Avatar of paulfioravanti

paulfioravanti's solution

to Leap in the Elixir Track

Published at Jun 20 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
  @leap_year 4
  @centurial_year 100
  @leap_cycle_length @leap_year * @centurial_year

  defguardp is_leap_year(year) when rem(year, @leap_year) == 0
  defguardp is_not_centurial_year(year) when rem(year, @centurial_year) != 0
  defguardp is_leap_cycle_year(year) when rem(year, @leap_cycle_length) == 0

  defguardp is_leap(year)
            when is_leap_year(year) and
                   (is_not_centurial_year(year) or is_leap_cycle_year(year))

  @doc """
  Returns whether 'year' is a leap year.

  A leap year 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
  @spec leap_year?(non_neg_integer) :: boolean
  def leap_year?(year) when is_leap(year), do: true
  def leap_year?(_year), do: false

Community comments

Find this solution interesting? Ask the author a question to learn more.

What can you learn from this solution?

A huge amount can be learned from reading other people’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?