From: Jacob Fugal <lukfugl@...>
Date: 2006-11-25T06:03:38+09:00
Subject: Re: what is *self?

On 11/24/06, josefK <josefK@kafka.org> wrote:
> I encountered this added method to Array (I don't recall
> the author - sorry ,)
> It converts an Array to a Hash by providing a block to compute the
> values given the array elements as keys:
>
> class Array
>    def to_h(&block)
>      Hash[*self.collect{|v| [v,block.call(v)]}.flatten]
>    end
> end

Vidar explained the general concept of the splat fairly well, but the
following clarification of order of operations should help understand
this particular application:

  # assuming self = [ 1, 2, 3 ] and block = proc{ |x| x**2 }
  self.
    collect{|v| [v,block.call(v)]}. # => [[1, 1], [2, 4], [3, 9]]
    flatten # => [1, 1, 2, 4, 3, 9]

Without the star, the full Method body would be (equivalently):

  Hash[[1, 1, 2, 4, 3, 9]]

This complains about an odd number of elements because it's receiving
only one element -- the array. Adding the "splat" in essence removes
the innermost pair of square brackets:

  Hash[1, 1, 2, 4, 3, 9]

Hash#[] can then use the list as key/value pairs.

As I said, Vidar explained the operation of the splat well, I just
wanted to point out *which* array the star was operating on -- the
result of the expression "self.collect{ ... }.flatten", not to self
itself (no pun intended).

Jacob Fugal