From: Benoit Daloze <eregontp@...>
Date: 2010-12-12T23:42:34+09:00
Subject: [ruby-core:33686] Re: [feature:trunk] Enumerable#categorize

On 12 December 2010 12:13, Tanaka Akira <akr@fsij.org> wrote:
> Hi.
>
> How about a method for converting enumerable to hash?
>
> �enum.categorize([opts]) {|elt| [key1, ..., val] } -> hash
>
> categorizes the elements in _enum_ and returns a hash.
>
> The block is called for each elements in _enum_.
> The block should return an array which contains
> one or more keys and one value.
>
> �p (0..10).categorize {|e| [e % 3, e % 5] }
> �#=> {0=>[0, 3, 1, 4], 1=>[1, 4, 2, 0], 2=>[2, 0, 3]}

I find this clearer (but categorize looks better) :

(0..10).group_by { |e| e%3 }.each_with_object({}) { |(k,v),h| h[k] =
v.map { |e| e%5 } }

(Unfortunately, the 'map' on a Hash is not very nice, so I guess you
better 'group' by hand and 'map' at the same time like Martin showed)

> Martin: I admit that it is more elegant if the initialization and the final line can be avoided, but I think that's an issue that can be dealt with separately.

Enumerable#each_with_object is handy in this case:
(0..10).each_with_object({}) { |e,h| (h[e%3] ||= []) << e%5 }

Or, with Ryan's variant:
(0..10).each_with_object(Hash.new { |h,k| h[k]=[] }) { |e,h| h[e%3] << e%5 }
# I wish there was a shorter way for a self-duplicating Hash's default value

For the rest, I agree with Martin.

Regards,
B.D.