C++ Program To Find Sum of Fibonacci Numbers at Even Indexes Upto N Terms

// C++ Program to find sum of 
// even-indiced Fibonacci numbers
#include <bits/stdc++.h>
using namespace std;

// Computes value of first 
// fibonacci numbers and 
// stores the even-indexed sum
int calculateEvenSum(int n)
{
    if (n <= 0)
        return 0;

    int fibo[2 * n + 1];
    fibo[0] = 0, fibo[1] = 1;

    // Initialize result
    int sum = 0;

    // Add 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 the alternating sum
    return sum;
}

// Driver code
int main()
{

    // Get n
    int n = 8;

    // Find the even-indiced sum
    cout << "Even indexed Fibonacci Sum upto " << 
             n << " terms: " << calculateEvenSum(n) << 
             endl;

    return 0;
}

// C++ Program to find even indexed 
// Fibonacci Sum 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 even-indexed  
// Fibonacci Sum
int calculateEvenSum(int n)
{
    return (fib(2 * n + 1) - 1);
}

// Driver code
int main()
{
    // Get n
    int n = 8;

    // Find the alternating sum
    cout << "Even indexed Fibonacci Sum upto " << 
             n << " terms: " << calculateEvenSum(n) << 
             endl;

    return 0;
}