Flatten Exercise for Theorem

.ruby-version

2.5.3

Readme.md

Theorem Exercise

Write some code, that will flatten an array of arbitrarily nested arrays of integers into a flat array of integers. e.g. [[1,2,[3]],4] -> [1,2,3,4].

Solution

Excercise::Flattener is a class with a flatten function, that takes care of solving the exercise.

Installation

You need to have Ruby installed, as well as the bundler gem.

Then run:

bundle install

Run tests

To run the test use: rspec flattener_spec.rb

Comments

The project folder structure would change (similar to a gem layout) if it wasn't for the gist limitations.

There is not much to comment about the solution, a recursive function checking that the arguments match the prerequisites and raising exceptions if they are not. I decided to use Array.reduce in favor of readability, code length and finally performance.

flattener.rb

module Exercise
  class Flattener

    def flatten(array)
      raise ArgumentError, 'argument should be an Array' unless array.is_a? Array
      _flatten(array)
    end

    private
    def _flatten(array_or_integer)
      array_or_integer.reduce([]) do |memo, element|
        unless [Array, Integer].any? { |klass| element.is_a?(klass) }
          raise ArgumentError, 'array elements should be either Arrays or Integers'
        end
        if element.is_a?(Array)
          memo + _flatten(element) #concat
        else
          memo << element #push
        end
      end
    end

    # one liner version
    # array_or_integer.reduce([]) { |memo, i| i.is_a?(Array) ? memo + _flatten(i) : memo << i}

  end
end

flattener_spec.rb

require './flattener'

describe Exercise::Flattener do

  let(:flattener) { described_class.new }

  it 'flattens arrays with several levels of nested arrays' do
    array1 = [1, 2, 3]
    expect(flattener.flatten(array1)).to eq(array1.flatten)

    array2 = [1, [2], 3]
    expect(flattener.flatten(array2)).to eq(array2.flatten)

    array3 = [1, 2, [3 , 4, [5, 6]]]
    expect(flattener.flatten(array3)).to eq(array3.flatten)

    array4 = [1, 2, [3 , 4, [5, [[6], 7, 8]]]]
    expect(flattener.flatten(array4)).to eq(array4.flatten)
  end

  it 'throws ArgumentError' do
    invalid_argument1 = ['a', [1, [true]]]
    expect{flattener.flatten(invalid_argument1)}.to raise_error(ArgumentError)

    invalid_argument2 = 1
    expect{flattener.flatten(invalid_argument2)}.to raise_error(ArgumentError)
  end

end

Gemfile

source "https://rubygems.org"

group :test do
  gem "rspec"
end