YOUR FREE GUIDE TO PROGRAMMING RUBY

FROMWWW.SAPPHIRESTEEL.COM

IN ASSOCIATION WITH BITWISE MAGAZINE

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The Little Book Of Ruby

Copyright © 2006 Dark Neon Ltd.

All rights reserved.

written by

Huw Collingbourne

Download source code from:

http://www.sapphiresteel.com

You may freely copy and distribute this eBook as long as you do not modify the text or remove this copyright notice. You must not make any charge for this eBook.

The Little Book of Ruby is produced in association with Rosedown Mill Ltd., makers of the Steel IDE for Visual Studio (www.sapphiresteel.com) and Bitwise Magazine – the online magazine for developers

(www.bitwisemag.com).

First edition: June 2006

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Table of Contents

Welcome To The Little Book Of Ruby...........................................................................5

Learn Ruby In Ten Chapters…..............................................................................5

What Is Ruby? .......................................................................................................5

What Is Rails?. .......................................................................................................5

Download Ruby plus an Editor..............................................................................6

Get The Source Code Of The Sample Programs. ..................................................6

Running Ruby Programs........................................................................................6

How To Use This Book.......................................... ................................................7

Making Sense Of The Text....................................................................................7

Chapter One......................................................................................................... ............8

Strings and Embedded Evaluation. ......................................................................11

Methods...............................................................................................................12

Numbers...............................................................................................................14

Testing a Condition: if … then............................................................................. 15

Chapter Two. .................................................................................................................17

Instances and Instance Variables..........................................................................18

Constructors – new and initialize................................................. ........................21

Inspecting Objects.............................................................................. ..................22

Chapter Three. ...............................................................................................................25

Superclasses and Subclasses. ...............................................................................28

Chapter Four .................................................................................................................30

Accessor Methods................................................................................................30

Attribute Readers and Writers...................................................... ........................32

Attributes Create Variables. .................................................................................34

Calling Methods of a Superclass.........................................................................37

Class Variables. ....................................................................................................38

Chapter Five..................................................................................................................40

Using Arrays. .......................................................................................................40

Creating Arrays....................................................................................................41

Multi-Dimensional Arrays...................................................................................43

Iterating Over Arrays. ..........................................................................................45

Indexing Into Arrays............................................................................................46

Chapter Six. ...................................................................................................................48

Creating Hashes...................................................................................................48

Indexing Into A Hash...........................................................................................50

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Hash Operations...................................................................................................51

Chapter Seven...............................................................................................................53

For Loops. ............................................................................................................53

Blocks..................................................................................................................57

While Loops.........................................................................................................58

While Modifiers. ..................................................................................................59

Until Loops. .........................................................................................................62

Chapter Eight..................................................................................................... ............64

If..Then..Else........................................................................................................65

And..Or..Not............................................................................................. ............67

If..Elsif. ................................................................................................................68

Unless................................................................................................. ..................70

If and Unless Modifiers.......................................................................................70

Case Statements...................................................................................................72

Chapter Nine.................................................................................................................75

A Module Is Like A Class…................................................................................75

Module Methods..................................................................................................76

Modules as Namespaces......................................................................................77

Module ‘Instance Methods’.................................................................................79

Included Modules or ‘Mixins’.............................................................................79

Including Modules From Files.............................................................................81

Pre-Defined Modules. ..........................................................................................82

Chapter Ten...................................................................................................................83

Saving Data..........................................................................................................83

YAML..................................................................................................................84

Files......................................................................................................................85

Moving On.... .......................................................................................................85

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Welcome To The Little Book Of Ruby

Learn Ruby In Ten Chapters…

• Chapter One :

Strings and Methods

• Chapter Two:

Classes and Objects

• Chapter Three:

Class Hierarchies

• Chapter Four:

Accessors, Attributes, Class Variables

• Chapter Five:

Arrays

• Chapter Six:

Hashes

• Chapter Seven:

Loops and Iterators

• Chapter Eight:

Conditional Statements

• Chapter Nine:

Modules and Mixins

• Chapter Ten:

Saving Files, Moving On…

What Is Ruby?

Ruby is a cross-platform interpreted language which has many features in common with other ‘scripting’ languages such as Perl and Python. However, its version of object orientation is more thorough than those languages and, in many respects, it has more in common with the great-granddaddy of ‘pure’

OOP languages, Smalltalk. The Ruby language was created by Yukihiro Matsumoto (commonly known as ‘Matz’) and it was first released in 1995.

What Is Rails?

Currently much of the excitement surrounding Ruby can be attributed to a web development framework called Rails – popularly known as ‘Ruby On Rails’. While Rails is an impressive framework, it is not the be-all and end-all The Little Book Of Ruby :: Welcome To The Little Book Of Ruby :: www.sapphiresteel.com :: page 6

of Ruby. Indeed, if you decide to leap right into Rails development without first mastering Ruby, you may find that you end up with an application that you don’t even understand. While the Little Book of Ruby won’t cover the special features of Rails, it will give you the grounding you need to understand Rails code and write your own Rails applications.

Download Ruby plus an Editor

You can download the latest version of Ruby from www.ruby-lang.org. Be sure to download the binaries (not merely the source code). The simplest way to get Ruby installed on a PC is by using the Ruby Installer for Windows:

http://rubyinstaller.rubyforge.org/wiki/wiki.pl – this includes the SciTE code editor. Visual Studio 2005 users can download our free Visual Studio Ruby In Steel editor and debugger plugin from: www.sapphiresteel.com.

