# thekeele's solution

## to Flatten Array in the Elixir Track

Published at May 06 2019 · 0 comments
Instructions
Test suite
Solution

#### Note:

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!"
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
``````

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

## 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 FlattenArray do
@doc """
Accept a list and return the list flattened without nil values.

## Examples

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

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)]
end``````

### What can you learn from this solution?

A huge amount can be learned from reading other peopleโ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?