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Ruby in Twenty Minutes

Now let’s create a greeter object and use it:

irb(main):035:0> greeter = Greeter.new("Pat")
=> #<Greeter:0x16cac @name="Pat">
irb(main):036:0> greeter.say_hi
Hi Pat!
=> nil
irb(main):037:0> greeter.say_bye
Bye Pat, come back soon.
=> nil

Once the greeter object is created, it remembers that the name is Pat. Hmm, what if we want to get at the name directly?

irb(main):038:0> greeter.@name
SyntaxError: (irb):38: syntax error, unexpected tIVAR, expecting '('

Nope, can’t do it.

Under the Object’s Skin

Instance variables are hidden away inside the object. They’re not terribly hidden, you see them whenever you inspect the object, and there are other ways of accessing them, but Ruby uses the good object-oriented approach of keeping data sort-of hidden away.

So what methods do exist for Greeter objects?

irb(main):039:0> Greeter.instance_methods
=> [:say_hi, :say_bye, :instance_of?, :public_send,
    :instance_variable_get, :instance_variable_set,
    :instance_variable_defined?, :remove_instance_variable,
    :private_methods, :kind_of?, :instance_variables, :tap,
    :is_a?, :extend, :define_singleton_method, :to_enum,
    :enum_for, :<=>, :===, :=~, :!~, :eql?, :respond_to?,
    :freeze, :inspect, :display, :send, :object_id, :to_s,
    :method, :public_method, :singleton_method, :nil?, :hash,
    :class, :singleton_class, :clone, :dup, :itself, :taint,
    :tainted?, :untaint, :untrust, :trust, :untrusted?, :methods,
    :protected_methods, :frozen?, :public_methods, :singleton_methods,
    :!, :==, :!=, :__send__, :equal?, :instance_eval, :instance_exec, :__id__]

Whoa. That’s a lot of methods. We only defined two methods. What’s going on here? Well this is all of the methods for Greeter objects, a complete list, including ones defined by ancestor classes. If we want to just list methods defined for Greeter we can tell it to not include ancestors by passing it the parameter false, meaning we don’t want methods defined by ancestors.

irb(main):040:0> Greeter.instance_methods(false)
=> [:say_hi, :say_bye]

Ah, that’s more like it. So let’s see which methods our greeter object responds to:

irb(main):041:0> greeter.respond_to?("name")
=> false
irb(main):042:0> greeter.respond_to?("say_hi")
=> true
irb(main):043:0> greeter.respond_to?("to_s")
=> true

So, it knows say_hi, and to_s (meaning convert something to a string, a method that’s defined by default for every object), but it doesn’t know name.

Altering Classes—It’s Never Too Late

But what if you want to be able to view or change the name? Ruby provides an easy way of providing access to an object’s variables.

irb(main):044:0> class Greeter
irb(main):045:1>   attr_accessor :name
irb(main):046:1> end
=> nil

In Ruby, you can reopen a class and modify it. The changes will be present in any new objects you create and even available in existing objects of that class. So, let’s create a new object and play with its @name property.

irb(main):047:0> greeter = Greeter.new("Andy")
=> #<Greeter:0x3c9b0 @name="Andy">
irb(main):048:0> greeter.respond_to?("name")
=> true
irb(main):049:0> greeter.respond_to?("name=")
=> true
irb(main):050:0> greeter.say_hi
Hi Andy!
=> nil
irb(main):051:0> greeter.name="Betty"
=> "Betty"
irb(main):052:0> greeter
=> #<Greeter:0x3c9b0 @name="Betty">
irb(main):053:0> greeter.name
=> "Betty"
irb(main):054:0> greeter.say_hi
Hi Betty!
=> nil

Using attr_accessor defined two new methods for us, name to get the value, and name= to set it.

Greeting Anything and Everything, MegaGreeter Neglects None!

This greeter isn’t all that interesting though, it can only deal with one person at a time. What if we had some kind of MegaGreeter that could either greet the world, one person, or a whole list of people?

Let’s write this one in a file instead of directly in the interactive Ruby interpreter IRB.

To quit IRB, type “quit”, “exit” or just hit Control-D.

#!/usr/bin/env ruby

class MegaGreeter
  attr_accessor :names

  # Create the object
  def initialize(names = "World")
    @names = names

  # Say hi to everybody
  def say_hi
    if @names.nil?
      puts "..."
    elsif @names.respond_to?("each")
      # @names is a list of some kind, iterate!
      @names.each do |name|
        puts "Hello #{name}!"
      puts "Hello #{@names}!"

  # Say bye to everybody
  def say_bye
    if @names.nil?
      puts "..."
    elsif @names.respond_to?("join")
      # Join the list elements with commas
      puts "Goodbye #{@names.join(", ")}.  Come back soon!"
      puts "Goodbye #{@names}.  Come back soon!"

if __FILE__ == $0
  mg = MegaGreeter.new

  # Change name to be "Zeke"
  mg.names = "Zeke"

  # Change the name to an array of names
  mg.names = ["Albert", "Brenda", "Charles",
              "Dave", "Engelbert"]

  # Change to nil
  mg.names = nil

Save this file as “ri20min.rb”, and run it as “ruby ri20min.rb”. The output should be:

Hello World!
Goodbye World.  Come back soon!
Hello Zeke!
Goodbye Zeke.  Come back soon!
Hello Albert!
Hello Brenda!
Hello Charles!
Hello Dave!
Hello Engelbert!
Goodbye Albert, Brenda, Charles, Dave, Engelbert.  Come
back soon!

There are a lot of new things thrown into this final example that we can take a deeper look at.