How can I get permutations of a list?

Question

How can I get permutations of a list in Elixir?

Eg, for [\"a\", \"b\", \"c\"], I would expect:

# [[\"a\", \"b\", \"c\"], [\"a\", \"c\",          


        

Answer 1

There's a slightly different approach, it also supports specifing the desired length for the result lists:

defmodule Permutations do
  def shuffle(list), do: shuffle(list, length(list))

  def shuffle([], _), do: [[]]
  def shuffle(_,  0), do: [[]]
  def shuffle(list, i) do
    for x <- list, y <- shuffle(list, i-1), do: [x|y]
  end
end

Running:

iex(24)> Permutations.shuffle ["a", "b", "c"]
[["a", "a", "a"], ["a", "a", "b"], ["a", "a", "c"], ["a", "b", "a"],
 ["a", "b", "b"], ["a", "b", "c"], ["a", "c", "a"], ["a", "c", "b"],
 ["a", "c", "c"], ["b", "a", "a"], ["b", "a", "b"], ["b", "a", "c"],
 ["b", "b", "a"], ["b", "b", "b"], ["b", "b", "c"], ["b", "c", "a"],
 ["b", "c", "b"], ["b", "c", "c"], ["c", "a", "a"], ["c", "a", "b"],
 ["c", "a", "c"], ["c", "b", "a"], ["c", "b", "b"], ["c", "b", "c"],
 ["c", "c", "a"], ["c", "c", "b"], ["c", "c", "c"]]

iex(25)> Permutations.shuffle ["a", "b", "c"], 2
[["a", "a"], ["a", "b"], ["a", "c"], ["b", "a"], ["b", "b"], ["b", "c"],
 ["c", "a"], ["c", "b"], ["c", "c"]]

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