What does the “yield” keyword do in Ruby?

Question

I encountered the following Ruby code:

class MyClass
    attr_accessor :items
    ...
    def each
        @items.each{|item| yield item}
    end
    ...
end         


        

Answer 1

When you write a method that takes a block, you can use the yield keyword to execute the block.

As an example, each could have been implemented in the Array class like this:

class Array
  def each
    i = 0
    while i < self.size
      yield( self[i] )
      i = i + 1
    end
  end
end

MyClass#each takes a block. It executes that block once for each item in the instance's items array, passing the current item as an argument.

It might be used like this:

instance = MyClass.new
instance.items = [1, 2, 3, 4, 5]
instance.each do |item|
  puts item
end