How does the `&:name` parameter work?

map.rb

cities = City.all
cities.map(&:name) # ['City 1', 'City 2']

How does it work? What is the '&' char for?

It is an alias for something like

cities = City.all
cities.map { |city| city.name }

In this case, it will map your cities list to list of the city names.

protip:

[1, 3, 10, 20, 34, 54, 60].inject(:+)

Right, but I'd like to understand how to use the '&' parameter. Is it interpreted as a block, maybe?

& is some kind of alias to a Proc. It's in the Ruby language itself, by the way.

class City
  attr_accessor :name
  def initialize(name)
    @name = name
  end
end
puts [City.new('City 1'), City.new('City 2')].map(&:name)

I found the answer here:

If the last argument to a method is preceded by an ampersand, Ruby assumes that it is a Proc object.
It removes it from the parameter list, converts the Proc object into a block, and associates it with the method.

Thanks.

It uses the to_proc method, so it's possible to do this:

class Greeter
  def to_proc
    lambda { "hello world" }
  end
end

def greet
  yield
end

greet &Greeter.new #=> "hello world"

And this:

class String
  def to_proc
    text = self
    lambda { eval text }
  end
end

instance_eval &"2 + 2" #=> 4

Nice! Thanks