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

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.