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rootulp's solution

to Leap in the Ruby Track

Published at Jul 13 2018 · 1 comment
Test suite


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.


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


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.


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."

  alias gregorian_leap? leap?
  alias julian_leap? leap?

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."

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

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

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

  # 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,
  # not your solution.
  # 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
  # If you are curious, read more about constants on RubyDoc:
  # http://ruby-doc.org/docs/ruby-doc-bundle/UsersGuide/rg/constants.html

  def test_bookkeeping
    assert_equal 3, BookKeeping::VERSION
# Year
class Year
  def self.leap?(year)
    if year % 400 == 0
    elsif year % 100 == 0
    elsif year % 4 == 0

Community comments

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Avatar of chris-wilson-7374

You've effectively got if true true elsif true false #...

Can you think of a way you can combine these into a simpler expression?

What can you learn from this solution?

A huge amount can be learned from reading other peopleโ€™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?
  • Are there new concepts here that you could read more about to improve your understanding?