CSC/ECE 517 Fall 2012/ch1b 1w60 ac

SaaS - 3.8 yield()

The yield() Function

What Is It?

Simply put, the yield function in Ruby passes control to a user-defined code block. As simple as that statement is, it can be quite confusing, so here is a quick example<ref>http://www.tutorialspoint.com/ruby/ruby_blocks.htm</ref>:

def test
   puts "You are in the method"
   yield
   puts "You are again back to the method"
   yield
end
test {puts "You are in the block"}

Executing this code results in:

You are in the method
You are in the block
You are again back to the method
You are in the block

First, take a look at the last line of code. You'll notice that the parameter to the test function is actually a code block. Next, look at where the text "You are in the block" appears: after the "You are in the method" text then again after the "You are again back to the method" text. Notice how this corresponds to where the yield statements are in the code? That's because the yield statement is calling the code block, then returning control to the test method.

Syntax

As you may have guessed from the previous example, the yield function's syntax is quite simple. In it's most basic form, the yield function can be called with:

yield

In this form, the yield function will simply execute the code block passed to the function it's called from. You can also pass parameters to the yield function which passes those parameters to the code block, like so:

yield parameter

Passing parameters allows you to call the same code block, but with different input. For example<ref>http://www.tutorialspoint.com/ruby/ruby_blocks.htm</ref>:

def test
   yield 5
   puts "You are in the method test"
   yield 100
end
test {|i| puts "You are in the block #{i}"}

Executing this code results in:

You are in the block 5
You are in the method test
You are in the block 100

See how the same code block results in different output yielded to the first and second times? That's because the values 5 and 10 are passed each time, respectively, and output by the code block.

Examples

Tree Traversal

<ref>https://docs.google.com/viewer?url=http%3A%2F%2Fcourses.ncsu.edu%2Fcsc517%2Fcommon%2Fhomework%2Ftests%2Fa1.pdf</ref>

class Tree
  attr_reader :value
  def initialize(value)
    @value = value
    @children = []
  end
  
  def <<(value)
    subtree = Tree.new(value)
    @children << subtree
    return subtree
  end
  
  def each
    yield value
    @children.each do |child_node|
      child_node.each { |e| yield e }
    end
  end
end

t = Tree.new()
... # code to populate the tree
t.each {|x| puts x}

In this example, the yield function is used to allow the tree owner to do whatever they want to each element of a tree. This example just prints the name of each node of the tree, but passing a more complex code block would let you do whatever you like with each child. In this example, we create a class that includes Enumerable, providing access to the each method, among others. So, when

References

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