Get The Source Code Of The Sample Programs

All the programs in every chapter in this book are available for download as a Zip archive from www.sapphiresteel.com. When you unzip the programs you will find that they are grouped into a set of directories – one for each chapter.

If you are using Ruby In Steel, you will be able to load all the programs as a single solution, with the programs for each chapter arranged on the branches of a tree in the Project Manager.

Running Ruby Programs

It is often useful to keep a Command window open in the source directory containing your Ruby program files. Assuming that the Ruby interpreter is correctly pathed on your system, you will then be able to run programs by entering ruby <program name> like this: ruby 1helloworld.rb

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If you are using Ruby In Steel you can run the programs in the interactive console by pressing CTRL+F5 or run them in the debugger by pressing F5.

How To Use This Book

This book is a step-by-step tutorial to programming in Ruby and you can follow it chapter by chapter, reading the text and running the sample programs. On the other hand, if you prefer to ‘dip in’, you may want to try out some of the programs in whichever order takes your fancy; then refer back to the text for explanations. There are no monolithic applications in this book –just small, self-contained sample programs – so it’s easy to skip from chapter to chapter if you wish…

Making Sense Of The Text

In The Little Book Of Ruby, any Ruby source code is written like this: def saysomething

puts( "Hello" )

end

When there is a sample program to accompany the code, the program name is shown in a little box like this:

helloname.rb

Explanatory notes (which generally provide some hints or give a more in-depth explanation of some point mentioned in the text) are shown in a shaded box like this:

This is an explanatory note. You can skip it if you like – but if you do so, you may miss something of interest…!

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Chapter One

IN WHICH WE TIE UP SOME STRINGS, ADD UP SOME NUMBERS, MESS ABOUT WITH METHODS AND DIAGNOSE

SOME CONDITIONS…

From the fact that you are reading this little book, I think it is safe to deduce that you want to program Ruby – and, if you are anything like me, you will be impatient to get on with it. OK, let’s not hang around. I’ll assume that you already have Ruby installed. If not, you’ll need to do that first, as explained in the Introduction…

Now, let’s start coding. Fire up your editor and enter the following: puts 'hello world'

Now run the program (In Steel, press CTRL+F5; in SciTE, press F5). All being well, Ruby should display “hello world”.

If you are using an editor which lacks an interactive console, you may have to run programs from the command prompt. To do this, open a command window (enter CMD from the Start/Run menu in Windows) and navigate to the directory containing the source code then enter ruby followed by the program name, like this: ruby helloworld.rb

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This must just about the shortest ‘hello world’ program in the history of programming so we’ll immediately move on to a way of getting input from the user…

The obvious next step is to ‘get’ a string. As you might guess, the Ruby method for this is gets.

uppercase.rb

Objects and Methods

Ruby is a highly OOP (Object Oriented Programming) language.

Everything from an integer to a string is considered to be an object.

And each object has built in ‘methods’ which can be used to do various useful things. To use a method, you need to put a dot after the object, then append the method name. For example, Here I am using the upcase method to display the string, “hello world” in uppercase:

puts( "hello world".upcase )

Some methods such as puts and gets are available everywhere and don’t need to be associated with a specific object. Technically speaking, these methods are provided by Ruby’s Kernel module and they are included in all Ruby objects. When you run a Ruby application, an object called main is automatically created and this object provides access to the Kernel methods.

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helloname.rb

The helloname.rb program prompts the user for his or her name – let’s suppose it’s “Fred” - and then displays a greeting: “Hello Fred”. Here’s the code:

print( 'Enter your name: ' )

name = gets()

puts( "Hello #{name}" )

While this is still very simple, there are a few important details that need to be explained. First, notice that I’ve used print rather than puts to display the prompt. This is because puts adds a linefeed at the end whereas print does not; in the present case I want the cursor to remain on the same line as the prompt.

On the next line I use gets() to read in a string when the user presses Enter.

This string is assigned to the variable, name. I have not pre-declared this variable, nor have I specified its type. In Ruby you can create variables as and when you need them and Ruby ‘infers’ their types. In the present case I have assigned a string to name so Ruby knows that the type of the name variable must be a string.

Note: Ruby is case sensitive. A variable called myvar is different from one called myVar. A variable such as name in our sample project must begin with a lowercase character.

Incidentally, the brackets following gets() are optional as are the brackets enclosing the strings after print and puts; the code would run just the same if you removed the brackets. However, Ruby is increasingly moving towards the The Little Book Of Ruby :: Chapter One :: www.sapphiresteel.com :: page 11

use of brackets – particularly when passing arguments to methods. Brackets help to avoid potential ambiguity in code and, in some cases, the Ruby interpreter will warn you if you omit them. While some Ruby programmers like to omit brackets whenever possible, I am not one of them; you will, therefore, find brackets used liberally in my programs.

Strings and Embedded Evaluation

The last line in the helloname.rb program is rather interesting: puts( "Hello #{name}" )

Here the name variable is embedded into the string itself. This is done by placing the variable between two curly braces preceded by a hash (‘pound’) character #{ }. This kind of ‘embedded’ evaluation only works with strings delimited by double quotes.

It isn’t only variables which can be embedded in double-quoted strings. You can also embed non-printing characters such as newlines “\n” and tabs “\t”.

You can even embed bits of program code and mathematical expressions. Let’s assume that you have a method called showname, which returns the string

‘Fred’. The following string would, in the process of evaluation, call the showname method and, as a result, it would display the string “Hello Fred”: string_eval.rb

puts "Hel o #{showname}"

See if you can figure out what would be displayed by the following: puts( "\n\t#{(1 + 2) * 3}" )

Run the string_eval.rb program to see if you were right.

