# Sum of Fibonacci Numbers

• Difficulty Level : Medium
• Last Updated : 27 Jan, 2023

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 straightforward, find all the Fibonacci numbers till f(n) and then add them up.

## C++

 `// C++ Program to find sum of Fibonacci numbers``#include``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

 ``

## Javascript

 ``

Output

```Sum of Fibonacci numbers is : 7
```

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

Method 2 : Without using extra space

## C++

 `#include ``using` `namespace` `std;` `int` `main() {` `    ``int` `n = 4, a = 0, b = 0, sumf = 1;``    ``if` `(n<=0)``      ``sumf = 0;``    ``int` `curr = 1;``    ``for``(``int` `i = 1;i

## Python3

 `#By using this code we can calculate sum of fibonocci numbers without using extra space``n ``=` `4``a ``=` `0``b ``=` `0``sumf ``=` `1``if` `n<``=``0``:``  ``sumf ``=` `0``curr ``=` `1``for` `i ``in` `range``(``1``,n):``  ``a ``=` `b``  ``b ``=` `curr``  ``curr ``=` `a``+``b``  ``sumf ``+``=` `curr``print``(``"The sum of fibonocci numbers is:"``,sumf)`

Output

```The sum of fibonocci numbers is:7
```

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

Method 3 (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) â€“ 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 ``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

 `// Java Program to find sum of Fibonacci numbers in``// O(Log n) time.``import` `java.util.*;` `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)``{``  ` `    ``// 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]>``0``)``        ``return` `f[n];` `    ``int` `k = ((n & ``1``)>``0``)? (n+``1``)/``2` `: n/``2``;` `    ``// Applying above formula [Note value n&1 is 1``    ``// if n is odd, else 0].``    ``f[n] = (n & ``1``)>``0``? (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``static` `int` `calculateSum(``int` `n)``{``    ``return` `fib(n+``2``) - ``1``;``}` `// Driver program to test above function``public` `static` `void` `main(String[] args)``{``    ``int` `n = ``4``;``    ``System.out.print(``"Sum of Fibonacci numbers is : "``         ``+ calculateSum(n) +``"\n"``);``}``}` `// This code is contributed by gauravrajput1`

## Python3

 `# 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

 ``

## Javascript

 ``

Output

```Sum of Fibonacci numbers is : 7
```

Time Complexity: O(logn)
Auxiliary Space: O(MAX)
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 review-team@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.