Fibonacci sequence that turns off computer.

Questions / Help
,
1

Recently I started learning elixir for fun and as I’m new to programming in general it is not going too well.
Yesterday I found on the internet an exercise that required me to summarize all Fibonacci numbers less than 4_000_000. I tried doing it like that:

defmodule Fibonacci do
  def fibonacci(number) do
    Enum.reverse(fibonacci_do(number))
  end

  def fibonacci_do(1), do: [0]
  def fibonacci_do(2), do: [1 | fibonacci_do(1)]

  def fibonacci_do(number) when number > 2 do
    [x, y | _] = all = fibonacci_do(number - 1)
    [x + y | all]
  end
end

defmodule Solve do
  def lineOfFi(limit) do
    fib_numbers = Fibonacci.fibonacci_do(limit)
    Enum.sum(Enum.filter(fib_numbers, &(&1 < limit and rem(&1, 2) == 0)))
  end
end

result = Solve.lineOfFi(4_000_000)
IO.puts(result)

The first part is code that I found on the internet (it was supposed to be fast), as calculating the sequence itself completely defeated me. Second part I based on a problem that I solved few days ago.
Unfortunately, this code contains a hidden bug or is so unoptimized that after 5 or 6 minutes of running, it crushes my computer.
I wonder what could be done to improve optimization of this code, that it will be possible to run it at all.

2

Welcome to Elixir community @Konbor704, the problem here is that this code is trying to calculate 4 million fibonacci numbers, NOT fibonacci numbers less than 4_000_000.
See this line:

fib_numbers = Fibonacci.fibonacci_do(limit)

So, your Elixir program is doing a long computation because Fibonacci numbers tend to grow very fast.

If you want to calculate fibonacci numbers, you can use the Stream and Enum modules, specifically look at Stream.unfold:

# This one-liner creates an infinite "list" of numbers from
# which you can extract as many numbers as you want.
# The elements will be calculated on-demand by calling
# the passed function
Stream.unfold({0, 1}, fn {a, b} -> {a, {b, a + b}} end)

Now if you want the first 100 Fibonacci numbers, you can do:

Stream.unfold({0, 1}, fn {a, b} -> {a, {b, a + b}} end)
|> Enum.take(100)

If you want all fibonacci numbers less than 4 million, you can do:

Stream.unfold({0, 1}, fn {a, b} -> {a, {b, a + b}} end)
|> Enum.take_while(fn number -> number < 4_000_000 end)

Further, if you want to sum the numbers, you can pipe the above output to Enum.sum.

Enjoy learning Elixir and have fun solving problems. :slight_smile:

EDIT: I would also suggest looking at the Elixir pipe operator ( |> operator). It is one of the most loved features of Elixir and you’ll soon find out how you can improve your code to make it more readable and how it also helps to understand the parameter positions for a function in a given module (thus allowing you to easily understand and making it easier to memorize standard library module functions!)

3 Likes
3

Thank you so much for explanation.

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