What is *self?

ruby-talk
1

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

#exp
a=[1,2,3]
h=a.to_h{|e| 2**e}
h.each{|k,v| puts "key=#{k} val=#{v}"}

#->key=1 val=2 key=2 val=4 key=3 val=8

Without the '*' on self it generates an error:
array2hash.rb:3:in `[]': odd number of arguments for Hash (ArgumentError)
         from array2hash.rb:3:in `to_h'
         from array2hash.rb:8

What does the '*' do to correct the error?

Does it push self up a level to Array somehow?

Does this notation generalize?

Thanks,

Mark

2

josefK 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

"*" is the "splat" operator when used as a prefix. It takes the
following array, and turns into a list of parameters. Without it the
argument to Hash becomes a single Array, while it expects a list of
keys and values.

And yes, it does generalize. Try
  p [1,2,3]
  p *[1,2,3]
in irb to get a better idea of the result. The first prints a single
array, the second prints _three separate values_

Vidar

3

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

···

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

4

Vidar Hokstad wrote:

josefK 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

"*" is the "splat" operator when used as a prefix. It takes the
following array, and turns into a list of parameters. Without it the
argument to Hash becomes a single Array, while it expects a list of
keys and values.

And yes, it does generalize. Try
  p [1,2,3]
  p *[1,2,3]
in irb to get a better idea of the result. The first prints a single
array, the second prints _three separate values_

Vidar

OK - I see - it does not affect 'self' just the output
I thought it was a way to modify 'self'

I guess 'super' and 'super.super...' is the generalization
I mistakenly thought was happening.

Thanks much,

Mark