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Guto Navagar
Guto Navagar

Reputation: 65

Elixir language: syntax error when trying to unzip a list. What is missing?

Anyone who can help.

I am a beginner in Elixir Language. I am trying to perform an “unzip” operation and the “iex” environment shows syntax error “syntax error before: ')'” at position “53” of the script, I am writing. I know the algorithm works because I tested it on Haskell, but I cannot find where my syntax error is and what to do to resolve it.

This

defmodule Alf do
  def lastx([]), do: raise ArgumentError, message: "empty list"
  def lastx([x]), do: x
  def lastx([x | xs]), do: lastx(xs)

  def initx([]), do: raise ArgumentError, message: "empty list"
  def initx([x]), do: []
  def initx([x | xs]), do: [x | initx(xs)]

  def revx([]), do: []
  def revx(xs), do: [lastx(xs) | revx(initx(xs))]

  def zipx([], b), do: []
  def zipx(a, []), do: []
  def zipx([x | a], [y | b]), do: [{x, y} | zipx(a, b)]

  def fstx(x, _), do: x

  def sndx(_, y), do: y
end

Here is the part that is not working properly:

defmodule Alf do
  def unzipx_bs(xs, ys, []), do: (revx(xs), revx(ys))
  def unzipx_bs(xs, ys, [z | zs]), do: unzipx_bs([fstx z | xs], [sndx z | ys], zs)
end

The “unzipx_bs” script will later be set to private and will have a script called “unzipx” as the access interface that will perform the so-called “unzipx_bs [], [], [{1, 4}, {2, 5}, {3 , 6}]” to generate a result similar to “[1, 2, 3], [4, 5, 6]”.

Upvotes: 1

Views: 87

Answers (1)

Paweł Obrok
Paweł Obrok

Reputation: 23184

I assume you wanted your unzipx_bs function to return a pair. Tuples are written with {} in elixir, so it should be {revx(xs), revx(ys)}. I would further propose that you unpack your z right there in the argument list, giving:

def unzipx_bs(xs, ys, []), do: {revx(xs), revx(ys)}
def unzipx_bs(xs, ys, [{x, y} | zs]), do: unzipx_bs([x | xs], [y | ys], zs)

Then:

Alf.unzipx_bs([], [], [{1, 2}, {3, 4}, {5, 6}])
# => {[1, 3, 5], [2, 4, 6]}

Upvotes: 2

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