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Comments…

Lines beginning with a # character are treated as comments (they are ignored by the Ruby interpreter):

# This is a comment

Methods

In the previous example, I blithely introduced a Ruby method without explaining precisely what it is and the syntax needed to create it. I’ll correct that omission now.

object.rb

A method is so called because it provide a method (that is, ‘a way’) for an object to respond to messages. In OOP terminology, you send a message to an object by asking it to do something. So let’s imagine that you have an object called ob which has a method called saysomething; this is how you would send it a saysomething message:

ob.saysomething

Let’s suppose that the saysomething method looks like this:

def saysomething

puts( "Hello" )

end

The result is, that when you send ob a saysomething message it responds with the saysomething method and displays “Hello”.

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OK, so that’s the ‘pure OOP’ way of describing this stuff. A not-so-pure OOP

way of describing this would be to say that saysomething is like a function which is bound to the object and can be called using dot notation: ob.saysomething.

method.rb

In Ruby a method is declared with the keyword def followed by a method name which should begin with a lowercase letter, like this:

def showstring

puts( "Hello" )

end

You may optionally put one or more arguments, separated by commas, after the method name:

def showname( aName )

puts( "Hello #{aName}" )

end

def return_name( aFirstName, aSecondName )

return "Hello #{aFirstName} #{aSecondName}"

end

The brackets around the arguments are optional. The following syntax is also permissible:

def return_name2 aFirstName, aSecondName

return "Hello #{aFirstName} #{aSecondName}"

end

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As explained previously, for the sake of clarity, I am very much prejudiced in favour of brackets but you can omit them if you wish.

mainob.rb

If methods belong to objects, which object owns any ‘free-standing’

methods that you write in your code? As I mentioned earlier, Ruby automatically creates an object named main when you run a program and it is to this object that any free-standing methods belong.

Numbers

Numbers are just as easy to use as strings. For example, let’s suppose you want to calculate the selling price or ‘grand total’ of some item based on its ex-tax value or ‘subtotal’.

To do this you would need to multiply the subtotal by the applicable tax rate and add the result to the value of the subtotal. Assuming the subtotal to be $100 and the tax rate to be 17.5%, this Ruby code would do the calculation and display the result:

subtotal = 100.00

taxrate = 0.175

tax = subtotal * taxrate

puts "Tax on $#{subtotal} is $#{tax}, so grand total is $#{subtotal+tax}"

Obviously, it would be more useful if it could perform calculations on a variety of subtotals rather than calculating the same value time after time!.

Here is a simple version of a Tax Calculator that prompts the user to enter a subtotal:

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taxrate = 0.175

print "Enter price (ex tax): "

s = gets

subtotal = s.to_f

tax = subtotal * taxrate

puts "Tax on $#{subtotal} is $#{tax}, so grand total is $#{subtotal+tax}"

Here s.to_f is a method of the String class. It attempts to convert the string to a floating point number. For example, the string “145.45” would be converted to the floating point number, 145.45. If the string cannot be converted, 0.0 is returned. So, for instance, “Hello world”.to_f would return 0.0.

Testing a Condition: if … then

The problem with the simple tax calculator code shown above is that it accepts minus subtotals and calculates minus tax on them – a situation upon which the Government is unlikely to look favourably! I therefore need to check for minus figures and, when found, set them to zero. This is my new version of the code:

tax_calculator.rb

taxrate = 0.175

print "Enter price (ex tax): "

s = gets

subtotal = s.to_f

if (subtotal < 0.0) then

subtotal = 0.0

end

tax = subtotal * taxrate

puts "Tax on $#{subtotal} is $#{tax}, so grand total is $#{subtotal+tax}"

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The Ruby if test is similar to an if test in other programming languages. Note, however, that the brackets are once again optional, as is the keyword then.

However, if you were to write the following, with no line break after the test condition, the then would be obligatory:

if (subtotal < 0.0) then subtotal = 0.0 end

Note that the end keyword that terminates the if block is not optional. Forget to add it and your code will not run.

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Chapter Two

DEFINING CLASSES, CREATING OBJECTS AND PEEKING INSIDE THEM…

So far we’ve used a number of ‘standard’ Ruby objects such as numbers and strings. Let’s now see how to create new types of objects of our very own. As in most other OOP languages, a Ruby object is defined by a class. The class is like a blueprint from which individual objects are constructed. This is a very simple class:

class MyClass

end

And this is how I would create a usable object from it:

ob = MyClass.new

Not that I can do a great deal with my ob object – for the simple reason that I haven’t programmed anything in the MyClass class, from which it is created.

object_class.rb

Actually, if you create an ‘empty’ class like MyClass, the objects created from it will not be totally useless. All Ruby classes automatically inherit the features of the Object class. So my ob object can make use of Object methods such as class (which tells an object display its class):

puts ob.class

#=> displays: “MyClass”

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To make MyClass a bit more useful, I need to give it a method or two. In this example (which was mentioned briefly in the last chapter), I’ve added a method called saysomething:

class MyClass

def saysomething

puts( "Hello" )

end

end

Now, when I create a MyClass object, I can call this method in order to get that object to say “Hello”:

ob = MyClass.new

ob.saysomething

Instances and Instance Variables

Let’s create some more useful objects. No home (or computer program) should be without a dog. So let’s make ourselves a Dog class:

class Dog

def set_name( aName )

@myname = aName

end

end

Note that the class definition begins with the keyword class (all lower case) and is followed by the name of the class itself, which must begin with an uppercase letter. My Dog class contains a single method, set_name. This takes an incoming argument, aName. The body of the method assigns the value of aName to a variable called @myname.

