ðŸŽ‰ Exercism Research is now launched. Help Exercism, help science and have some fun at research.exercism.io ðŸŽ‰

# shmibs's solution

## to Flatten Array in the Elixir Track

Published at Jul 13 2018 · 0 comments
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.

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!"
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.

## Source

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.

### flatten_array_test.exs

``````if !System.get_env("EXERCISM_TEST_EXAMPLES") do
end

ExUnit.start()
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]
end

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

@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]
end

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

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

@tag :pending
test "returns an empty list if all values in nested list are nil" do
assert FlattenArray.flatten([nil, [nil], [nil, [nil]]]) == []
end
end``````
``````defmodule Flattener do
# still got to be a better way X_X
@doc """
Accept a list and return the list flattened without nil values.

## Examples

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

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

"""
@spec flatten(list) :: list
def flatten([]), do: []
def flatten([[]|t]), do: flatten(t)
def flatten([nil|t]), do: flatten(t)
def flatten([h|t]) when is_list(h), do: flatten(h) ++ flatten(t)
def flatten([h|t]), do: [h|flatten(t)]
end``````