exercism/elixir/lucas-numbers

Lucas Numbers

Welcome to Lucas Numbers on Exercism's Elixir Track. If you need help running the tests or submitting your code, check out HELP.md. If you get stuck on the exercise, check out HINTS.md, but try and solve it without using those first :)

Introduction

Streams

All functions in the Enum module are eager. When performing multiple operations on enumerables with the Enum module, each operation is going to generate an intermediate result.

The Stream module is a lazy alternative to the eager Enum module. It offers many of the same functions as Enum, but instead of generating intermediate results, it builds a series of computations that are only executed once the stream is passed to a function from the Enum module.

Streams implement the Enumerable protocol and are composable -- you can chain them together to create more complex functionality.

Instructions

You are a huge fan of the Numberphile Youtube channel and you just saw a cool video about the Lucas Number Sequence. You want to create this sequence using Elixir.

While designing your function, you want to make use of lazy evaluation, so that you can generate as many numbers as you want, but only if you need to -- So you decide to use a stream:

1. Generate the base cases

You know that the sequence has two starting numbers which don't follow the same rule. Write two base case clauses to return these numbers:

LucasNumbers.generate(1)
# => [2]

LucasNumbers.generate(2)
# => [2, 1]

2. Create the generalized case

For any sequence longer than 2, you know that you need to add the previous two numbers to get the next number and so on. Write the generalized case.

LucasNumbers.generate(3)
# => [2, 1, 3]

LucasNumbers.generate(4)
# => [2, 1, 3, 4]

3. Catch bad arguments

Later, you find someone is using your function and having problems because they are using incorrect arguments. Add a guard clause to raise an error if a non-integer or an integer less than 1 is used to generate the sequence:

LucasNumbers.generate("Hello World")
# => ** (ArgumentError) count must be specified as an integer >= 1

Source

Created by

• @neenjaw

Contributed to by

• @angelikatyborska