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# pgaspar's solution

## to Leap in the Ruby Track

Published at Jul 13 2018 · 3 comments
Instructions
Test suite
Solution

#### Note:

This solution was written on an old version of Exercism. The tests below might not correspond to the solution code, and the exercise may have changed since this code was written.

Given a year, report if it is a leap year.

The tricky thing here is that a leap year in the Gregorian calendar occurs:

``````on every year that is evenly divisible by 4
except every year that is evenly divisible by 100
unless the year is also evenly divisible by 400
``````

For example, 1997 is not a leap year, but 1996 is. 1900 is not a leap year, but 2000 is.

If your language provides a method in the standard library that does this look-up, pretend it doesn't exist and implement it yourself.

## Notes

Though our exercise adopts some very simple rules, there is more to learn!

For a delightful, four minute explanation of the whole leap year phenomenon, go watch this youtube video.

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 leap_test.rb
``````

To include color from the command line:

``````ruby -r minitest/pride leap_test.rb
``````

## Source

JavaRanch Cattle Drive, exercise 3 http://www.javaranch.com/leap.jsp

## Submitting Incomplete Solutions

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

### leap_test.rb

``````require 'minitest/autorun'
require_relative 'leap'

# Common test data version: 1.1.0 7f4d0d8
class Date
def leap?
raise RuntimeError, "Implement this yourself instead of using Ruby's implementation."
end

alias gregorian_leap? leap?
alias julian_leap? leap?
end

class YearTest < Minitest::Test
def test_year_not_divisible_by_4_common_year
# skip
refute Year.leap?(2015), "Expected 'false', 2015 is not a leap year."
end

def test_year_divisible_by_4_not_divisible_by_100_leap_year
skip
assert Year.leap?(2020), "Expected 'true', 2020 is a leap year."
end

def test_year_divisible_by_100_not_divisible_by_400_common_year
skip
refute Year.leap?(2100), "Expected 'false', 2100 is not a leap year."
end

def test_year_divisible_by_400_leap_year
skip
assert Year.leap?(2000), "Expected 'true', 2000 is a leap year."
end

# Problems in exercism evolve over time, as we find better ways to ask
# questions.
# The version number refers to the version of the problem you solved,
#
# Define a constant named VERSION inside of the top level BookKeeping
# module, which may be placed near the end of your file.
#
# In your file, it will look like this:
#
# module BookKeeping
#   VERSION = 1 # Where the version number matches the one in the test.
# end
#
# http://ruby-doc.org/docs/ruby-doc-bundle/UsersGuide/rg/constants.html

def test_bookkeeping
skip
assert_equal 3, BookKeeping::VERSION
end
end``````
``````class Year
# on every year that is evenly divisible by 4
# except every year that is evenly divisible by 100
# unless the year is also evenly divisible by 400

def self.leap?(year)
(divisible_by?(year, 4) && !divisible_by?(year, 100)) ||
(divisible_by?(year, 100) && divisible_by?(year, 400))
end

class << self
private

def divisible_by?(number, factor)
(number % factor).zero?
end
end
end

module BookKeeping
VERSION = 3
end``````

Nice solution !!

Why did you both self.leap? and class << self

Why not ? class Year class << self def self.leap?(year) (divisible_by?(year, 4) && !divisible_by?(year, 100)) || (divisible_by?(year, 100) && divisible_by?(year, 400)) end private

``````def divisible_by?(number, factor)
(number % factor).zero?
end
``````

etc

More out of curiosity

Solution Author
commented over 2 years ago

Thank you! No particular reason :)

I wanted to make divisible_by? private, and that's why I used the class << self syntax, because simply using the private keyword doesn't work for class methods in Ruby. I didn't remember to also include leap? in it, but it would also work, yep ? And maybe look a bit better too.

Fair enough! I came across private_class_method which may pretty things up if you like the self.method syntax ? class Year -- snip-- def self.divisible_by?(number, factor) (number % factor).zero? end -- snip--

private_class_method : divisible_by?

### What can you learn from this solution?

A huge amount can be learned from reading other peopleâ€™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.

• What compromises have been made?