freeCodeCamp/guide/english/certifications/javascript-algorithms-and-data-structures/intermediate-algorithm-scripting/sum-all-odd-fibonacci-numbers/index.md

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Sum All Odd Fibonacci Numbers

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Problem Explanation:

You will need to gather all the Fibonacci numbers and then check for the odd ones. Once you get the odd ones then you will add them all. The last number should be the number given as a parameter if it actually happens to be an off Fibonacci number.

Relevant Links

• Fibonacci number

Hint: 1

To get the next number of the series, you need to add the current one to the previous and that will give you the next one.

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Hint: 2

To check if a number is even all you have to check is if number % 2 == 0.

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Hint: 3

As you get the next odd one, don't forget to add it to a global variable that can be returned at the end. result += currNumber; will do the trick.

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Spoiler Alert!

Solution ahead!

Basic Code Solution:

function sumFibs(num) {
    var prevNumber = 0;
    var currNumber = 1;
    var result = 0;
    while (currNumber <= num) {
        if (currNumber % 2 !== 0) {
            result += currNumber;
        }

        currNumber += prevNumber;
        prevNumber = currNumber - prevNumber;
    }

    return result;
}

// test here
sumFibs(4);

Run Code

Code Explanation:

• Create a variable to keep record of the current and previous numbers along with the result that will be returned.
• Use a while loop to make sure we do not go over the number given as parameter.
• We use the modulo operand to check if the current number is odd or even. If it is even, add it to the result.
• Complete the Fibonacci circle by rotating getting the next number and swapping values after.
• Return the result.

Relevant Links

• JS while Loop

Intermediate Code Solution:

function sumFibs(num) {
    // Perform checks for the validity of the input
    if (num < 0) return -1;
    if (num === 0 || num === 1) return 1;

    // Create an array of fib numbers till num
    const arrFib = [1, 1];
    let nextFib = 0;
    
    // We put the new Fibonacci numbers to the front so we
    // don't need to calculate the length of the array on each
    // iteration
    while((nextFib = arrFib[0] + arrFib[1]) <= num) {
        arrFib.unshift(nextFib);
    }

    // Sum only the odd numbers and return the value
    // First, reverse the array to avoid starting acc with the first/greater number when it's even
    return arrFib.reverse().reduce((acc, curr) => {
        return acc + curr * (curr % 2);
    }, 0);
}

// test here
sumFibs(4);

Run Code

Code Explanation:

• Create an array of fibonacci numbers till num.
• Use reduce() method to find the sum of odd members of array.
• Return the sum.

Relevant Links

• JS Array Prototype Push
• JS For Loops Explained
• JS Array Prototype Reduce

NOTES FOR CONTRIBUTIONS:

• DO NOT add solutions that are similar to any existing solutions. If you think it is similar but better, then try to merge (or replace) the existing similar solution.
• Add an explanation of your solution.
• Categorize the solution in one of the following categories — Basic, Intermediate and Advanced.