An alternating array is a list of any length in which two (not necessarily different) values are alternating (all even-indexed items are equal, and all odd-indexed items are equal). Given an array, return true if it is alternating. Examples -------- [] -> True [1] -> True [1,1] -> True [1,2,1] -> True [10,5,10,5,10] -> True [2,2,3,3] -> False [5…

alternating_array.ex

defmodule AlternatingArray do
  @moduledoc """
  An alternating array is a list of any length in which two (not necessarily different)
  values are alternating (all even-indexed items are equal, and all odd-indexed items are equal).
  Given an array, return true if it is alternating.

  Examples
  --------
  [] -> True
  [1] -> True
  [1,1] -> True
  [1,2,1] -> True
  [10,5,10,5,10] -> True
  [2,2,3,3] -> False
  [5,4,3,5,4,3] -> False
  
  Link: https://buttondown.com/cassidoo/archive/you-only-get-one-life-so-you-might-as-well-feel/
  """

  def is_alternating(arr) do
    check_positions(arr, 0)
  end

  defp check_positions([], _index), do: true
  defp check_positions([_], _index), do: true
  defp check_positions([_first, _second], _index), do: true

  defp check_positions([first, second, third | rest], index) do
    if first == third do
      check_positions([second, third | rest], index + 1)
    else
      false
    end
  end
end

test_cases = [
  {[], true},
  {[1], true},
  {[1, 1], true},
  {[1, 2, 1], true},
  {[10, 5, 10, 5, 10], true},
  {[2, 2, 3, 3], false},
  {[5, 4, 3, 5, 4, 3], false}
]

Enum.each(test_cases, fn {arr, expected} ->
  result = AlternatingArray.is_alternating(arr)
  passed = result == expected
  IO.inspect("Array: #{inspect(arr)}, Expected: #{expected}, Got: #{result}, Passed: #{passed}")
end)

output.txt

$iex alternate_array.ex
Erlang/OTP 27 [erts-15.2.6] [source] [64-bit] [smp:10:10] [ds:10:10:10] [async-threads:1] [jit] [dtrace]

"Array: [], Expected: true, Got: true, Passed: true"
"Array: [1], Expected: true, Got: true, Passed: true"
"Array: [1, 1], Expected: true, Got: true, Passed: true"
"Array: [1, 2, 1], Expected: true, Got: true, Passed: true"
"Array: [10, 5, 10, 5, 10], Expected: true, Got: true, Passed: true"
"Array: [2, 2, 3, 3], Expected: false, Got: false, Passed: true"
"Array: [5, 4, 3, 5, 4, 3], Expected: false, Got: false, Passed: true"