[ruby-talk:7476] Re: flatten_by(n)

David Alan Black <[e-mail]> wrote:
>
>Hello --
>
>In honor of all the stuff about flattening in the thread ostensibly
>about Perl (but happily not), I thought I'd share this, for the
>possible reading/hacking pleasure of my fellow rubes:
>
>   class Array
>     def flatten_by(n)
>       res = []
>       return self if n < 1
>       each do |e|
>	if e.is_a? Array
>	  e.each do |ee|
>	    res.push ee
>	    res = res.flatten_by(n-1)
>	  end
>	else
>	  res.push e
>	end
>       end
>       res
>     end
>   end
>
>
>   a = [1, [2, [3], []], [5], [4, [[6]]]]
>
>   a.flatten_by(0) => [1, [2, [3], []], [5], [4, [[6]]]]
>   a.flatten_by(1) => [1, 2, [3], [], 5, 4, [[6]]]
>   a.flatten_by(2) => [1, 2, 3, 5, 4, [6]]
>   a.flatten_by(3) => [1, 2, 3, 5, 4, 6]
>
>
>And now.... (Ben, are you watching?) .... the grand test:

Yup, watching...

>   class Array
>     def flat_map(&block)
>       map(&block) .flatten_by(1)
>     end
>   end
>
>   [1,2,[3,4],5] .flat_map do |e| [e, "hello"] end
>
>     => [1, "hello", 2, "hello", [3, 4], "hello", 5, "hello"]
>
>
>It actually *could* be useful....  at least if, like me, you are
>obsessed with turning arrays into hashes in every possible way:
>
>   class Array
>     def to_h_all_true
>       Hash[*self.flat_map do |e| [e,true] end]
>     end
>   end
>
>   [1, [2, [3], []], [5], [4, [[6]]]] .to_h_all_true
>    => {[5]=>true, 1=>true, [2, [3], []]=>true, [4, [[6]]]=>true}
>
>
>Comments/optimizations/whatever welcome.

Nice.

Is there any reason that your flatten_by function takes a
parameter?  I would think it would be faster to have it just
flatten once (don't do the recursive call).  If someone wants
to flatten 6 times, they can call it 6 times.

I am away from my home box again, but something like this
should work:

  class Array
    def flatten_once
      res = []
      each do |e|
        if e.is_a? Array
          res += e
        else
          res.push e
        end
      end
      res
    end
  end

Cheers,
Ben
_________________________________________________________________
Get your FREE download of MSN Explorer at http://explorer.msn.com