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4d47's solution

to Leap in the Crystal Track

Published at Jul 13 2018 · 0 comments
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.


Follow the setup instructions for Crystal here:


More help installing can be found here:


Making the Test Suit Pass

Execute the tests with:

$ crystal spec

In each test suite all but the first test have been skipped.

Once you get a test passing, you can unskip the next one by changing pending to it.


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 "spec"
require "../src/*"

struct Time
  def leap_year?
    raise "That's too easy! Implement this method in your own way!"

describe "Leap" do
  describe "#leap?" do
    it "marks 1996 as a leap year" do
      Year.leap?(1996).should be_true

    pending "marks 1997 not as a leap year" do
      Year.leap?(1997).should be_false

    pending "marks 1998 not as a leap year" do
      Year.leap?(1998).should be_false

    pending "marks 1900 not as a leap year" do
      Year.leap?(1900).should be_false

    pending "marks 2400 as a leap year" do
      Year.leap?(2400).should be_true

    pending "marks 2000 as a leap year" do
      Year.leap?(2000).should be_true
module Year
  extend self

  def leap?(n : Int) : Bool
    n.divisible_by?(4) && !n.divisible_by?(100) || n.divisible_by?(400)

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