0;
    for (int i = 1; i <= n; i++)
        sum += 1.0/factorial(i);
    return sum;
}
  
// Driver program to test above functions
int main()
{
    int n = 5;
    cout << sum(n);
    return 0;
}


Java




// A simple Java program to compute 
// sum of series 1/1! + 1/2! + .. + 1/n!
import java.io.*;
  
class GFG {
      
    // Utility function to find
    static int factorial(int n)
    {
        int res = 1;
        for (int i = 2; i <= n; i++)
        res *= i;
        return res;
    }
      
    // A Simple Function to return value
    // of 1/1! + 1/2! + .. + 1/n!
    static double sum(int n)
    {
        double sum = 0;
        for (int i = 1; i <= n; i++)
            sum += 1.0/factorial(i);
        return sum;
    }
  
    // Driver program 
    public static void main (String[] args) 
    {
        int n = 5;
        System.out.println(sum(n));
    }
}
  
// This code is contributed by Ajit.


Python3




# Python3 program to compute sum of series
# 1/1! + 1/2! + .. + 1/n!
  
# Function to find factorail of a number 
def factorial(n):
    res = 1
    for i in range(2, n + 1):
            res *= i
    return res
          
# A Simple Function to return value 
# of 1/1! + 1/2! + .. + 1/n!
def sum(n):
    s = 0.0
      
    for i in range(1, n + 1):
        s += 1.0 / factorial(i)
    print(s)
  
# Driver program to test above functions
n = 5
sum(n)
  
# This code is contributed by Danish Raza


C#




// A simple C# program to compute sum 
// of series 1/1! + 1/2! + .. + 1/n!
using System;
  
class GFG {
      
    // Utility function to find
    static int factorial(int n)
    {
        int res = 1;
        for (int i = 2; i <= n; i++)
            res *= i;
              
        return res;
    }
      
    // A Simple Function to return value
    // of 1/1! + 1/2! + .. + 1/n!
    static double sum(int n)
    {
        double sum = 0;
        for (int i = 1; i <= n; i++)
            sum += 1.0/factorial(i);
              
        return sum;
    }
  
    // Driver program 
    public static void Main () 
    {
        int n = 5;
          
        Console.WriteLine(sum(n));
    }
}
  
// This code is contributed by Sam007.


PHP




<?php
// A simple PHP program to compute
// sum of series 1/1! + 1/2! + .. + 1/n!
  
// Utility function to find
function factorial($n)
{
    $res = 1;
    for ($i = 2; $i <= $n; $i++)
        $res *= $i;
    return $res;
}
  
// A Simple Function to return 
// value of 1/1! + 1/2! + .. + 1/n!
function sum($n)
{
    $sum = 0;
    for ($i = 1; $i <= $n; $i++)
        $sum += 1.0 / factorial($i);
    return $sum;
}
  
// Driver Code
$n = 5;
echo(sum($n));
  
// This code is contributed by Ajit.
?>



Output:

1.71667

The time complexity of above solution is O(n * n!) which is huge.



An Efficient Solution can find the sum in O(n) time. The idea is to calculate factorial in same loop as sum. Following is the implementation of this idea.

C++




// A simple C++ program to compute sum of series 1/1! + 1/2! + .. + 1/n!
#include <iostream>
using namespace std;
  
// An Efficient Function to return value of 1/1! + 1/2! + .. + 1/n!
double sum(int n)
{
    double sum = 0;
    int fact = 1;
    for (int i = 1; i <= n; i++)
    {
       fact *= i;         // Update factorial
       sum += 1.0/fact;   // Update series sum
    }
    return sum;
}
  
// Driver program to test above functions
int main()
{
    int n = 5;
    cout << sum(n);
    return 0;
}


Java




// A simple Java program to compute 
// sum of series 1/1! + 1/2! + .. + 1/n!
import java.io.*;
  
class GFG {
      
    // An Efficient Function to return 
    // value of 1/1! + 1/2! + .. + 1/n!
    static double sum(int n)
    {
        double sum = 0;
        int fact = 1;
        for (int i = 1; i <= n; i++)
        {
            // Update factorial
            fact *= i;
              
            // Update series sum
            sum += 1.0/fact; 
        }
        return sum;
    }
  
    // Driver program 
    public static void main (String[] args) 
    {
        int n = 5;
        System.out.println(sum(n));
    }
}
  
// This code is contributed by Ajit.


Python3




# Python3 program to compute sum of series 
# 1/1! + 1/2! + .. + 1/n!
  
# Function to return value of
# 1/1! + 1/2! + .. + 1/n!
def sum(n):
    sum = 0
    fact = 1
  
    for i in range(1, n + 1):
  
        # Update factorial
        fact *=
  
        # Update series sum
        sum += 1.0/fact 
  
    print(sum)
  
# Driver program to test above functions
n = 5
sum(n)
  
# This code is contributed by Danish Raza


C#




// A simple C# program to compute sum
// of series 1/1! + 1/2! + .. + 1/n!
using System;
  
class GFG {
      
    // An Efficient Function to return 
    // value of 1/1! + 1/2! + .. + 1/n!
    static double sum(int n)
    {
        double sum = 0;
        int fact = 1;
          
        for (int i = 1; i <= n; i++)
        {
              
            // Update factorial
            fact *= i;
              
            // Update series sum
            sum += 1.0 / fact; 
        }
        return sum;
    }
  
    // Driver program 
    public static void Main () 
    {
        int n = 5;
          
        Console.WriteLine(sum(n));
    }
}
  
// This code is contributed by Sam007.


PHP




<?php
// A simple PHP program to 
// compute sum of series 
// 1/1! + 1/2! + .. + 1/n!
  
// An Efficient Function to 
// return value of 1/1! + 
// 1/2! + .. + 1/n!
function sum($n
{
    $sum = 0;
    $fact = 1;
    for ($i = 1; $i <= $n; $i++)
    {
        // Update factorial
        $fact *= $i;    
          
        // Update series sum
        $sum += 1.0 / $fact
    }
    return $sum;
}
  
// Driver Code
$n = 5;
echo sum($n);
  
// This code is contributed by vt_m.
?>



Output:

1.71667

This article is contributed by Rahul Gupta. Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above.

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