Why do I need asterisk before an array?

Question

I have no idea if this is a Hash issue or an Array issue, but I don't figure out why asterisk (*) sign is required in the third example to get a hash filled with data. Without it, it outputs an empty hash.

# -*- coding: utf-8 -*-
require 'pp'
pp [[:first_name, 'Shane'], [:last_name, 'Harvie']]
# => [[:first_name, "Shane"], [:last_name, "Harvie"]]

pp [[:first_name, 'Shane'], [:last_name, 'Harvie']].flatten
# => [:first_name, "Shane", :last_name, "Harvie"]

pp Hash[*[[:first_name, 'Shane'], [:last_name, 'Harvie']].flatten]
# => {:first_name=>"Shane", :last_name=>"Harvie"}

pp Hash[[[:first_name, 'Shane'], [:last_name, 'Harvie']].flatten]
# => {}

Below is an explanation of the method.

--------------------------------------------------------------- Hash::[]
     Hash[ [key =>|, value]* ]   => hash

     From Ruby 1.8
------------------------------------------------------------------------
     Creates a new hash populated with the given objects. Equivalent to
     the literal +{ _key_, _value_, ... }+. Keys and values occur in
     pairs, so there must be an even number of arguments.

        Hash["a", 100, "b", 200]       #=> {"a"=>100, "b"=>200}
        Hash["a" => 100, "b" => 200]   #=> {"a"=>100, "b"=>200}
        { "a" => 100, "b" => 200 }     #=> {"a"=>100, "b"=>200}

Answer 1

The splat operator (that is, the *) turns what would otherwise be an array into a list for assignment within the hash. You have to give the [] operator an actual list to turn into the key/value pairs of a hash. (See below for a link to a short description of the splat operator which actually can do this (unwind an array into a list) or the reverse (gather a list into an array).)

The way you did it above, you give Hash[] an odd number of items, namely a single array. (Think of what [[:first_name, 'Shane'], [:last_name, 'Harvie']].flatten produces. It yields [:first_name, 'Shane', :last_name, 'Havie'].) As the docs you quoted say, the [] operator must have an even number of elements. Note that the following (though useless) does work:

>> Hash[[[:first_name, 'Shane'], [:last_name, 'Harvie']].flatten, 1]
=> {[:first_name, "Shane", :last_name, "Harvie"]=>1}

(It's unclear to me why you don't get the "odd number of arguments for Hash" error when using the code you have above - as you do if you try Hash[1].)

A simpler example may make it clearer. First, passing in one item, an array. The opening up the array with * to hand Hash[] a list of items:

>> Hash[['foo', 'bar', 'bizz', 'buzz']]
=> {}
>> Hash[*['foo', 'bar', 'bizz', 'buzz']]
=> {"foo"=>"bar", "bizz"=>"buzz"}

See this blog post for a fuller explanation. You might also find this short write-up about the splat operator useful.