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Variables beginning with the @ character are ‘instance variables’ –that means that they belong to individuals objects – or ‘instances’ of the class. It is not necessary to pre-declare variables.

I can create instances of the Dog class (that is, ‘dog objects’) by calling the new method. Here I am creating two dog objects (remember that class names begin uppercase letters; object names begin with lowercase letters): mydog = Dog.new

yourdog = Dog.new

At the moment, these two dogs have no names. So the next thing I do is call the set_name method to give them names:

mydog.set_name( 'Fido' )

yourdog.set_name( 'Bonzo' )

Having given names to the dogs, I need to have some way to find out their names later on. Each dog needs to know its own name, so let’s give it a get_name method:

def get_name

return @myname

end

The return keyword here is optional. Ruby methods will always return the last expression evaluated. For the sake of clarity (and to avoid unexpected results from methods of more complexity than this one!) we shall make a habit of explicitly returning any values which we plan to use. Finally, let’s give the dog some behaviour by asking it to talk. Here is the finished class definition: The Little Book Of Ruby :: Chapter Two :: www.sapphiresteel.com :: page 20

dogs_and_cats.rb

class Dog

def set_name( aName )

@myname = aName

end

def get_name

return @myname

end

def talk

return 'woof!'

end

end

Now, we can create a dog, name it, display its name and ask it to talk like this: mydog = Dog.new

mydog.set_name( 'Fido' )

puts(mydog.get_name)

puts(mydog.talk)

For the sake of variety – and to show that I am not biased against our feline friends - I have also added a Cat class in my program, dogs_and_cats.rb. The Cat class is similar to the Dog class apart from the fact that its talk method, naturally enough, returns a miaow instead of a woof.

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This program contains an error. The object named someotherdog never has a value assigned to its @name variable. Fortunately, Ruby doesn’t blow up when we try to display this dog’s name. Instead it just prints ‘nil’. We’ll shortly look at a simple way of making sure that errors like this don’t happen again…

Constructors – new and initialize

treasure.rb

For now, let’s take a look at another example of a user-defined class. Load up treasure.rb. This is an adventure game in the making. It contains two classes, Thing and Treasure. The Thing class is very similar to the Dog class from the last program – well, apart from the fact that it doesn’t woof, that is.

The Treasure class has a few interesting extras, however. First of all, it hasn’t got get_name and set_name methods. Instead, it contains a method named initialize which takes two arguments whose values are assigned to the @name and @description variables:

def initialize( aName, aDescription )

@name

= aName

@description = aDescription

end

When a class contains a method named initialize this is automatically called when an object is created using the new method. It is a good idea to use an initialize method to set the values of an object’s instance variables. This has two clear benefits over setting each instance variable using methods such set_name. First of all, a complex class may contain numerous instance The Little Book Of Ruby :: Chapter Two :: www.sapphiresteel.com :: page 22

variables and you can set the values of all of them with the single initialize method rather than with many separate ‘set’ methods; secondly, if the variables are all automatically initialised at the time of object creation, you will never end up with an ‘empty’ variable (like the nil value returned when we tried to display the name of someotherdog in the previous program).

Note: The new method creates an object so it can be thought of as the object’s ‘constructor’. However, you should not normally implement your own version of the new method (this is possible but it is generally not advisable). Instead, when you want to perform any

‘setup’ actions – such as assigning values to an object’s internal variables - you should do so in a method named initialize. Ruby executes the initialize method immediately after a new object is created.

Finally, I have created a method called to_s which is intended to return a string representation of a Treasure object. The method name, to_s, is not arbitrary. The same method name is used throughout the standard Ruby class hierarchy. In fact, the to_s method is defined for the Object class itself which is the ultimate ancestor of all other classes in Ruby. By redefining the to_s method, I have added new behaviour which is more appropriate to the Treasure class than the default method. In other words, I have ‘overridden’ its to_s method.

Inspecting Objects

Incidentally, notice too that I have ‘looked inside’ the Treasure object, t1, using the inspect method:

t1.inspect

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The inspect method is defined for all Ruby objects. It returns a string containing a human-readable representation of the object. In the present case, it displays something like this:

#<Treasure:0x28962f8 @description="an Elvish weapon forged of gold",

@name="Sword">

This begins with the class name, Treasure; this is followed by a number, which may be different from the one shown above – this is Ruby’s internal identification code for this particular object; then there are the names and values of the object’s variables.

p.rb

Ruby also provides the p method as a shortcut to inspecting and displaying objects:

p( anobject )

to_s.rb

To see how to_s can be used with a variety of objects and to test how a Treasure object would be converted to a string in the absence of an overridden to_s method, try out the to_s.rb program.

As you will see, classes such as Class, Object, String and Treasure, simply return their names when the to_s method is called. An object, such as the Treasure object, t, returns its identifier – which is the same identifier returned by the inspect method.

Looking over my treasure.rb program I can’t help thinking that its code is a bit repetitive. After all, why have a Thing class which contains a name and a Treasure class which also contains a name (the @name instance variable), each The Little Book Of Ruby :: Chapter Two :: www.sapphiresteel.com :: page 24

of which are coded independently? It would make more sense to regard a Treasure as a ‘type of Thing’. If I were to develop this program into a complete adventure game, other objects such as Rooms and Weapons might be yet other

‘types of Thing’. It is clearly time to start working on a proper class hierarchy.

