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Sum of Fibonacci Numbers

  • Difficulty Level : Medium
  • Last Updated : 25 Mar, 2021

Given a number positive number n, find value of f0 + f1 + f2 + …. + fn where fi indicates i’th Fibonacci number. Remember that f0 = 0, f1 = 1, f2 = 1, f3 = 2, f4 = 3, f5 = 5, … 
Examples : 
 

Input  : n = 3
Output : 4
Explanation : 0 + 1 + 1 + 2  = 4

Input  :  n = 4
Output :  7
Explanation : 0 + 1 + 1 + 2 + 3  = 7

 

 
Method 1 (O(n)) 
Brute Force approach is pretty straight forward, find all the Fibonacci numbers till f(n) and then add them up. 
 

C++




// C++ Program to find sum of Fibonacci numbers
#include<bits/stdc++.h>
using namespace std;
 
// Computes value of first fibonacci numbers
int calculateSum(int n)
{
    if (n <= 0)
       return 0;
 
    int fibo[n+1];
    fibo[0] = 0, fibo[1] = 1;
 
    // Initialize result
    int sum = fibo[0] + fibo[1];
 
    // Add remaining terms
    for (int i=2; i<=n; i++)
    {
        fibo[i] = fibo[i-1]+fibo[i-2];
        sum += fibo[i];
    }
 
    return sum;
}
 
// Driver program to test above function
int main()
{
    int n = 4;
    cout << "Sum of Fibonacci numbers is : "
         << calculateSum(n) << endl;
    return 0;
}

Java




// Java Program to find
// sum of Fibonacci numbers
 
import java.io.*;
 
class GFG {
     
    // Computes value of first
    // fibonacci numbers
    static int calculateSum(int n)
    {
        if (n <= 0)
           return 0;
      
        int fibo[]=new int[n+1];
        fibo[0] = 0; fibo[1] = 1;
      
        // Initialize result
        int sum = fibo[0] + fibo[1];
      
        // Add remaining terms
        for (int i=2; i<=n; i++)
        {
            fibo[i] = fibo[i-1]+fibo[i-2];
            sum += fibo[i];
        }
      
        return sum;
    }
      
    // Driver program to test above function
    public static void main(String args[])
    {
        int n = 4;
        System.out.println("Sum of Fibonacci" +
        " numbers is : "+ calculateSum(n));
    }
}
 
// This code is contributed by Nikita tiwari.

Python3




# Python 3 Program to find
# sum of Fibonacci numbers
 
 
# Computes value of first
# fibonacci numbers
def calculateSum(n) :
    if (n <= 0) :
        return 0
  
    fibo =[0] * (n+1)
    fibo[1] = 1
  
    # Initialize result
    sm = fibo[0] + fibo[1]
  
    # Add remaining terms
    for i in range(2,n+1) :
        fibo[i] = fibo[i-1] + fibo[i-2]
        sm = sm + fibo[i]
         
    return sm
 
 
# Driver program to test
# above function
n = 4
print("Sum of Fibonacci numbers is : " ,
      calculateSum(n))
 
# This code is contributed
# by Nikita tiwari.

C#




// C# Program to find
// sum of Fibonacci numbers
using System;
 
class GFG
{
     
    // Computes value of first
    // fibonacci numbers
    static int calculateSum(int n)
    {
        if (n <= 0)
        return 0;
     
        int []fibo = new int[n + 1];
        fibo[0] = 0; fibo[1] = 1;
     
        // Initialize result
        int sum = fibo[0] + fibo[1];
     
        // Add remaining terms
        for (int i = 2; i <= n; i++)
        {
            fibo[i] = fibo[i - 1] + fibo[i - 2];
            sum += fibo[i];
        }
     
        return sum;
    }
     
    // Driver Code
    static void Main()
    {
        int n = 4;
        Console.WriteLine( "Sum of Fibonacci" +
                              " numbers is : "+
                              calculateSum(n));
    }
}
 
