67 lines
2.4 KiB
Markdown
67 lines
2.4 KiB
Markdown
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# Lucas Numbers
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Welcome to Lucas Numbers on Exercism's Elixir Track.
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If you need help running the tests or submitting your code, check out `HELP.md`.
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If you get stuck on the exercise, check out `HINTS.md`, but try and solve it without using those first :)
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## Introduction
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## Streams
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All functions in the [`Enum` module][exercism-enum] are _eager_. When performing multiple operations on enumerables with the `Enum` module, each operation is going to generate an intermediate result.
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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.
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Streams implement the _Enumerable [protocol][exercism-protocols]_ and are composable -- you can chain them together to create more complex functionality.
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[exercism-enum]: https://exercism.org/tracks/elixir/concepts/enum
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[exercism-protocols]: https://exercism.org/tracks/elixir/concepts/protocols
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## Instructions
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You are a huge fan of the [Numberphile Youtube channel](https://www.youtube.com/watch?v=PeUbRXnbmms) and you just saw a cool video about the _Lucas Number Sequence_. You want to create this sequence using Elixir.
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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:
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## 1. Generate the base cases
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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:
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```elixir
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LucasNumbers.generate(1)
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# => [2]
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LucasNumbers.generate(2)
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# => [2, 1]
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```
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## 2. Create the generalized case
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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.
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```elixir
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LucasNumbers.generate(3)
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# => [2, 1, 3]
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LucasNumbers.generate(4)
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# => [2, 1, 3, 4]
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```
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## 3. Catch bad arguments
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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:
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```elixir
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LucasNumbers.generate("Hello World")
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# => ** (ArgumentError) count must be specified as an integer >= 1
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```
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## Source
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### Created by
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- @neenjaw
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### Contributed to by
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- @angelikatyborska
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