That’s what we shall do in the next lesson…

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Chapter Three

CLASS HIERARCHIES…

We ended the last lesson by creating two new classes: a Thing and a Treasure .

In spite of the fact that these two classes shared some features (notably both had a ‘name’), there was no connection between them. Now, these two classes are so trivial that this tiny bit of repetition doesn’t really matter much.

However, when you start writing real programs of some complexity, your classes will frequently contain numerous variables and methods; and you really don’t want to keep recoding the same old stuff over and over again.

It makes sense to create a class hierarchy in which a class which is a ‘special type’ of some other class simply ‘inherits’ the features of that other class. In our simple adventure game, for instance, a Treasure is a special type of Thing so the Treasure class should inherit the features of the Thing class.

Class Hierarchies – Ancestors and Descendants: In this book, I often talk about ‘descendant’ classes ‘inheriting’ features from their

‘ancestor’ classes. These terms deliberately suggest a kind a family relationship between ‘related’ classes. In Ruby, each class only has one parent. A class may, however, descend from a long and distinguished family tree with many generations of grandparents, great-grandparents and so on…

The behaviour of Things in general will be coded in the Thing class itself. The Treasure class will automatically ‘inherit’ all the features of the Thing class, so we won’t need to code them all over again. It will then add some additional features, specific to Treasures.

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As a general rule, when creating a class hierarchy, the classes with the most generalised behaviour are higher up the hierarchy than classes with more specialist behaviour. So a Thing class with just a name and a description, would be the ancestor of a Treasure class which has a name, a description and, additionally, a value; the Thing class might also be the ancestor of some other specialist class such as a Room which has a name, a description and also exits

– and so on…

One Parent, Many Children

This diagram shows a Thing class which has a name and a description (in a Ruby program, these might be internal variables such as @name and @description plus some methods to access them). The Treasure and Room classes both descend from the Thing class so they automatically ‘inherit’ a name and a description. The Treasure class adds one new item: value – so it now has name, description and value; The Room class adds exits – so it has name, description and exits.

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adventure1.rb

Let’s see how to create a descendant class in Ruby. Load up the adventure1.rb program. This starts simply enough with the definition of a Thing class which has two instance variables, @name and @description. These variables are assigned values in the initialize method when a new Thing object is created.

Instance variables generally cannot (and should not) be directly accessed from the world outside the class itself due the principle of encapsulation.

“Encapsulation” is a term that refers to the ‘modularity’ of an object.

Put simply, it means that only the object itself can mess around with its own internal state. The outside world cannot. The benefit of this is that the programmer is able to change the implementation of methods without having to worry that some external code elsewhere in the program relies upon some specific detail of the previous implementation.

In order to obtain the value of each variable in a Thing object we need a get accessor method such as get_name; in order to assign a new value we need a set accessor method such as set_name:

def get_name

return @name

end

def set_name( aName )

@name = aName

end

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Superclasses and Subclasses

Now look at the Treasure class. Notice how this is declared: class Treasure < Thing

The angle bracket, < , indicates that Treasure is a ‘subclass’, or descendant, of Thing and therefore it inherits the data (variables) and behaviour (methods) from the Thing class. Since the get_name, set_name, get_description and set_description methods already exist in the ancestor class (Thing) these don’t need to be re-coded in the descendant class (Treasure).

The Treasure class has one additional piece of data, its value (@value) and I have written get and set accessors for this. When a new Treasure object is created, its initialize method is automatically called. A Treasure object has three variables to initialize (@name, @description and @value), so its initialize method takes three arguments:

def initialize( aName, aDescription, aValue )

The first two arguments are passed, using the super keyword, to the initialize method of the superclass (Thing) so that the Thing class’s initialize method can deal with them:

super( aName, aDescription )

When used inside a method, the super keyword calls a method with the same name in the ancestor or ‘super’ class.

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The current method in the Treasure class is called initialize so when code inside this method passes the two arguments ( aName, aDescription ) to super it is actually passing them to the initialize method of its superclass, Thing.

If the super keyword is used on its own, without any arguments being specified, all the arguments sent to the current method are passed to the ancestor method.

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Chapter Four

ACCESSORS, ATTRIBUTES AND CLASS VARIABLES…

Now, getting back to the little adventure game work I was programming earlier on… I still don’t like the fact that the classes are full of repetitive code due to all those get and set accessors. Let me see what I can do to remedy that.

Accessor Methods

Instead of accessing the value of the @description instance variable with two different methods, get_description and set_description, like this…

puts( t1.get_description )

t1.set_description( “Some description” )

…it would be so much nicer to retrieve and assign values just as you would retrieve and assign values to and from a simple variable, like this: puts( t1.description )

t1.description = “Some description”

In order to be able to do this, I need to modify the Treasure class definition.

