How do methods use hash arguments in Ruby?

问题:

I saw hash arguments used in some library methods as I've been learning.

E.g.,

list.search(:titles, genre: 'jazz', duration_less_than: 270)

Can someone explain how a method uses arguments like this, and how you could create a method that makes use of Hash arguments?

回答1:

Example:

def foo(regular, hash={})
    puts "regular: #{regular}"
    puts "hash: #{hash}"
    puts "a: #{hash[:a]}"
    puts "b: #{hash[:b]}"
end

foo("regular argument", a: 12, :b => 13)

I use hash={} to specify that the last argument is a hash, with default value of empty hash. Now, when I write:

foo("regular argument", a: 12, :b => 13)

It's actually a syntactic sugar for:

foo("regular argument", {a: 12, :b => 13})

Also, {a: 12} is syntactic sugar for {:a => 12}.

When all of this is combined together, you get a syntax that looks similar to named arguments in other languages.

回答2:

In Ruby 2.x, you can use ** hash splat:

def foo( ordered_argument, **named_arguments )
  puts "Ordered argument: #{ordered_argument}"
  puts "Named arguments: #{named_arguments}"
end

foo( :titles, genre: 'jazz', duration_less_than: 270 )
#=> Ordered argument: titles
#=> Named arguments: {:genre=>"jazz", :duration_less_than=>270}

回答3:

When a Ruby method call's argument list ends in one or more key-value pairs, like foo: 'bar' or 'foo' => 1, Ruby collects them all into a single hash and passes that hash as the last parameter. You can see that yourself in irb:

irb(main):002:0> puts foo: 'bar', baz: 'quux'
{:foo=>"bar", :baz=>"quux"}
=> nil

Thus, you can add a final, optional parameter to a method you're writing to receive this hash. You'll usually want to default it to an empty hash. You can call the parameter anything you want, but options is a common name:

def my_method(a, b, c, options = {})
  ...
end

One useful trick if you're using Rails: It's often handy to treat plain strings and symbols as equivalent. Rails adds a symbolize_keys! method to Hash to convert all string keys to symbols:

def my_method(a, b, c, options = {})
  options.symbolize_keys!
  ...
end

回答4:

I would do one of two options:

1- if a got a large number of arguments to pass into a method I would use a hash like this:

some_method({titles => 'titulo', genre => 'jazz', duration_less_than => 270})

or

my_hash = {titles => 'titulo', genre => 'jazz', duration_less_than => 270}
some_method(my_hash)

and

def some_method(hash_options)
  #important code
end

2- option will be more 'traditional'

some_method('titulo', 'jazz', 270)

def some_method(titles, genre, duration_less_than)
#important code
end

回答5:

Since Ruby 2.0 you can use keyword arguments ^[1][2] as opposed to the single Hash parameter.

def foo(keyword_arg: 'bar')
  keyword_arg
end

And here's how it behaves.

> foo
=> "bar"
> foo(keyword_arg: 'baz')
=> "baz"

1. http://robots.thoughtbot.com/ruby-2-keyword-arguments
2. http://brainspec.com/blog/2012/10/08/keyword-arguments-ruby-2-0/

回答6:

This is how I do it:

def my_method(title, args)
  puts title
  puts args

passing parameters:

my_method('test title', a: 'foo', b: 'bar')
  # => test title
  # => '{:a => 'foo', :b => 'bar'}