Ruby 1

Cay S. Horstmann

Why Ruby?

• Rails
• Web framework for agile development
• 2004, David Heinemeier Hansson
• Convention over configuration
• DRY (Don't Repeat Yourself)
• ActiveRecord dynamically turns database records into classes
• Rich tool infrastructure (ex. Rake, Ruby-based build tool)
• Hugely popular with web developers, particularly startups (ex. Twitter)
• The language makes it all possible

Ruby Features

• Good reference: http://www.ruby-doc.org/docs/ProgrammingRuby/
• Dynamically typed

  x = 1
  x = "1"  # x can hold an int or a string

• Duck typing

  def quack(x)  # no parameter type
    print(x + x + x)
  end
  
  quack("q")  # qqq
  quack(2)  # 6
  quack(true)  # error: no + for boolean

Blocks

• Block encapsulates code that can be passed to a function

  [1, 2, 3].each { |a| print 2 * a, " " }  # prints 2 4 6

• Kind of like closures, but not quite—see here
• One block can be passed to a method, as anonymous arg
• Block is invoked with yield

  def filter(a)
    r = []
    for i in a 
      if yield(i) then r.push(i) end  # invoke block
    end
    return r
  end
  filter([1,4,9,16,25]) { |x| x % 2 == 0 }  # returns [4,16]

More about Functions

• Optional parentheses, optional return

  def double(x) return 2*x end 
  double(2)  # returns 4
  double 2  # also ok, no ()
  def average(x,y) 0.5*(x + y) end  # last expression is returned
  average 2,4  

• Named and default arguments

  print("Hello", :size=>18, :style=>:italic, :weight=>:bold, :face=>"Courier")

  Function receives a map of all => pairs

  def print(str, params = {})  # params is empty map if no args supplied
    if !params[:size].nil? then ... end
    ...
  end

  :size is a symbol, a unique string (called atom in Lisp)

Classes

• Instance variables prefixed with @
• attr_accessor adds fields and getters/setters
• Constructor called initialize

  class Tree
    attr_accessor :children, :name
    def initialize(name, children = [])
      @name=name
      @children=children
    end
    def visit(&block)  # & captures otherwise anonymous block
      block.call(self)  # self is implicit parameter ("this" in C++/Java)
      children.each { |c| c.visit(&block) }
    end
  end
  t = Tree.new("Cay", [Tree.new("Charlotte"), Tree.new("Emily")])
  t.visit { |n| puts(n.name) }

Open Classes

• Can add to existing classes

  class Object
    def quack
      print(self + self + self)
    end
  end
  
  2.quack  # prints 6
  "q".quack # prints qqq
  2.class.quack  # error: undefined method `+' for Fixnum:Class 

• But that's not all—see the lecture on metaprogramming for much more

Lab

• Format of modules: Short lecture, lab, review
• You work with a buddy
• You need to bring a laptop (Windows, Linux, or Mac OS)
• One of you (the coder) codes, the other (the scribe) types up answers
• Switch roles each lab
• The scribe submits lab work to the Sakai site
• Don't worry about the English in the reports. Mostly code and a bit of English is fine.

Step 1

1. Install Ruby. It's easy with Linux, Mac OS, or Cygwin
   • sudo apt-get ruby
   • sudo port install ruby
   • apt-cyg install ruby
2. Start the interactive Ruby console by typing irb.
3. Make an array containing a number, a string, and a Boolean value and store it in a variable a. How did you do that?
4. Run the quack function on all elements of the array, using a block. How did you do that?

Step 2

• Write an implementation of a function map that applies a block to all elements of an array and returns an array of the results. For example,

  map([1,2,3]) { |x| x * x }

  should return

  [1,4,9]

  Hint: Adapt the filter function

  What is the code of your function?

• Add optional parameters :sorted and :descending (true or false). For example,

  map([1,3,2], :descending => true) { |x| x * x }

  returns the mapped values in the opposite order:

  [4,9,1]

Step 3

• Construct a tree

         120
        /   \
      10    12
     /  \  / | \
    2   5 2  2  3

  How do you do that?

• How do you compute the sum of all values in the tree (using the visit method, of course)
• Add a method to the tree class that prints the tree in Lisp style, like this:

  (120 (10 (2) (5)) (12 (2) (2) (3)))

  How do you do add this to the tree class (without changing the original class)?