One way of doing this would be by rewriting the accessor methods for

@description as follows:

def description

return @description

end

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def description=( aDescription )

@description = aDescription

end

accessors.rb

I have added accessors similar to the above in the accessors.rb program. There are two differences from my previous version. First, both of the accessors are called description rather than get_description and set_description; secondly the set accessor appends an equals sign ( = ) to the method name. It is now possible to assign a new string like this:

t.description = "a bit faded and worn around the edges"

And you can retrieve the value like this:

puts( t.description )

Note: When you write a set accessor in this way, you must append the = character directly to the method name, not merely place it somewhere between the method name and the arguments. So this is correct:

def name=( aName )

But this is an error:

def name =( aName )

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Attribute Readers and Writers

In fact, there is a simpler and shorter way of achieving the same result. All you have to do is use two special methods, attr_reader and attr_writer, followed by a symbol like this:

attr_reader :description

attr_writer :description

You should add this code inside your class definition but outside of any methods, like this:

class Thing

attr_reader :description

attr_writer :description

# some methods here…

end

Symbols: In Ruby, a symbol is a name preceded by a colon. Symbol is defined in the Ruby class library to represent names inside the Ruby interpreter. Symbols have a number of special uses. For example, when you pass one or more symbols as arguments to attr_reader (while it may not be obvious, attr_reader is, in fact, a method of the Module class), Ruby creates an instance variable and a get accessor method to return the value of that variable; both the instance variable and the accessor method will have the same name as the specified symbol.

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Calling attr_reader with a symbol has the effect of creating an instance variable with a name matching the symbol and a get accessor for that variable.

Calling attr_writer similarly creates an instance variable with a set accessor.

Here, the variable would be called @description. Instance variables are considered to the ‘attributes’ of an object, which is why the attr_reader and attr_writer methods are so named.

accessors2.rb

The accessors2.rb program contains some working examples of attribute readers and writers in action. Notice that Thing class defines a short-form set accessor (using attr_writer plus a symbol) for the @name variable: attr_writer :name

But it has a long-form get accessor – an entire hand-coded method – for the same variable:

def name

return @name.capitalize

end

The advantage of writing a complete method like this is that it gives you the opportunity to do some extra processing rather than simply reading and writing an attribute value. Here the get accessor uses the String class’s capitalize method to return the string value of @name with its initials letters in uppercase.

The @description attribute needs no special processing at all so I have used both attr_reader and attr_writer to get and set the value of the

@description variable.

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Attributes or Properties? Don’t be confused by the terminology. In Ruby, an ‘attribute’ is the equivalent of what many other programming languages call a ‘property’.

When you want to read and to write a variable, the attr_accessor method provides a shorter alternative to using both attr_reader and attr_writer. I have made use of this to access the value attribute in the Treasure class: attr_accessor :value

This is equivalent to:

attr_reader :value

attr_writer :value

Attributes Create Variables

Earlier I said that calling attr_reader with a symbol actually creates a variable with the same name as the symbol. The attr_accessor method also does this.

In the code for the Thing class, this behaviour is not obvious since the class has an initialize method which explicitly creates the variables. The Treasure class, however, makes no reference to the @value variable in its initialize method: The Little Book Of Ruby :: Chapter Four :: www.sapphiresteel.com :: page 35

class Treasure < Thing

attr_accessor :value

def initialize( aName, aDescription )

super( aName, aDescription )

end

end

The only indication that an @value variable exists at all is this accessor definition which declares a value attribute:

attr_accessor :value

My code down at the bottom of the source file sets the value of each Treasure object:

t1.value = 800

Even though it has never been formally declared, the @value variable really does exist, and we are able to retrieve its numerical value using the get accessor:

t1.value

To be absolutely certain that the attribute accessor really has created @value, you can always look inside the object using the inspect method. I have done so in the final two code lines in this program:

puts "This is treasure1: #{t1.inspect}"

puts "This is treasure2: #{t2.inspect}"

The Little Book Of Ruby :: Chapter Four :: www.sapphiresteel.com :: page 36

accessors3.rb

Attribute accessors can initialize more than one attribute at a time if you send them a list of symbols in the form of arguments separated by commas, like this:

attr_reader :name, :description

attr_writer(:name, :description)

attr_accessor(:value, :id, :owner)

As always, in Ruby, brackets around the arguments are optional.

adventure2.rb

Now let’s see how to put attribute readers and writers to use in my adventure game. Load up the adventure2.rb program. You will see that I have created two readable attributes in the Thing class: name and description. I have also made description writeable; however, as I don’t plan to change the names of any Thing objects, the name attribute is not writeable:

attr_reader( :name, :description )

attr_writer( :description )

I have created a method called to_s which returns a string describing the Treasure object. Recall that all Ruby classes have a to_s method as standard.

The to_s method in the Thing class overrides (and so replaces) the default one. You can override existing methods when you want to implement new behaviour appropriate to the specific class type.

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Cal ing Methods of a Superclass

I have decided that my game will have two classes descending from Thing.

The Treasure class adds a value attribute which can be both read and written.

Note that its initialize method calls its superclass in order to initialize the name and description attributes before initializing the new @value variable: super( aName, aDescription )

@value = aValue

Here, if I had omitted the call to the superclass, the name and description attributes would never be initialized. This is because Treasure.initialize overrides Thing.initialize; so when a Treasure object is created, the code in Thing.initialize will not automatically be executed.

In some Ruby books, a hash or pound sign may be shown between the class name and a method name like this: Treasure#initialize.

This is purely a convention of documentation (one which I prefer to ignore) and is not real Ruby syntax. I guess it’s just a case of “You say tomayto and I say tomahto; you say Treasure#initialize and I say Treasure.initialize”. Heck, let’s not fight about this - it’s only punctuation…!

On the other hand, the Room class, which also descends from Thing, currently has no initialize method; so when a new Room object is created Ruby goes scrambling back up the class hierarchy in search of one. The first initialize method it finds is in Thing; so a Room object’s name and description attributes are initialised there.

