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# angelikatyborska's solution

## to Strain in the Ruby Track

Published at Jul 13 2018 · 1 comment
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

• 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

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

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

skip
end

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

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

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``````
``````class Array
def keep(&block)
Strain.keep(self, &block)
end

end
end

module Strain
def self.strain(collection, &predicate)
collection.each_with_object([]) do |item, new_collection|
new_collection << item if predicate.call(item)
end
end

def self.keep(collection, &predicate)
strain(collection) { |item| predicate.call(item) }
end

strain(collection) { |item| !predicate.call(item) }
end
end``````