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thekeele's solution

to Leap in the Elixir Track

Published at Nov 05 2019 · 0 comments
Test suite

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.

Running tests

Execute the tests with:

$ mix test

Pending tests

In the test suites, all but the first test have been skipped.

Once you get a test passing, you can unskip the next one by commenting out the relevant @tag :pending with a # symbol.

For example:

# @tag :pending
test "shouting" do
  assert Bob.hey("WATCH OUT!") == "Whoa, chill out!"

Or, you can enable all the tests by commenting out the ExUnit.configure line in the test suite.

# ExUnit.configure exclude: :pending, trace: true

If you're stuck on something, it may help to look at some of the available resources out there where answers might be found.


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.


defmodule LeapTest do
  use ExUnit.Case

  # @tag :pending
  test "vanilla leap year" do
    assert Year.leap_year?(1996)

  @tag :pending
  test "any old year" do
    refute Year.leap_year?(1997), "1997 is not a leap year."

  @tag :pending
  test "century" do
    refute Year.leap_year?(1900), "1900 is not a leap year."

  @tag :pending
  test "exceptional century" do
    assert Year.leap_year?(2400)


ExUnit.configure(exclude: :pending, trace: true)
defmodule Year do
  @spec leap_year?(non_neg_integer) :: boolean

  defguard is_leap_year?(year)
           when rem(year, 400) === 0
           or rem(year, 100) !== 0
           and rem(year, 4) === 0

  def leap_year?(year) when is_leap_year?(year), do: true
  def leap_year?(_), do: false

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