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I'm learning Ruby via RubyMonk. In the solution to the waiter robot problem, there's a line of code that creates a new hash using an array:

o = Hash[*order]

given

order = [:table, 1, :sandwich, "denver", :drink, "mango shake"]

I understand what's being done here and how the splat operator works. However, I'm confused about the syntax for creating this hash. RubyDoc says that ::[] is really the method being called, so I was able to determine that o = Hash::[](*order) is the same thing. But why can this be shortened to just Hash[*order]? Is this a special construct that gets interpreted by the parser, or is there another reason? Along the same lines, why don't any of the following work?

o = Hash.new
o.[](*order)

or

o = Hash.new
o[*order]

or even something like o = {}[*order]

I know these shouldn't work; I'm just not sure why. I think I'm confused by the usage of Hash[*order] without first instantiating a hash with Hash.new. Is this an example of the difference between class methods and instance methods?

(As a side note, it seems to me that o = {*order} should work, but it doesn't.)

Can someone explain what's going on here, and if there are alternate ways to add values from an array into a hash?

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2 Answers 2

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3

Is this an example of the difference between class methods and instance methods?

Exactly.

But why can this be shortened to just Hash[*order]?

Ruby interprets some_object[] as a call to the method named [] on some_object. This isn't special for Hashes, you can implement a [] method in any class of your own and use that syntax.

Can someone explain what's going on here, and if there are alternate ways to add values from an array into a hash?

Hash[*order] calls a class method (Hash#[]) which creates a new Hash. o.[](*order) doesn't work for the same reason you can't call new on it: {}.new doesn't make any sense. You can't call a class method on an instance of the class.

You can add values with merge:

o = Hash.new
o.merge(Hash[*order])

o = {*order} doesn't work because {} is the syntax for a Hash literal, and putting *order in there doesn't make sense.

Hash(*order) is Kernel#Hash, a method that expects only one argument.

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4 Comments

Thanks, that helps. Why then can't you do Hash(*order)? I'm confused as to how it makes sense to use square brackets here.
@sgroves That syntax calls Kernel#Hash, linked to in the last line of my updated answer.
Hash[] is a convenience class method, likewise when you call object[] it's a different method than Hash[]. Checkout the Hash class docs and look for the two different [] methods. Also [] is a special parse case in the language, it basically does object.[](args).
Ahh, thanks Azolo. Knowing that the [] means something special to the interpreter makes it clear to me.
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When you write Hash(*order) you're actually calling the Hash method in the Kernel module, which is not the same as calling the [] method in the Hash class. See the docs for Kernel#Hash to see what's going on under the hood there.

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Hash(*order) calls Kernel#Hash. Hash[*order] in fact does call the public class method on Hash linked to in the question.
Thanks for spotting that. I've updated my answer since I meant to write Hash(*order) to begin with.

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