Ruby in Twenty Minutes

Introduction

This is a small Ruby tutorial that should take no more than 20 minutes to complete. It makes the assumption that you already have Ruby installed. (If you do not have Ruby on your computer install it before you get started.)

Interactive Ruby

Ruby comes with a program that will show the results of any Ruby statements you feed it. Playing with Ruby code in interactive sessions like this is a terrific way to learn the language.
Open up IRB (which stands for Interactive Ruby).

• If you’re using Mac OS X open up Terminal and type irb, then hit enter.
• If you’re using Linux, open up a shell and type irb and hit enter.
• If you’re using Windows, open Interactive Ruby from the Ruby section of your Start Menu.

irb(main):001:0>

Ok, so it’s open. Now what?
Type this: "Hello World"

irb(main):001:0> "Hello World"
=> "Hello World"

Ruby Obeyed You!

What just happened? Did we just write the world’s shortest “Hello World” program? Not exactly. The second line is just IRB’s way of telling us the result of the last expression it evaluated. If we want to print out “Hello World” we need a bit more:

irb(main):002:0> puts "Hello World"
Hello World
=> nil

puts is the basic command to print something out in Ruby. But then what’s the => nil bit? That’s the result of the expression. puts always returns nil, which is Ruby’s absolutely-positively-nothing value.

Your Free Calculator is Here

Already, we have enough to use IRB as a basic calculator:

irb(main):003:0> 3+2
=> 5

Three plus two. Easy enough. What about three times two? You could type it in, it’s short enough, but you may also be able to go up and change what you just entered. Try hitting the up-arrow on your keyboard and see if it brings up the line with 3+2 on it. If it does, you can use the left arrow key to move just after the + sign and then use backspace to change it to a * sign.

irb(main):004:0> 3*2
=> 6

Next, let’s try three squared:

irb(main):005:0> 3**2
=> 9

In Ruby ** is the way you say “to the power of”. But what if you want to go the other way and find the square root of something?

irb(main):006:0> Math.sqrt(9)
=> 3.0

Ok, wait, what was that last one? If you guessed, “it was figuring out the square root of nine,” you’re right. But let’s take a closer look at things. First of all, what’s Math?

Modules Group Code by Topic

Math is a built-in module for mathematics. Modules serve two roles in Ruby. This shows one role: grouping similar methods together under a familiar name. Math also contains methods like sin() and tan().
Next is a dot. What does the dot do? The dot is how you identify the receiver of a message. What’s the message? In this case it’s sqrt(9), which means call the method sqrt, shorthand for “square root” with the parameter of 9.
The result of this method call is the value 3.0. You might notice it’s not just 3. That’s because most of the time the square root of a number won’t be an integer, so the method always returns a floating-point number.
What if we want to remember the result of some of this math? Assign the result to a variable.

irb(main):007:0> a = 3 ** 2
=> 9
irb(main):008:0> b = 4 ** 2
=> 16
irb(main):009:0> Math.sqrt(a+b)
=> 5.0

As great as this is for a calculator, we’re getting away from the traditional Hello World message that beginning tutorials are supposed to focus on…
--------------
What if we want to say “Hello” a lot without getting our fingers all tired? We need to define a method!

irb(main):010:0> def hi
irb(main):011:1> puts "Hello World!"
irb(main):012:1> end
=> :hi

The code def hi starts the definition of the method. It tells Ruby that we’re defining a method, that its name is hi. The next line is the body of the method, the same line we saw earlier: puts "Hello World". Finally, the last line end tells Ruby we’re done defining the method. Ruby’s response => :hi tells us that it knows we’re done defining the method. This response could be => nil for Ruby 2.0 and earlier versions. But, it’s not important here, so let’s go on.

The Brief, Repetitive Lives of a Method

Now let’s try running that method a few times:

irb(main):013:0> hi
Hello World!
=> nil
irb(main):014:0> hi()
Hello World!
=> nil

Well, that was easy. Calling a method in Ruby is as easy as just mentioning its name to Ruby. If the method doesn’t take parameters that’s all you need. You can add empty parentheses if you’d like, but they’re not needed.
What if we want to say hello to one person, and not the whole world? Just redefine hi to take a name as a parameter.

irb(main):015:0> def hi(name)
irb(main):016:1> puts "Hello #{name}!"
irb(main):017:1> end
=> :hi
irb(main):018:0> hi("Matz")
Hello Matz!
=> nil

So it works… but let’s take a second to see what’s going on here.