// This code is contributed by Anuj_67

PHP




<?php
// PHP Program to find sum
// of Fibonacci numbers
 
// Computes value of first
// fibonacci numbers
function calculateSum($n)
{
    if ($n <= 0)
    return 0;
 
    $fibo[0] = 0;
    $fibo[1] = 1;
 
    // Initialize result
    $sum = $fibo[0] + $fibo[1];
 
    // Add remaining terms
    for($i = 2; $i <= $n; $i++)
    {
        $fibo[$i] = $fibo[$i - 1] +
                    $fibo[$i - 2];
        $sum += $fibo[$i];
    }
 
    return $sum;
}
 
    // Driver Code
    $n = 4;
    echo "Sum of Fibonacci numbers is : ",
          calculateSum($n),"\n";
 
// This code is contributed by aj_36
?>

Javascript




<script>
// Javascript Program to find sum
// of Fibonacci numbers
 
// Computes value of first
// fibonacci numbers
function calculateSum(n)
{
    let fibo = [];
    if (n <= 0)
    return 0;
 
    fibo[0] = 0;
    fibo[1] = 1;
 
    // Initialize result
    let sum = fibo[0] + fibo[1];
 
    // Add remaining terms
    for(let i = 2; i <= n; i++)
    {
        fibo[i] = fibo[i - 1] +
                    fibo[i - 2];
        sum += fibo[i];
    }
 
    return sum;
}
 
    // Driver Code
    let n = 4;
    document.write(`Sum of Fibonacci numbers is :
        ${calculateSum(n)} <br>`);
 
// This code is contributed by _saurabh_jaiswal
</script>

Output : 
 

Sum of Fibonacci numbers is : 7

 
Method 2 (O(Log n)) 
The idea is to find relationship between the sum of Fibonacci numbers and n’th Fibonacci number.
F(i) refers to the i’th Fibonacci number. 
S(i) refers to sum of Fibonacci numbers till F(i), 
 



We can rewrite the relation F(n+1) = F(n) + F(n-1) as below
F(n-1)    = F(n+1)  -  F(n)

Similarly,
F(n-2)    = F(n)    -  F(n-1)
.          .           .
.          .             .
.          .             .
F(0)      = F(2)    -  F(1)
-------------------------------

Adding all the equations, on left side, we have 
F(0) + F(1) + … F(n-1) which is S(n-1).
Therefore, 
S(n-1) = F(n+1) – F(1) 
S(n-1) = F(n+1) – 1 
S(n) = F(n+2) – 1 —-(1)
In order to find S(n), simply calculate the (n+2)’th Fibonacci number and subtract 1 from the result.
F(n) can be evaluated in O(log n) time using either method 5 or method 6 in this article (Refer to methods 5 and 6).
Below is the implementation based on method 6 of this 
 

C++




// C++ Program to find sum of Fibonacci numbers in
// O(Log n) time.
#include <bits/stdc++.h>
using namespace std;
const int MAX = 1000;
 
// Create an array for memoization
int f[MAX] = {0};
 
// Returns n'th Fibonacci number using table f[]
int fib(int n)
{
    // Base cases
    if (n == 0)
        return 0;
    if (n == 1 || n == 2)
        return (f[n] = 1);
 
    // If fib(n) is already computed
    if (f[n])
        return f[n];
 
    int k = (n & 1)? (n+1)/2 : n/2;
 
    // Applying above formula [Note value n&1 is 1
    // if n is odd, else 0].
    f[n] = (n & 1)? (fib(k)*fib(k) + fib(k-1)*fib(k-1))
           : (2*fib(k-1) + fib(k))*fib(k);
 
    return f[n];
}
 
// Computes value of first Fibonacci numbers
int calculateSum(int n)
{
    return fib(n+2) - 1;
}
 
// Driver program to test above function
int main()
{
    int n = 4;
    cout << "Sum of Fibonacci numbers is : "
         << calculateSum(n) << endl;
    return 0;
}

Java] 


