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## to Sublist in the Elixir Track

Published at Jul 13 2018 · 1 comment
Instructions
Test suite
Solution

#### Note:

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 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
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``````

Find this solution interesting? Ask the author a question to learn more.
Solution Author
commented over 4 years ago

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.

### What can you learn from this solution?

A huge amount can be learned from reading other peopleâ€™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?