maths.livemd

Maths

Module

defmodule Math do
  # permutations
  def permutations([]) do
    [[]]
  end

  def permutations(list) do
    for h <- list, t <- permutations(list -- [h]), do: [h | t]
  end

  # combine array with itself to array of size k
  def combinations(_, 0), do: [[]]
  def combinations([], _), do: []

  def combinations([x | xs], r) do
    for(y <- combinations(xs, r - 1), do: [x | y]) ++ combinations(xs, r)
  end

  # dot product of arrays
  def product([]), do: [[]]

  def product([list | rest]) do
    for x <- list, y <- product(rest), do: [x | y]
  end
end

Usage

[1, 2, 3, 4]
|> Math.permutations()

[1, 2, 3, 4]
|> Math.combinations(2)

array1 = [1, 2, 3, 4]
array2 = [5, 6]
array3 = [7, 8, 9]
Math.product([array1, array2, array3])