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Class Variables

There are a few other interesting things going on in this program. Right at the top of the Thing class you will see this:

@@num_things = 0

The two @ characters at the start of this variable name, @@num_things, define this to be a ‘class variable’. The variables we’ve used inside classes up to now have been instance variables, preceded by a single @, like @name.

Whereas each new object (or ‘instance’) of a class assigns its own values to its own instance variables, all objects derived from a specific class share the same class variables. I have assigned 0 to the @@num_things variable to ensure that it has a meaningful value at the outset.

Here, the @@num_things class variable is used to keep a running total of the number of Thing objects in the game. It does this simply by incrementing the class variable (it uses += to add 1 to it) in the initialize method every time a new object is created:

@@num_things +=1

If you look lower down in my code, you will see that I have created a Map class to contain an array of rooms. This includes a version of the to_s method which prints information on each room in the array. Don’t worry about the implementation of the Map class; we’ll be looking at arrays and their methods shortly.

The Little Book Of Ruby :: Chapter Four :: www.sapphiresteel.com :: page 39

This diagram shows a Thing class (the rectangle) which contains a class variable, @@num_things and an instance variable, @name. The three oval shapes represent ‘Thing objects’ – that is, ‘instances’ of the Thing class. When one of these objects assigns a value to its instance variable, @name, that value only affects the @name variable in the object itself – so here, each object has a different value for @name.

But when an object assigns a value to the class variable,

@@num_things, that value ‘lives inside’ the Thing class and is

‘shared’ by all instances of that class. Here @@num_things equals 3

and that is true for all the Thing objects.

Find the code down at the bottom of the file and run the program in order to see how we have created and initialised all the objects and used the class variable, @@num_things, to keep a tally of all the Thing objects that have been created.

The Little Book Of Ruby :: Chapter Five :: www.sapphiresteel.com :: page 40

Chapter Five

ARRAYS…

Up to now, we’ve generally been using objects one at a time. In this chapter we’ll find out how to create a list of objects. We’ll start by looking at the most common type of list structure – an array.

Using Arrays

array0.rb

What is an Array?

An Array is a sequential collection of items in which each item can be indexed.

In Ruby, (unlike many other languages) a single Array can hold items of mixed data types such as strings, integers and floats or even a method-call which returns some value:

a1 = [1,'two', 3.0, array_length( a0 ) ]

The first item in an array has the index 0, which means that the final item has an index equal to the total number of items in the array minus 1. Given the array, a1, shown above, this is how to obtain the values of the first and last items:

a1[0]

# returns 1st item (at index 0)

a1[3]

# returns 4th item (at index 3)

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We’ve already used arrays a few times – for example, in adventure2.rb in chapter 4 we used an array to store a map of Rooms:

mymap = Map.new([room1,room2,room3])

Creating Arrays

In common with many other programming languages, Ruby uses square brackets to delimit an array. You can easily create an array, fill it with some comma-delimited values and assign it to a variable:

arr = ['one','two','three','four']

array1.rb

As with most other things in Ruby, arrays are objects. They are defined, as you might guess, by the Array class and, just like strings, they are indexed from 0.

You can reference an item in an array by placing its index between square brackets. If the index is invalid, nil is returned:

arr = ['a', 'b', 'c']

puts(arr[0]) # shows ‘a’

puts(arr[1]) # shows ‘b’

puts(arr[2]) # shows ‘c’

puts(arr[3]) # nil

The Little Book Of Ruby :: Chapter Five :: www.sapphiresteel.com :: page 42

array2.rb

It is permissible to mix data types in an array and even to include expressions which yield some value. Let’s assume that you have already created this method:

def hello

return "hello world"

end

You can now declare this array:

x = [1+2, hello, `dir`]

Here, the first element is the integer, 3 and the second is the string “hello world” (returned by the method hello). If you run this on Windows, the third array element will be a string containing a directory listing. This is due to the fact that `dirìs a back-quoted string which is executed by the operating system. The final ‘slot’ in the array is, therefore, filled with the value returned by the dir command which happens to be a string of file names. If you are running on a different operating system, you may need to substitute an appropriate command at this point.

dir_array.rb

Creating an Array of File Names: A number of Ruby classes have methods which return arrays of values. For example, the Dir class, which is used to perform operations on disk directories, has the entries method. Pass a directory name to the method and it returns a list of files in an array:

Dir.entries( 'C:\\' )

# returns an array of files in C:\

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If you want to create an array of strings but can’t be bothered typing all the quotation marks, a shortcut is to put unquoted text separated by spaces between round brackets preceded by %w like this:

y = %w( this is an array of strings )

You can also create arrays using the usual object construction method, new.

Optionally, you can pass an integer to new to create an empty array of a specific size (with each element set to nil), or you can pass two arguments – the first to set the size of the array and the second to specify the element to place at each index of the array, like this:

a = Array.new

# an empty array

a = Array.new(2)

# [nil,nil]

a = Array.new(2,"hel o world") # ["hello world","hello world"]

Multi-Dimensional Arrays

To create a multi-dimensional array, you can create one array and then add other arrays to each of its ‘slots’. For example, this creates an array containing two elements, each of which is itself an array of two elements: a = Array.new(2)

a[0]= Array.new(2,'hello')

a[1]= Array.new(2,'world')

Or you could nest arrays inside one another using square brackets. This creates an array of four arrays, each of which contains four integers: The Little Book Of Ruby :: Chapter Five :: www.sapphiresteel.com :: page 44

a = [ [1,2,3,4],

[5,6,7,8],

[9,10,11,12],

[13,14,15,16] ]

In the code shown above, I have placed the four ‘sub-arrays’ on separate lines.

