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adri's solution

to Sublist in the Elixir Track

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Instructions
Test suite
Solution

Given two lists determine if the first list is contained within the second list, if the second list is contained within the first list, if both lists are contained within each other or if none of these are true.

Specifically, a list A is a sublist of list B if by dropping 0 or more elements from the front of B and 0 or more elements from the back of B you get a list that's completely equal to A.

Examples:

  • A = [1, 2, 3], B = [1, 2, 3, 4, 5], A is a sublist of B
  • A = [3, 4, 5], B = [1, 2, 3, 4, 5], A is a sublist of B
  • A = [3, 4], B = [1, 2, 3, 4, 5], A is a sublist of B
  • A = [1, 2, 3], B = [1, 2, 3], A is equal to B
  • A = [1, 2, 3, 4, 5], B = [2, 3, 4], A is a superlist of B
  • A = [1, 2, 4], B = [1, 2, 3, 4, 5], A is not a superlist of, sublist of or equal to B

Running tests

Execute the tests with:

$ elixir sublist_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!"
end

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.

Submitting Incomplete Solutions

It's possible to submit an incomplete solution so you can see how others have completed the exercise.

sublist_test.exs

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

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

defmodule SublistTest do
  use ExUnit.Case

  test "empty equals empty" do
    assert Sublist.compare([], []) == :equal
  end

  @tag :pending
  test "empty is a sublist of anything" do
    assert Sublist.compare([], [nil]) == :sublist
  end

  @tag :pending
  test "anything is a superlist of empty" do
    assert Sublist.compare([nil], []) == :superlist
  end

  @tag :pending
  test "1 is not 2" do
    assert Sublist.compare([1], [2]) == :unequal
  end

  @tag :pending
  test "comparing massive equal lists" do
    l = Enum.to_list(1..1_000_000)
    assert Sublist.compare(l, l) == :equal
  end

  @tag :pending
  test "sublist at start" do
    assert Sublist.compare([1, 2, 3], [1, 2, 3, 4, 5]) == :sublist
  end

  @tag :pending
  test "sublist in middle" do
    assert Sublist.compare([4, 3, 2], [5, 4, 3, 2, 1]) == :sublist
  end

  @tag :pending
  test "sublist at end" do
    assert Sublist.compare([3, 4, 5], [1, 2, 3, 4, 5]) == :sublist
  end

  @tag :pending
  test "partially matching sublist at start" do
    assert Sublist.compare([1, 1, 2], [1, 1, 1, 2]) == :sublist
  end

  @tag :pending
  test "sublist early in huge list" do
    assert Sublist.compare([3, 4, 5], Enum.to_list(1..1_000_000)) == :sublist
  end

  @tag :pending
  test "huge sublist not in huge list" do
    assert Sublist.compare(Enum.to_list(10..1_000_001), Enum.to_list(1..1_000_000)) == :unequal
  end

  @tag :pending
  test "superlist at start" do
    assert Sublist.compare([1, 2, 3, 4, 5], [1, 2, 3]) == :superlist
  end

  @tag :pending
  test "superlist in middle" do
    assert Sublist.compare([5, 4, 3, 2, 1], [4, 3, 2]) == :superlist
  end

  @tag :pending
  test "superlist at end" do
    assert Sublist.compare([1, 2, 3, 4, 5], [3, 4, 5]) == :superlist
  end

  @tag :pending
  test "1 and 2 does not contain 3" do
    assert Sublist.compare([1, 2], [3]) == :unequal
  end

  @tag :pending
  test "partially matching superlist at start" do
    assert Sublist.compare([1, 1, 1, 2], [1, 1, 2]) == :superlist
  end

  @tag :pending
  test "superlist early in huge list" do
    assert Sublist.compare(Enum.to_list(1..1_000_000), [3, 4, 5]) == :superlist
  end

  @tag :pending
  test "strict equality needed" do
    assert Sublist.compare([1], [1.0, 2]) == :unequal
  end

  @tag :pending
  test "recurring values sublist" do
    assert Sublist.compare([1, 2, 1, 2, 3], [1, 2, 3, 1, 2, 1, 2, 3, 2, 1]) == :sublist
  end

  @tag :pending
  test "recurring values unequal" do
    assert Sublist.compare([1, 2, 1, 2, 3], [1, 2, 3, 1, 2, 3, 2, 3, 2, 1]) == :unequal
  end
end
defmodule Sublist do
  @doc """
  Returns whether the first list is a sublist or a superlist of the second list
  and if not whether it is equal or unequal to the second list.
  """
  def compare(a, b) do
    cond do
      a == b -> :equal
      b == [nil] -> :sublist
      a == [nil] -> :superlist
      is_sublist(b,a) -> :superlist
      is_sublist(a,b) -> :sublist
      a != b -> :unequal
    end
  end

  defp is_sublist(a, b) do
    Stream.chunk(b, length(a), 1)
    |> Enum.member?(a)
  end
end

What can you learn from this solution?

A huge amount can be learnt 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 I could read more about to develop my understanding?

Community comments

See what others have said about this solution
about 2 years ago
adri says

After seeing the solution by @dtt101 I went with a slower (by about 2x on the test suite runtime) but much more readable solution using Stream.chunk instead of recursion.