Ruby tutorial code

I gave a little talk about Ruby to folks here at Sharpcast and thought I'd share the handout I prepared. It covers most of the basics of Ruby.

#!/bin/env ruby
# Ruby basics by Adam S. Rosien.  2-Aug-2006.

# Comments are prefixed by #

# Everything is an object.  No primitives.  nil is even an object.
#
# Methods are modeled as messages.  This is known as "duck" typing:
# "If it walks like a duck and quacks like a duck, it must be a duck."
# Interfaces are kind of moot; you just send a message and see if it responds.
# Method don't require parenthesis unless needed for an ambiguity.
#
# Some naming conventions:
#   ClassNames
#   method_names and variable_names
#   methods_asking_question? return true or false
#   slightly_dangerous_method! that might have side-effects
1.to_s             # '1'
'foo'.nil?         # false
nil.nil?           # true
' foo '.strip!     # modifies string in place, removing leading/trailing spaces

# What's an object's class?
'foo'.class              # String
'foo'.class.class        # Class
'foo'.class.class.class  # Class

# Traverse the class hierarchy.
String.superclass  # Object
Object.superclass  # nil

# Symbols: objects used to represent unique names in the intepreter.
asym = :sym

# Other reflection: You can query an object about messages and send them, i.e., make a method call.
'foo'.respond_to? :asdf    # false
'foo'.respond_to? :length  # true
'foo'.send :length         # 3

# Basic types
i = 500
f = 500.1
s = 'I want my MTV on channel 9.'
# Use double quotes and #{} to expand template.
s = "I want my MTV on channel #{i}." # "I want my MTV on channel 500"

# Arrays
array = [1, 2, 3]
array.size  # 3
array[1]    # 2

# Hashes, i.e., maps
hash = { 'foo' => 'bar', 'baz' => 999 }
hash['foo']  # 'bar'
hash['baz']  # 999
hash[1234]   # nil

# Classes
class Foo
        # Define a class method.
        def Foo.all_the_time
                "Foo all the time!"
        end

        # Called when object is created.
        def initialize
                # Instance variables are prefixed with '@'.
                @bar = 'bar'
        end

        # Define a method named bar that returns the value of the instance variable @bar.
        def bar
                # No 'return' needed.  Methods return the value of the last expression.
                @bar
        end
end

# Objects are created by calling 'new' on their class.
foo = Foo.new

# Instance method.
foo.bar  # 'bar'

# Class method.
Foo.all_the_time  # 'Foo all the time!"

# You can define a method on an individual object!  (Known as "singleton method")
def foo.baz
        'baz'
end
foo.baz  # 'baz'

# You can even add or change methods on any other class!  (This can be a good thing.)
class String
        # self is equivalent to 'this' in Java or C++.
        # to_s returns the string representation of an object.
        def wiggly
                "~~~#{self.to_s}~~~"
        end
end
"ruby".wiggly  # '~~~ruby~~~'

# If an object doesn't respond to a message, the 'method_missing' method is called.
# You can override this method in your classes for magical behavior.
class Foo
        # sym is the name of the method called, args is an array of arguments.
        def method_missing sym, *args
                puts "I can't #{sym}!"
        end
end

Foo.new.throw_ninja_stars  # "I can't throw_ninja_stars!"

# Inheritance
class Bar < Foo
end

# Blocks: the most useful feature of Ruby.
# A block is passed to a method which may 'yield' values to it.

# The each method of an array takes a block, and yields each element of the array to it.
# Using braces block syntax.
array.each {|elem| puts elem }  # 1
                                # 2
                                # 3

# Using do-end block syntax.
array.each do |elem|
        puts elem
end  # 1
     # 2
     # 3

# Defining a method that takes a block:
class Foo
        # Yield the value of @bar to the block three times.
        def bar_three_times
                yield @bar
                yield @bar
                yield @bar
        end
end
Foo.new.bar_three_times {|bar| puts bar}  # bar
                                          # bar
                                          # bar

# Useful methods that take blocks.
[1,2,3].map {|elem| elem + 1}            # [2,3,4]
[1,2,3].find_all {|elem| elem % 2 == 0}  # [2]
[1,2,3].reject {|elem| elem % 2 == 0}    # [1,3]
[3,1,6,3,6,4,8].sort.uniq                # [1,3,4,6,8]
['foosieseses', 'barsieses', 'bazzies'].sort_by {|elem| elem.length} # ["bazzies", "barsieses", "foosieseses"]

# Inject takes an initial value for an accumulator, passing the accumulator and element to the block.
# The accumulator is set to the value the block returns.
[1,2,3].inject(0) {|sum, elem| sum + elem}    # 6
[2,3,4].inject(1) {|prod, elem| prod * elem}  # 24

{'a' => 1, 'b' => 2}.each_pair {|k,v| puts "#{k} = #{v}"}  # a = 1
                                                           # b = 2
# Metaprogramming: powerful mojo.  Code that writes code.
# Why?  So you don't have to.

# It's a pain to have to write getters and setters for instance variables.
# Ruby defines helper class methods to do this for you.
class Foo
        attr_reader :bar    # Defines method 'bar' that returns @bar.
        attr_writer :bar    # Defines method 'bar=' that sets @bar to a value.
        attr_accessor :bar  # Reader and writer.
end
f = Foo.new
f.bar  # 'bar'
f.bar = 'foo'
f.bar  # 'foo'

# How might these kinds of methods be written?
class Bar
        # The *syms says that there are a variable number of method arguments,
        # so just turn them into an array.
        def Bar.attr_reader *syms
                syms.each do |sym|
                        # Evaluate this string as ruby in the context of the current class.
                        class_eval "def #{sym}; @#{sym}; end"
                end
        end

        def initialize
                @asdf = 'asdf'
                @ghij = 'ghij'
        end

        attr_reader :asdf, :ghij
end
b = Bar.new
b.asdf  # 'asdf'

# You can also evaluate a string as ruby in the context of an instance, rather than a class.
b.instance_eval "asdf"  # 'asdf'