Program to find Nth odd Fibonacci Number

Last Updated : 01 Apr, 2021

Given an integer N. The task is to find the N^th odd Fibonacci number.
The odd number fibonacci series is as:

1, 1, 3, 5, 13, 21, 55, 89, 233, 377, 987, 1597………….and so on.
Note: In the above series we have omited even terms from the general fibonacci sequence.

Examples:

Input: N = 3
Output: 3

Input: N = 4
Output: 5

Approach:
On observing carefully, it can be deduced that every third Fibonacci number is even, so the N^th odd Fibonacci number is the {(3*N+1)/2}^th term in the general Fibonacci sequence.
Below is the implementation of the above approach:

C++

// C++ program for Nth odd fibonaci number
 
#include <bits/stdc++.h>
using namespace std;
 
// Function to find nth odd fibonaci number
int oddFib(int n)
{
    n = (3 * n + 1) / 2;
    int a = -1, b = 1, c, i;
    for (i = 1; i <= n; i++) {
        c = a + b;
        a = b;
        b = c;
    }
    return c;
}
 
// Driver Code
int main()
{
    int n = 4;
     
    cout << oddFib(n);
     
    return 0;
}

Java

// Java program for Nth odd fibonaci number
class GFG
{
     
    // Function to find nth odd fibonaci number
    static int oddFib(int n)
    {
        n = (3 * n + 1) / 2;
         
        int a = -1, b = 1, c = 0, i;
         
        for (i = 1; i <= n; i++)
        {
            c = a + b;
            a = b;
            b = c;
        }
        return c;
    }
     
    // Driver Code
    public static void main (String[] args)
    {
        int n = 4;
         
        System.out.println(oddFib(n));
    }
}
 
// This code is contributed by AnkitRai01

Python3

# Python3 program for Nth odd fibonaci number
 
# Function to find nth odd fibonaci number
def oddFib(n):
    n = (3 * n + 1) // 2
 
    a = -1
    b = 1
    c = 0
    for i in range(1, n + 1):
        c = a + b
        a = b
        b = c
 
    return c
 
# Driver Code
n = 4
 
print(oddFib(n))
 
# This code is contributed by mohit kumar

C#

// C# program for Nth odd fibonaci number
using System;
     
class GFG
{
     
    // Function to find nth odd fibonaci number
    static int oddFib(int n)
    {
        n = (3 * n + 1) / 2;
         
        int a = -1, b = 1, c = 0, i;
         
        for (i = 1; i <= n; i++)
        {
            c = a + b;
            a = b;
            b = c;
        }
        return c;
    }
     
    // Driver Code
    public static void Main (String[] args)
    {
        int n = 4;
         
        Console.WriteLine(oddFib(n));
    }
}
 
// This code is contributed by 29AjayKumar

Javascript

<script>
 
// JavaScript program for Nth odd fibonaci number
 
// Function to find nth odd fibonaci number
function oddFib(n)
{
    n = (3 * n + 1) / 2;
    var a = -1, b = 1, c, i;
    for (i = 1; i <= n; i++) {
        c = a + b;
        a = b;
        b = c;
    }
    return c;
}
 
// Driver Code
var n = 4;
document.write(oddFib(n));
 
</script>

Output:

Time Complexity: O(N)

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