What I’ve left out

TryRuby is a Ruby course for beginners. Its not possible to tell you everything about Ruby in 15 minutes. Here is an overview of some of the things that I have left out.

Else-if

There is an elsif statement so you can do this:

# elsif example
if a == 1
  do_thing1
elsif b == 2
  do_thing2
else
  do_thing3
end

Case

A variation on the if-elsif-else theme. Useful when you have to test a variable against a lot of different values:

# Case example 1
case a
when 1
  do_thing1
when 2
  do_thing2
when 3
  do_thing3
else
  do_thing4
end

And there is an alternate form:

# Case example 2
case
when a == 1
  do_thing1
when b == 2
  do_thing2
when a >= 3
  do_thing3
else
  do_thing4
end

Regular expressions

Sometimes you have a string and you want to test if that string contains some text. For instance if ‘user_123’ starts with ‘user_’. You could use a test like my_string[0..4] == ‘user_’, but there is a much more elegant and flexible way to do this kind of string testing, called regular expressions.

Reason this isn’t in TryRuby lessons is the fact that regular expressions have a syntax that is so strange at first, it might make your head explode. If you see a complex regular expression for the first time, it is like a monkey has been randomly bashing away on a keyboard.

The regex for the example mentioned above is relatively simple:

my_string.match( /^user_(\d+)/ )

This will return nil if my_string didn’t conform to the regex, or a matchdata object if it did. In the last case it will also give you the 123 part as the matched data.

The Ruby documentation has a pretty good explanation on regex’s. And googling on regex will give you millions of hits.

Method missing

When you try to use a method on your class that isn’t defined, Ruby will look for a method_missing method in your class. If present, the method_missing method will be called and handed the name of the method you wanted to call in the first place as a parameter.

Now you can handle the missing method as if it existed. For instance you might want to implement a sort_by_variable method. Where variable is the name of one of your class variables.

Inspection

Ruby can give you a lot of information about objects when a program is running. What an object is and what methods it responds to.

# Inspection
s = 'abc'
puts s.class
puts s.methods
puts s.respond_to?(:match)

Error handling

Reality is that all programs can run into errors. Errors that only raise their ugly heads when your program is running. And even when your code is flawless. That just happens.
You can let your program crash, but it is usually nicer if you let your program continue and politely tell that an error has occurred and it can’t continue what it was doing. Ruby has a couple of facilities for this purpose:

# Rescue
begin
  some_method_that_might_run_into_an_error
rescue => e
  puts "Error = #{e}"
end

And sometimes you might even want your program to raise an error (like when the user did something really stupid).

# Raise
raise "Yo dude seriously ? You can't enter zero for a divisor" if divisor == 0

IO

You will often need to read or write diskfiles. That is easy in Ruby. For instance reading can be done with IO.Read.

# Read a file
file_contents = IO.Read("my_file")

See the documentation for more info.

Spaceship operator

Ruby supports this strange thing: <=> called the spaceship operator. It returns -1, 0 or 1 depending on whether the left hand side is smaller, equal to or bigger than the right hand side. Useful for sorting.

  # Spaceship operator
  'a' <=> 'b'   => Returns -1

Class inheritance

When you are coding objects (classes) that model some real life situation, you will often encounter objects that are a subset of other objects. And there are many of such subsets. This is where class inheritance comes to the rescue.

A very simple (and incomplete) example:

# Class inheritance
class Animal
  attr_reader :number_of_legs
end

class Cow < Animal
  #@number_of_legs = 4
end

class Chicken < Animal
  #@number_of_legs = 2
end

The Cow and Chicken classes can use all attributes and methods that are defined for Animal.

A course or a book on Object Oriented programming can teach you all about this import concept.

Instance variables

In the lessons we mentioned object variables. Actually these should be called instance variables, since these variables belong to one instance of a type of object.

Modules and mixins

Sometimes you have two classes that have a lot of similar code, but they cannot inherit from a common parent. To avoid having the same code in two places (something you really should avoid) you can place the common code in a module and mix that module into your class.

Or you can mix-in an existing module like Enumerable:

  # Mix-in
  class Blurbalizer
    include Enumerable

    attr_reader :blurbs

    def initialize
      @blurbs = []
    end

    def each(&block)
      @blurbs.each{ |blurb| block.call(blurb) }
    end
  end

  Blurbalizer.new.methods

As long as you define method each, the Enumerable mixin now provides a lot of methods for free, like map and sort.

Meta programming

Unlike most other languages Ruby lets you do something called Meta programming. This means that a Ruby program can change itself while it is running. You can use this for example to read a table from a database, check the fieldnames of the table and create a method to access each field by name.

Garbage collection

In your code you are using variables all the time. Some variables are used from the start to finish, others only for a very short time. For instance variables created inside methods. But where do these variables go after you have discarded them ? Do they magically disappear ?
No, these variables, and the associated computer memory, are collected by the garbage collector.

At regular intervals the garbage collector throws away all these old, unused, unloved variables. And thus makes the associated computer memory free again, ready to be reused by your program.

Normally you never notice this cleanup. Except when you are running some time-critical program like a game. Every couple of seconds the game shows a bit of a hiccup.

Importing Ruby gems

Before you start coding to solve a programming problem it might be a good idea to see if somebody else already solved the same problem. The RubyGems website holds a vast collection of premade Ruby code. There is bound to be something there to give you a quick start to solve your coding problem. Gems are also very easy to install.

DRY

Not really a (Ruby) technique, but more a concept, DRY stands for Don’t Repeat Yourself. Meaning: do not write te same piece of code twice.
When you are coding you will often find you need to write a bit of code that is the same as, or very similar to, something you wrote before. Stop writing new code right then !!!
Instead find the existing similar code and extract it into a new method. Call the new method from the existing code and from the code you were about to write. Then test if everything works as expected.
Advantages of this way of coding are that you write less and easier to maintain code.

The price of concise readable code

Lastly, Ruby allows you to write very concise readable code. This comes with a price of course.
To make concise (= short) code the methods you use need to be able to do a lot of powerful stuff. This means that there have to be a lot of different methods, each one powerful in its own way. Learning to write proper Ruby code means that you will have to know a lot of different methods and this will take quite a bit of time.

Other programming languages might only have a short set of methods and are quicker to learn, but making a real program in those languages involves a lot more work since you would have to recreate the powerful methods that Ruby knows as standard.