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thekeele's solution

to Flatten Array in the Elixir Track

Published at May 06 2019 · 0 comments
Test suite


This exercise has changed since this solution was written.

Take a nested list and return a single flattened list with all values except nil/null.

The challenge is to write a function that accepts an arbitrarily-deep nested list-like structure and returns a flattened structure without any nil/null values.

For Example

input: [1,[2,3,null,4],[null],5]

output: [1,2,3,4,5]

Running tests

Execute the tests with:

$ elixir flatten_array_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

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


Interview Question https://reference.wolfram.com/language/ref/Flatten.html

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("flatten_array.exs", __DIR__)

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

defmodule FlattenArrayTest do
  use ExUnit.Case

  test "returns original list if there is nothing to flatten" do
    assert FlattenArray.flatten([1, 2, 3]) == [1, 2, 3]

  @tag :pending
  test "flattens an empty nested list" do
    assert FlattenArray.flatten([[]]) == []

  @tag :pending
  test "flattens a nested list" do
    assert FlattenArray.flatten([1, [2, [3], 4], 5, [6, [7, 8]]]) == [1, 2, 3, 4, 5, 6, 7, 8]

  @tag :pending
  test "removes nil from list" do
    assert FlattenArray.flatten([1, nil, 2]) == [1, 2]

  @tag :pending
  test "removes nil from a nested list" do
    assert FlattenArray.flatten([1, [2, nil, 4], 5]) == [1, 2, 4, 5]

  @tag :pending
  test "returns an empty list if all values in nested list are nil" do
    assert FlattenArray.flatten([nil, [nil], [nil, [nil]]]) == []
defmodule FlattenArray do
  @doc """
    Accept a list and return the list flattened without nil values.

    ## Examples

      iex> FlattenArray.flatten([1, [2], 3, nil])

      iex> FlattenArray.flatten([nil, nil])


  @spec flatten(deep_list) :: list when deep_list: [any | deep_list]
  def flatten([]), do: []
  def flatten([nil | cdr]), do: flatten(cdr)
  def flatten([car | cdr]) when is_list(car), do: flatten(car ++ cdr)
  def flatten([car | cdr]) when is_integer(car), do: [car | flatten(cdr)]

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