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

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-indices 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-indices 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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