# Python 3 Program to find sum of 
# Fibonacci numbers in O(Log n) time.

MAX = 1000

# Create an array for memoization
f = [0] * MAX

# Returns n'th Fibonacci number
# using table f[]
def fib(n):
    
    n = int(n)

    # Base cases
    if (n == 0):
        return 0
    if (n == 1 or n == 2):
        return (1) 

    # If fib(n) is already computed
    if (f[n] == True):
        return f[n] 

    k = (n+1)/2 if (n & 1) else n/2

    # Applying above formula [Note value n&1 
    # is 1 if n is odd, else 0].
    f[n] = (fib(k) * fib(k) + fib(k-1) * fib(k-1)) if (n & 1) else (2 * fib(k-1) + fib(k)) * fib(k)
    return f[n] 

# Computes value of first Fibonacci numbers
def calculateSum(n):

    return fib(n+2) - 1

# Driver program to test above function
n = 4
print("Sum of Fibonacci numbers is :", calculateSum(n)) 

# This code is contributed by
# Smitha Dinesh Semwal


C#




// C# Program to find sum
// of Fibonacci numbers in
// O(Log n) time.
using System;
 
class GFG {
    static int MAX = 1000;
 
    // Create an array for memoization
    static int []f = new int[MAX];
     
    // Returns n'th Fibonacci
    // number using table f[]
    static int fib(int n)
    {
        for(int i = 0;i < MAX;i++)
        f[i] = 0;
         
        //Arrays.fill(f, 0);
        // Base cases
        if (n == 0)
            return 0;
        if (n == 1 || n == 2)
            return (f[n] = 1);
     
        // If fib(n) is
        // already computed
        if (f[n] == 1)
            return f[n];
            int k;
        if((n & 1) == 1)
            k = (n + 1) / 2 ;
        else
            k = n / 2;
     
        // Applying above formula
        // [Note value n&1 is 1
        // if n is odd, else 0].
        if((n & 1) == 1)
            f[n] = (fib(k) * fib(k) + fib(k - 1)
                                   * fib(k - 1));
        else
            f[n] = (2 * fib(k - 1) + fib(k)) *
                                       fib(k);
     
        return f[n];
    }
     
    // Computes value of first
    // Fibonacci numbers
    static int calculateSum(int n)
    {
        return fib(n + 2) - 1;
    }
     
    // Driver Code
    public static void Main()
    {
        int n = 4;
        Console.Write( "Sum of Fibonacci numbers is : "
                                    + calculateSum(n));
         
    }
}
 
// This code is contributed by nitin mittal.

PHP




<?php
// PHP Program to find sum of Fibonacci
// numbers in O(Log n) time.
$MAX = 1000;
 
// Create an array for memoization
$f = array_fill(0, $MAX, 0);
 
// Returns n'th Fibonacci number
// using table f[]
function fib($n)
{
    global $f;
     
    // Base cases
    if ($n == 0)
        return 0;
    if ($n == 1 || $n == 2)
        return ($f[$n] = 1);
 
    // If fib(n) is already computed
    if ($f[$n])
        return $f[$n];
 
    $k = ($n & 1) ? ($n + 1) / 2 : $n / 2;
 
    // Applying above formula [Note value n&1
    // is 1 if n is odd, else 0].
    $f[$n] = ($n & 1) ?
             (fib($k) * fib($k) + fib($k - 1) * fib($k - 1)) :
                  (2 * fib($k - 1) + fib($k)) * fib($k);
 
    return $f[$n];
}
 
// Computes value of first Fibonacci numbers
function calculateSum($n)
{
    return fib($n + 2) - 1;
}
 
// Driver Code
$n = 4;
print("Sum of Fibonacci numbers is : " .
                      calculateSum($n));
 
// This code is contributed by mits
?>

Output : 
 

Sum of Fibonacci numbers is : 7

This article is contributed by Chirag Agarwal. If you like GeeksforGeeks and would like to contribute, you can also write an article and mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.
Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above
 

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