<< problem 1 - Multiples of 3 and 5 | Largest prime factor - problem 3 >> |

# Problem 2: Even Fibonacci numbers

(see projecteuler.net/problem=2)

Each new term in the Fibonacci sequence is generated by adding the previous two terms.

By starting with 1 and 2, the first 10 terms will be:

1, 2, 3, 5, 8, 13, 21, 34, 55, 89, ...

By considering the terms in the Fibonacci sequence whose values do not exceed four million,

find the sum of the even-valued terms.

# My Algorithm

As explained in the problem statement, you can compute all Fibonacci numbers in an iterative way:

F_i=F_{i-2}+F_{i-1}

My variables `a`

and `b`

stand for F_{i-2} and F_{i-1} whereas `next`

is F_i

After each iteration, `next=a+b`

and then `a`

becomes `b`

and `b`

becomes `next`

.

A number is even if there is no remainder when divided by 2.

In most programming languages it's written as `variable % 2 == 0`

Internally, your compiler might translate this to the more efficient `(variable & 1) == 0`

## Note

`unsigned long long`

is required to pass all Hackerrank tests.

# My code

… was written in C++11 and can be compiled with G++, Clang++, Visual C++. You can download it, too.

#include <iostream>
int main()
{
unsigned int tests;
std::cin >> tests;
while (tests--)
{
unsigned long long last;
std::cin >> last;
unsigned long long sum = 0;
// first Fibonacci numbers
unsigned long long a = 1;
unsigned long long b = 2;
// until we reach the limit
while (b <= last)
{
// even ?
if (b % 2 == 0)
sum += b;
// next Fibonacci number
auto next = a + b;
a = b;
b = next;
}
std::cout << sum << std::endl;
}
return 0;
}

This solution contains 5 empty lines, 4 comments and 1 preprocessor command.

# Interactive test

You can submit your own input to my program and it will be instantly processed at my server:

This is equivalent to`echo "1 1000" | ./2`

Output:

*Note:* the original problem's input `4000000`

__cannot__ be entered

because just copying results is a soft skill reserved for idiots.

*(this interactive test is still under development, computations will be aborted after one second)*

# Benchmark

The correct solution to the original Project Euler problem was found in less than 0.01 seconds on a Intel® Core™ i7-2600K CPU @ 3.40GHz.

(compiled for x86_64 / Linux, GCC flags: `-O3 -march=native -fno-exceptions -fno-rtti -std=c++11 -DORIGINAL`

)

See here for a comparison of all solutions.

**Note:** interactive tests run on a weaker (=slower) computer. Some interactive tests are compiled without `-DORIGINAL`

.

# Changelog

February 23, 2017 submitted solution

March 24, 2017 added comments

# Hackerrank

see https://www.hackerrank.com/contests/projecteuler/challenges/euler002

My code solves **5** out of **5** test cases (score: **100%**)

# Difficulty

Project Euler ranks this problem at **5%** (out of 100%).

Hackerrank describes this problem as **easy**.

*Note:*

Hackerrank has strict execution time limits (typically 2 seconds for C++ code) and often a much wider input range than the original problem.

In my opinion, Hackerrank's modified problems are usually a lot harder to solve. As a rule thumb: brute-force is rarely an option.

# Links

projecteuler.net/thread=2 - **the** best forum on the subject (*note:* you have to submit the correct solution first)

Code in various languages:

Python: www.mathblog.dk/project-euler-problem-2/ (written by Kristian Edlund)

Haskell: github.com/nayuki/Project-Euler-solutions/blob/master/haskell/p002.hs (written by Nayuki)

Java: github.com/nayuki/Project-Euler-solutions/blob/master/java/p002.java (written by Nayuki)

Mathematica: github.com/nayuki/Project-Euler-solutions/blob/master/mathematica/p002.mathematica (written by Nayuki)

C: github.com/eagletmt/project-euler-c/blob/master/1-9/problem2.c (written by eagletmt)

Go: github.com/frrad/project-euler/blob/master/golang/Problem002.go (written by Frederick Robinson)

Javascript: github.com/dsernst/ProjectEuler/blob/master/2 Even Fibonacci numbers.js (written by David Ernst)

Scala: github.com/samskivert/euler-scala/blob/master/Euler002.scala (written by Michael Bayne)

# Heatmap

green problems solve the original Project Euler problem and have a perfect score of 100% at Hackerrank, too.

yellow problems score less than 100% at Hackerrank (but still solve the original problem).

gray problems are already solved but I haven't published my solution yet.

blue problems are solved and there wasn't a Hackerrank version of it at the time I solved it or I didn't care about it because it differed too much.

red problems are solved but exceed the time limit of one minute or the memory limit of 256 MByte.

*Please click on a problem's number to open my solution to that problem:*

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I scored 13,183 points (out of 15300 possible points, top rank was 17 out of ≈60000 in August 2017) at Hackerrank's Project Euler+.

Look at my progress and performance pages to get more details.

My username at Project Euler is

**stephanbrumme**while it's stbrumme at Hackerrank.

# Copyright

I hope you enjoy my code and learn something - or give me feedback how I can improve my solutions.

All of my solutions can be used for any purpose and I am in no way liable for any damages caused.

You can even remove my name and claim it's yours. But then you shall burn in hell.

The problems and most of the problems' images were created by Project Euler.

Thanks for all their endless effort.

<< problem 1 - Multiples of 3 and 5 | Largest prime factor - problem 3 >> |