Please help a newbie porting Norvig's Sudoku solver

chx · September 16, 2017, 3:06pm

I have started porting http://norvig.com/sudoku.html this to Elixir to learn the basics. After a few false starts I got far with code that at least to my eyes doesn’t look particularly horrible.

github.com

chx/sudoku/blob/master/sudoku.exs

defmodule Sudoku.Helpers do
  # wrapping in a list is necessary to make it work with single characters
  # and charlists both.
  def cross(a, b) do
    for x <- List.wrap(a), y <- List.wrap(b), do: {x, y}
  end
  # this is for testing and inspecting.
  def to_list(a) when is_list(a), do: Enum.map(a, &to_list/1)
  def to_list(a) when is_map(a), do: for {k, v} <- a, into: %{}, do: {to_list(k), to_list(v)}
  def to_list(a) when is_tuple(a), do: to_list(Tuple.to_list(a))
  def to_list(a), do: a
end


defmodule Sudoku do
  import Sudoku.Helpers

  @digits   '123456789'
  @rows     'ABCDEFGHI'
  @cols     @digits

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It might be argued my Enum.reduce() loops are a bit cheating… and actually, are they? Is this very bad practice?

The bigger problem is I can’t really get the actual search working. Any pointers (or captures because this language doesn’t have pointers )?

Finally the display is not finished but that’s not a big challenge. I am still very confused by when and how does Elixir decide to print the integers represented by a charlist (like, how do you convert a charlist to a string and not get 4950 instead of AB etc) and when it decides to use the characters but I guess practice will help.

Thanks!

OvermindDL1 · September 16, 2017, 4:29pm

Enum.reduce is one of the 2 basic operations, the other being flat_map, always feel free to use it. ^.^

Can always carry an environment and pretend pointers are keys on a map and the values are what the pointers point to, just thread the environment in and out of all calls.

“If every element in a charlist is an integer and each integer is inside the Printable Range then display it like a String by default”, you can of course override the default to print it however you want too, see the options on the calls.

Or just put 0 on the list somewhere, it is not printable so will always cause the list to not be printable too. ^.^

chx · September 18, 2017, 10:24am

I have ported search() but it doesn’t quite work and I do not see where I am wrong (I surely am). If someone could look into it that’d be great. Thanks for your time!