What does the (unary) * operator do in this Ruby code?

Question

Given the Ruby code

line = "first_name=mickey;last_name=mouse;country=usa" 
record = Hash[*line.split(/=|;/)] 

I understand everything in the second line apart from the * operator - what is it doing and where is the documentation for this? (as you might guess, searching for this case is proving hard...)

Answer 1

The * is the splat operator.

It expands an Array into a list of arguments, in this case a list of arguments to the Hash.[] method. (To be more precise, it expands any object that responds to to_ary/to_a, or to_a in Ruby 1.9.)

To illustrate, the following two statements are equal:

method arg1, arg2, arg3
method *[arg1, arg2, arg3]

It can also be used in a different context, to catch all remaining method arguments in a method definition. In that case, it does not expand, but combine:

def method2(*args)  # args will hold Array of all arguments
end

Some more detailed information here.