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

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What is * args in Ruby?

In the code you posted, *args simply indicates that the method accepts a variable number of arguments in an array called args . It could have been called anything you want (following the Ruby naming rules, of course).

What are the operators in Ruby?

Ruby Arithmetic OperatorsAddition − Adds values on either side of the operator. Subtraction − Subtracts right hand operand from left hand operand. Multiplication − Multiplies values on either side of the operator. Division − Divides left hand operand by right hand operand.

What does === mean in Ruby?

In Ruby, the === operator is used to test equality within a when clause of a case statement.

What is << operator in Ruby?

As a general convention, << in Ruby means "append", i.e. it appends its argument to its receiver and then returns the receiver. So, for Array it appends the argument to the array, for String it performs string concatenation, for Set it adds the argument to the set, for IO it writes to the file descriptor, and so on.


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.


The splat operator unpacks an array passed to a function so that each element is sent to the function as an individual parameter.

A simple example:

>> def func(a, b, c)
>>   puts a, b, c
>> end
=> nil

>> func(1, 2, 3)  #we can call func with three parameters
1
2
3
=> nil

>> list = [1, 2, 3]
=> [1, 2, 3]

>> func(list) #We CAN'T call func with an array, even though it has three objects
ArgumentError: wrong number of arguments (1 for 3)
    from (irb):12:in 'func'
    from (irb):12

>> func(*list) #But we CAN call func with an unpacked array.
1
2
3
=> nil

That's it!


As everyone mentions, it's a "splat". Looking for Ruby syntax is impossible, and I've asked this in other questions. The answer to that part of the question is that you search on

asterisk in ruby syntax

in Google. Google is there for you, just put what you see into words.

Anyhoo, like a lot of Ruby code, that code is quite dense. The

line.split(/=|;/)

makes an array of SIX elements, first_name, mickey, last_name, mouse, country, usa. Then the splat is used to make that into a Hash. Now the Ruby people always send you to look at the Splat method, since everything is exposed in Ruby. I have no idea where it is, but once you have that, you'll see that it runs a for through the array and builds the hash.

You would look for the code in the core documentation. If you cannot find it (I could not), you would try to write some code like this (which works, but is NOT Ruby-like code):

line = "first_name=mickey;last_name=mouse;country=usa"
presplat = line.split(/=|;/)
splat = Hash.new
for i in (0..presplat.length-1)
    splat[presplat[i]] = presplat[i+1] if i%2==0
end

puts splat["first_name"]

and then the Ruby gang will be able to tell you why your code is silly, bad, or just plain wrong.

If you've read this far, take a read through the Hash documentation for initialization.

Basically a hash that is initialized with several arguments creates them as key value pairs:

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

So in your example this would lead to the following Hash:

{"first_name"=>"mickey", "last_name"=>"mouse", "county"=>"usa"}