lujanfernaud's solution to Strain on the Ruby track

Instructions

Test suite

Solution

Implement the keep and discard operation on collections. Given a collection and a predicate on the collection's elements, keep returns a new collection containing those elements where the predicate is true, while discard returns a new collection containing those elements where the predicate is false.

For example, given the collection of numbers:

1, 2, 3, 4, 5

And the predicate:

is the number even?

Then your keep operation should produce:

2, 4

While your discard operation should produce:

1, 3, 5

Note that the union of keep and discard is all the elements.

The functions may be called keep and discard, or they may need different names in order to not clash with existing functions or concepts in your language.

Restrictions

Keep your hands off that filter/reject/whatchamacallit functionality provided by your standard library! Solve this one yourself using other basic tools instead.

For installation and learning resources, refer to the exercism help page.

For running the tests provided, you will need the Minitest gem. Open a terminal window and run the following command to install minitest:

gem install minitest

If you would like color output, you can require 'minitest/pride' in the test file, or note the alternative instruction, below, for running the test file.

Run the tests from the exercise directory using the following command:

ruby strain_test.rb

To include color from the command line:

ruby -r minitest/pride strain_test.rb

Source

Conversation with James Edward Gray II https://twitter.com/jeg2

Submitting Incomplete Solutions

It's possible to submit an incomplete solution so you can see how others have completed the exercise.

strain_test.rb

require 'minitest/autorun'
require_relative 'strain'

class ArrayTest < Minitest::Test
  def test_empty_keep
    assert_equal [], [].keep { |e| e < 10 }
  end

  def test_keep_everything
    skip
    assert_equal [1, 2, 3], [1, 2, 3].keep { |e| e < 10 }
  end

  def test_keep_first_and_last
    skip
    assert_equal [1, 3], [1, 2, 3].keep(&:odd?)
  end

  def test_keep_neither_first_nor_last
    skip
    assert_equal [2, 4], [1, 2, 3, 4, 5].keep(&:even?)
  end

  def test_keep_strings
    skip
    words = %w(apple zebra banana zombies cherimoya zelot)
    result = words.keep { |word| word.start_with?('z') }
    assert_equal %w(zebra zombies zelot), result
  end

  def test_keep_arrays
    skip
    rows = [
      [1, 2, 3],
      [5, 5, 5],
      [5, 1, 2],
      [2, 1, 2],
      [1, 5, 2],
      [2, 2, 1],
      [1, 2, 5]
    ]
    result = rows.keep { |row| row.include?(5) }
    assert_equal [[5, 5, 5], [5, 1, 2], [1, 5, 2], [1, 2, 5]], result
  end

  def test_empty_discard
    skip
    assert_equal [], [].discard { |e| e < 10 }
  end

  def test_discard_nothing
    skip
    assert_equal [1, 2, 3], [1, 2, 3].discard { |e| e > 10 }
  end

  def test_discard_first_and_last
    skip
    assert_equal [2], [1, 2, 3].discard(&:odd?)
  end

  def test_discard_neither_first_nor_last
    skip
    assert_equal [1, 3, 5], [1, 2, 3, 4, 5].discard(&:even?)
  end

  def test_discard_strings
    skip
    words = %w(apple zebra banana zombies cherimoya zelot)
    result = words.discard { |word| word.start_with?('z') }
    assert_equal %w(apple banana cherimoya), result
  end

  def test_discard_arrays
    skip
    rows = [
      [1, 2, 3],
      [5, 5, 5],
      [5, 1, 2],
      [2, 1, 2],
      [1, 5, 2],
      [2, 2, 1],
      [1, 2, 5]
    ]
    result = rows.discard { |row| row.include?(5) }
    assert_equal [[1, 2, 3], [2, 1, 2], [2, 2, 1]], result
  end
end

module Enumerable
  def keep
    return to_enum unless block_given?

    to_a.each_with_object [] { |item, array| array << item if yield item }
  end

  def discard
    keep { |item| !yield item }
  end
end

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lujanfernaud's solution

to Strain in the Ruby Track