This is not obligatory but it does help to clarify the structure of the multi-dimensional array by displaying each sub-array as though it were a row, similar to the rows in a spreadsheet. When talking about arrays within arrays, it is convenient to refer to each nested array as a ‘row’ of the ‘outer’ array.

array_new.rb

You can also create an Array object by passing an array as an argument to the new method. Be careful, though. It is a quirk of Ruby that, while it is legitimate to pass an array argument either with or without enclosing round brackets, Ruby considers it a syntax error if you fail to leave a space between the new method and the opening square bracket – another good reason for making a firm habit of using brackets when passing arguments!

multi_array.rb

For some examples of using multi-dimensional arrays, load up the multi_array.rb program. This starts by creating an array, multiarr, containing two other arrays. The first of these arrays is at index 0 of multiarr and the second is at index 1:

multiarr = [['one','two','three','four'],[1,2,3,4]]

The Little Book Of Ruby :: Chapter Five :: www.sapphiresteel.com :: page 45

Iterating Over Arrays

You can access the elements of an array by iterating over them using a for loop. The loop will iterate over two elements here: namely, the two sub-arrays at index 0 and 1:

for i in multiarr

puts(i.inspect)

end

This displays:

["one", "two", "three", "four"]

[1, 2, 3, 4]

Iterators and for loops: The code inside a for loop is executed for each element in an expression. The syntax is summarized like this: for <one or more variables> in <expression> do

<code to run>

end

When more than one variable is supplied, these are passed to the code inside the for. end block just as you would pass arguments to a method. Here, for example, you can think of (a,b,c,d) as four arguments which are initialised, at each turn through the for loop, by the four values from a row of multiarr:

for (a,b,c,d) in multiarr

print("a=#{a}, b=#{b}, c=#{c}, d=#{d}\n" )

end

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Indexing Into Arrays

array_index.rb

You can index from the end of an array using minus figures, where -1 is the index of the last element; and you can also use ranges (values between a start index and an end index separated by two dots):

arr = ['h','e','l','l','o',' ','w','o','r','l','d']

print( arr[0,5] )

#=> ‘hello’

print( arr[-5,5 ] )

#=> ‘world’

print( arr[0. 4] )

#=> ‘hello’

print( arr[-5. -1] )

#=> ‘world’

Notice that, as with strings, when provided with two integers in order to return a number of contiguous items from an array, the first integer is the start index while the second is a count of the number of items ( not an index): arr[0,5]

# returns 5 chars - ["h", "e", "l", "l", "o"]

array_assign.rb

You can also make assignments by indexing into an array. Here, for example, I first create an empty array then put items into indexes 0, 1 and 3. The ‘empty’

slot at number 2 will be filled with a nil value:

arr = []

arr[0] = [0]

arr[1] = ["one"]

arr[3] = ["a", "b", "c"]

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# arr now contains:

# [[0], ["one"], nil, ["a", "b", "c"]]

Once again, you can use start-end indexes, ranges and negative index values: arr2 = ['h','e','l','l','o',' ','w','o','r','l','d']

arr2[0] = 'H'

arr2[2,3] = 'L', 'L'

arr2[4. 5] = 'O','-','W'

arr2[-4,4] = 'a','l','d','o'

# arr2 now contains:

# ["H", "e", "L", "L", "O", "-", "W", "a", "l", "d", "o"]

The Little Book Of Ruby :: Chapter Six :: www.sapphiresteel.com :: page 48

Chapter Six

HASHES…

While arrays provide a good way of indexing a collection of items by number, there may be times when it would be more convenient to index them in some other way. If, for example, you were creating a collection of recipes, it would be more meaningful to have each recipe indexed by name such as “Rich Chocolate Cake” and “Coq au Vin” rather than by numbers: 23, 87 and so on.

Ruby has a class that lets you do just that. It’s called a Hash. This is the equivalent of what some other languages call a ‘Dictionary’. Just like a real dictionary, the entries are indexed by some unique key (in a dictionary, this would be a word) and a value (in a dictionary, this would be the definition of the word).

Creating Hashes

hash1.rb

You can create a hash by creating a new instance of the Hash class: h1 = Hash.new

h2 = Hash.new("Some kind of ring")

Both the examples above create an empty Hash. A Hash object always has a default value – that is, a value that is returned when no specific value is found at a given index. In these examples, h2 is initialized with the default value,

“Some kind of ring”; h1 is not initialized with a value so its default value will be nil.

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Having created a Hash object, you can add items to it using an array-like syntax – that is, by placing the index in square brackets and using = to assign a value.

The obvious difference here being that, with an array, the index (the ‘key’) must be an integer; with a Hash, it can be any unique data item: h2['treasure1'] = 'Silver ring'

h2['treasure2'] = 'Gold ring'

h2['treasure3'] = 'Ruby ring'

h2['treasure4'] = 'Sapphire ring'

Often, the key may be a number or, as in the code above, a string. In principle, however, a key can be any type of object. Given some class, X, the following assignment is perfectly legal:

x1 = X.new('my Xobject')

h2[x1] = 'Diamond ring'

There is a shorthand way of creating Hashes and initializing them with key-value pairs. Just add a key followed by => and its associated value; each key-value pair should be separated by a comma and the whole lot placed inside a pair of curly brackets:

h1 = { 'room1'=>'The Treasure Room',

'room2'=>'The Throne Room',

'loc1'=>'A Forest Glade',

'loc2'=>'A Mountain Stream' }

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Unique Keys? