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to Flatten Array in the Elixir Track

Published at Aug 18 2019 · 0 comments
Test suite

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:

$ 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.


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.


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]]]) == []


ExUnit.configure(exclude: :pending, trace: true)
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(list) :: list
  def flatten(list), do: flatten(list, [])

  defp flatten([], acc), do: acc
  defp flatten([nil | tail], acc), do: flatten(tail, acc)

  defp flatten([head | tail], acc) when is_list(head) do
    acc = flatten(tail, acc)
    flatten(head, acc)

  defp flatten([head | tail], acc), do: [head | flatten(tail, acc)]

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