What do you do if you can not comprehend solution?

Learning Questions & Discussions Coding Challenges
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1

What do you do if you’ve done an exercise, it works, it’s not very inefficient and seems concise, then you go to the community solutions, sort by highest-rated user and see this?

defmodule Sublist do
  @doc """
  Returns whether the first list is a sublist or a superlist of the second list
  and if not whether it is equal or unequal to the second list.
  """
  def compare(a, b) when is_list(a) and is_list(b) do
    case {contains?(a, b), contains?(b,a)} do
      {true, true} -> :equal
      {true, false} -> :superlist
      {false, true} -> :sublist
      {false, false} -> :unequal
    end
  end

  # determines if list a contains list b. restore_a is needed to restore
  # already "eaten" members of a, when b couldn't be matched completely
  defp contains?(a, b, current_b \\ :initial, restore_a \\ nil)
  defp contains?(a, b, :initial, nil), do: contains?(a, b, b, nil)
  defp contains?(_, _, [], _), do: true
  defp contains?([], _, _, _), do: false
  defp contains?([x | a], b, [x | c], nil), do: contains?(a, b, c, a)
  defp contains?([x | a], b, [x | c], restore_a), do: contains?(a, b, c, restore_a)
  defp contains?([_ | a], b, _, nil), do: contains?(a, b, b, nil)
  defp contains?(_, b, _, restore_a), do: contains?(restore_a, b, b, nil)
end

To clarify I enjoy creating solutions, and having to think, and play, but there is border.

2

Create your own wrapper my_contains?/4 which would print the intermediate results and run it for several different inputs.

  def compare(a, b) when is_list(a) and is_list(b) do
    case {my_contains?(a, b), my_contains?(b,a)} do
      {true, true} -> :equal
      {true, false} -> :superlist
      {false, true} -> :sublist
      {false, false} -> :unequal
    end
  end

  defp my_contains?(a, b, current_b \\ :initial, restore_a \\ nil) do
    IO.inspect(a: a, b: b, current_b: current_b, restore_a: restore_a)
    contains?(a, b, current_b, restore_a)
  end

  defp contains?(a, b, current_b \\ :initial, restore_a \\ nil)
  defp contains?(a, b, :initial, nil), do: my_contains?(a, b, b, nil)
  defp contains?(_, _, [], _), do: true
  defp contains?([], _, _, _), do: false
  defp contains?([x | a], b, [x | c], nil), do: my_contains?(a, b, c, a)
  defp contains?([x | a], b, [x | c], restore_a), do: my_contains?(a, b, c, restore_a)
  defp contains?([_ | a], b, _, nil), do: my_contains?(a, b, b, nil)
  defp contains?(_, b, _, restore_a), do: my_contains?(restore_a, b, b, nil)```
1 Like
3

The code you cited seems fine to me, pretty clever use of tail recursion. I am not sure what do you mean. do you mean:

  • You don’t like this code. Of course you can have a opinion and choose to use your own code. or:
  • You like this code but wish some one to explain to you in digestible pieces. Then you did not say which part you do not understand. Or:
  • You don’t care about this code at all, all you care is to win the competition to provide the best solution. Then it is a matter of skill issue, right?
1 Like
4

It’s just too hard for me. I got overwhelmed
So, I’ve been getting better at reading community solutions, but still I don’t know in what order to read function clauses to understand it. Now sometimes i get it right away how sth works like i jump through lines with correct order to understand it fast. But How should you approach reading this kind of multiclause functions?

5

The correct reading order is from the top to bottom. If you want to improve your skill on tail recursion, write with it. Can you implement the enumerating functions in Enum, starting with Enum.reverse/1, with tail recursion? If you can write comfortably in tail recursion, it will click for you.

1 Like
6

Yes i did it many times, implementing Enum.reverse/1 is easy for me.
for example right now i wrote function to flatten list:

  defp fl(list, acc \\ [])

  defp fl([], acc), do: acc

  defp fl([el | tail], acc) when is_list(el),
    do: fl(el, acc) |> then(&fl(tail, &1))

  defp fl([el | tail], acc) when not is_list(el),
    do: fl(tail, [el | acc])
end

The problem is 5 minutes later when i look at the code i want to add something, e.x ommiting some values, i cannot comprehand what is happening
XD

7

Reading from top to bottom is not always the way (for me) to understand how function will behave.

8

nitpick, can’t help it.

defp fl([], acc), do: Enum.reverse(acc)
9 
  @spec flatten(list) :: list
  def flatten(list), do: fl(list) |> Enum.reverse()

  defp fl(list, acc \\ [])

  defp fl([], acc), do: acc

  defp fl([[_|_] = el | tail], acc), do: fl(el, acc) |> then(&fl(tail, &1))
  defp fl([nil | tail], acc), do: fl(tail, acc)
  defp fl([el | tail], acc),do: fl(tail, [el | acc])

I guess we should reverse at the end, not on intermediate states, and your suggestion would not work (I tested it).

How to tell if my above solution is proper tail-call recursion?

1 Like