Holding Spots in a String

What’s the #{name} bit? That’s Ruby’s way of inserting something into a string. The bit between the braces is turned into a string (if it isn’t one already) and then substituted into the outer string at that point. You can also use this to make sure that someone’s name is properly capitalized:

irb(main):019:0> def hi(name = "World")
irb(main):020:1> puts "Hello #{name.capitalize}!"
irb(main):021:1> end
=> :hi
irb(main):022:0> hi "chris"
Hello Chris!
=> nil
irb(main):023:0> hi
Hello World!
=> nil

A couple of other tricks to spot here. One is that we’re calling the method without parentheses again. If it’s obvious what you’re doing, the parentheses are optional. The other trick is the default parameter World. What this is saying is “If the name isn’t supplied, use the default name of "World"”.

Evolving Into a Greeter

What if we want a real greeter around, one that remembers your name and welcomes you and treats you always with respect. You might want to use an object for that. Let’s create a “Greeter” class.

irb(main):024:0> class Greeter
irb(main):025:1>   def initialize(name = "World")
irb(main):026:2>     @name = name
irb(main):027:2>   end
irb(main):028:1>   def say_hi
irb(main):029:2>     puts "Hi #{@name}!"
irb(main):030:2>   end
irb(main):031:1>   def say_bye
irb(main):032:2>     puts "Bye #{@name}, come back soon."
irb(main):033:2>   end
irb(main):034:1> end
=> :say_bye

The new keyword here is class. This defines a new class called Greeter and a bunch of methods for that class. Also notice @name. This is an instance variable, and is available to all the methods of the class. As you can see it’s used by say_hi and say_bye.
So how do we get this Greeter class set in motion?
-----------
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 open a class up again 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
  end

  # 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}!"
      end
    else
      puts "Hello #{@names}!"
    end
  end

  # 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!"
    else
      puts "Goodbye #{@names}.  Come back soon!"
    end
  end
end


if __FILE__ == $0
  mg = MegaGreeter.new
  mg.say_hi
  mg.say_bye

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

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

  # Change to nil
  mg.names = nil
  mg.say_hi
  mg.say_bye
end

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!

---------------

So, looking deeper at our new program, notice the initial lines, which begin with a hash mark (#). In Ruby, anything on a line after a hash mark is a comment and is ignored by the interpreter. The first line of the file is a special case, and under a Unix-like operating system tells the shell how to run the file. The rest of the comments are there just for clarity.
Our say_hi method has become a bit trickier:

# 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}!"
    end
  else
    puts "Hello #{@names}!"
  end
end

It now looks at the @names instance variable to make decisions. If it’s nil, it just prints out three dots. No point greeting nobody, right?

Cycling and Looping—a.k.a. Iteration

If the @names object responds to each, it is something that you can iterate over, so iterate over it and greet each person in turn. Finally, if @names is anything else, just let it get turned into a string automatically and do the default greeting.
Let’s look at that iterator in more depth:

@names.each do |name|
  puts "Hello #{name}!"
end

each is a method that accepts a block of code then runs that block of code for every element in a list, and the bit between do and end is just such a block. A block is like an anonymous function or lambda. The variable between pipe characters is the parameter for this block.
What happens here is that for every entry in a list, name is bound to that list element, and then the expression puts "Hello #{name}!" is run with that name.
Most other programming languages handle going over a list using the for loop, which in C looks something like:

for (i=0; i<number_of_elements; i++)
{
  do_something_with(element[i]);
}

This works, but isn’t very elegant. You need a throw-away variable like i, have to figure out how long the list is, and have to explain how to walk over the list. The Ruby way is much more elegant, all the housekeeping details are hidden within the each method, all you need to do is to tell it what to do with each element. Internally, the each method will essentially call yield "Albert", then yield "Brenda" and then yield "Charles", and so on.

Blocks, the Highly Sparkling Glint on the Edge of Ruby

The real power of blocks is when dealing with things that are more complicated than lists. Beyond handling simple housekeeping details within the method, you can also handle setup, teardown, and errors—all hidden away from the cares of the user.

# 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!"
  else
    puts "Goodbye #{@names}.  Come back soon!"
  end
end

The say_bye method doesn’t use each, instead it checks to see if @names responds to the join method, and if so, uses it. Otherwise, it just prints out the variable as a string. This method of not caring about the actual type of a variable, just relying on what methods it supports is known as “Duck Typing”, as in “if it walks like a duck and quacks like a duck…”. The benefit of this is that it doesn’t unnecessarily restrict the types of variables that are supported. If someone comes up with a new kind of list class, as long as it implements the join method with the same semantics as other lists, everything will work as planned.

Kicking Off the Script

So, that’s the MegaGreeter class, the rest of the file just calls methods on that class. There’s one final trick to notice, and that’s the line:

if __FILE__ == $0

__FILE__ is the magic variable that contains the name of the current file. $0 is the name of the file used to start the program. This check says “If this is the main file being used…” This allows a file to be used as a library, and not to execute code in that context, but if the file is being used as an executable, then execute that code.