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to Leap in the Elixir 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.

Running tests

Execute the tests with:

$ elixir leap_test.exs

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

For more detailed information about the Elixir track, please see the help page.


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.


if !System.get_env("EXERCISM_TEST_EXAMPLES") do
  Code.load_file("leap.exs", __DIR__)

ExUnit.configure(exclude: :pending, trace: true)

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)
defmodule Year do
  @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) do
    cond do
      rem(year, 400) == 0 -> true
      rem(year, 100) == 0 -> false
      rem(year, 4) == 0 -> true
      true -> false

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