What Does a Double * (Splat) Operator Do
Have You Seen a Function Declared Like This? Def Foo A, **B. .. End I Understand That a Single * Is the Splat Operator. What Does ** Mean? 4 Answers Ruby 2.0...
Have you seen a function declared like this?
def foo a, **b
...
end
I understand that a single * is the splat operator. What does ** mean?
4 Answers
Ruby 2.0 introduced keyword arguments, and ** acts like *, but for keyword arguments. It returns a Hash with key / value pairs.
For this code:
def foo(a, *b, **c)
[a, b, c]
end
Here's a demo:
> foo 10
=> [10, [], {}]
> foo 10, 20, 30
=> [10, [20, 30], {}]
> foo 10, 20, 30, d: 40, e: 50
=> [10, [20, 30], {:d=>40, :e=>50}]
> foo 10, d: 40, e: 50
=> [10, [], {:d=>40, :e=>50}]
That is the double splat operator which is available since Ruby 2.0.
It captures all keyword arguments (which can also be a simple hash, which was the idiomatic way to emulate keyword arguments before they became part of the Ruby language)
def my_method(**options)
puts options.inspect
end
my_method(key: "value")
The above code prints {key:value} to the console.
Just like the single splat operator captures all regular arguments, but instead of an array you get a hash.
Real-life example:
For example in Rails the cycle method looks like this:
def cycle(first_value, *values)
options = values.extract_options!
# ...
end
This method can be called like this: cycle("red", "green", "blue", name: "colors").
This is quite a common pattern: You accept a list of arguments and the last one is an options hash, which can be extract - for example - using ActiveSupport's extract_options!.
In Ruby 2.0 you can simplify these methods:
def cycle(first_value, *values, **options)
# Same code as above without further changes!
end
Admittedly it's only a minor improvement if you are already using ActiveSupport but for plain Ruby the code gains quite a lot of conciseness.
In addition, you can use it in caller side like this:
def foo(opts); p opts end
bar = {a:1, b:2}
foo(bar, c: 3)
=> ArgumentError: wrong number of arguments (given 2, expected 1)
foo(**bar, c: 3)
=> {:a=>1, :b=>2, :c=>3}
For your case, the above answers are accurate.
However, double splat operators can also be used in Ruby language for arithmetic operations as well. For example, x^y can be written as x**y.
Refer to this stackover question for more.
James Sterling reports on renewable energy developments, climate policy, ecological conservation, and green tech innovations around the globe.
