[Feature proposal] add counter / multiset function to Enum (or List) module

[David Whitlock]

Hi all,

In  many languages, there is a multiset functionality (in Python, this is called Counter), which produces a map with the counts of each item from a list. For example, you might want to do a word search of a text stream, and after converting the text to a list of words, you can then call the multiset / counter function and it would produce something like this:

    %{"the" => 34, "cat" => 2, "mat" => 7}

I've found this functionality very useful in the past, and I was wondering if it might be added to the standard library.

My proposal is to add something like the following function to the Enum module:

    def counter(items) do
      Enum.reduce items, %{}, fn key, map ->
        current = case :maps.find(key, map) do
          {:ok, value} -> value
          :error -> 0
        end
        :maps.put(key, current + 1, map)
      end
    end

This would take any list and return a map containing the 'counts' of each item. I don't see the need for creating a separate multiset type. It can just be used like any other map.

Please let me know what you think,
David

[José Valim]

Here is a simpler version with Map.update:

    Enum.reduce items, %{}, fn item, map ->

      Map.update(map, item, 1, & &1 + 1)

    end

To be quite honest, I don't think this particular case is important enough to warrant its own function. :)

Thank you David!

José Valim

www.plataformatec.com.br

Skype: jv.ptec

Founder and Director of R&D

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[David Whitlock]

Ok. Well, at least your solution can provide documentation for this :)