# C Program to Find the Sum of Fibonacci Numbers at Even Indexes up to N Terms

• Last Updated : 01 Aug, 2022

Here, we will build a C Program to Find the Sum of Fibonacci numbers at even indexes up to N terms

Fibonacci Numbers: 0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144, ……

Input:

`N = 5`

Output:

`88 // Sum of the elements at even indexes = 0 + 1 + 3 + 8 + 21 + 55 = 88`

## Approach:

This approach includes solving the problem directly by finding all Fibonacci numbers till 2n and adding up only the even indices numbers from the array.

Example:

## C

 `// C Program to find the sum of``// even-indiced Fibonacci numbers``#include `` ` `// Computes value of the first ``// fibonacci numbers and stores``// the even-indexed sum``int` `calculateEvenSum(``int` `n)``{``    ``// return 0 if n is equals or less than to 0``    ``if` `(n <= 0)``        ``return` `0;`` ` `    ``int` `fibo[2 * n + 1];``    ``fibo = 0, fibo = 1;`` ` `    ``// Initialize the result``    ``int` `sum = 0;`` ` `    ``// Adding the remaining terms``    ``for` `(``int` `i = 2; i <= 2 * n; i++) {``        ``fibo[i] = fibo[i - 1] + fibo[i - 2];`` ` `        ``// For even indices``        ``if` `(i % 2 == 0)``            ``sum += fibo[i];``    ``}`` ` `    ``// Return alternating sum``    ``return` `sum;``}`` ` `// Driver Code``int` `main()``{`` ` `    ``// Get n``    ``int` `n = 5;`` ` `    ``// calculateEvenSum(n) function is computed and return``    ``// the sum of even-indiced Fibonacci numbers.``    ``int` `sum = calculateEvenSum(n);`` ` `    ``// display result``    ``printf``(``"Even indexed Fibonacci Sum upto %d terms = %d"``,``           ``n, sum);`` ` `    ``return` `0;``}`

Output

`Even indexed Fibonacci Sum upto 5 terms = 88`

Time Complexity: O(N)

Space Complexity: O(1)

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