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:
And the predicate:
Then your keep operation should produce:
While your discard operation should produce:
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.
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
Conversation with James Edward Gray II https://twitter.com/jeg2
It's possible to submit an incomplete solution so you can see how others have completed the exercise.
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
A huge amount can be learned from reading other people’s code. This is why we wanted to give exercism users the option of making their solutions public.
Here are some questions to help you reflect on this solution and learn the most from it.
Level up your programming skills with 3,449 exercises across 52 languages, and insightful discussion with our volunteer team of welcoming mentors. Exercism is 100% free forever.
Sign up Learn More
Community comments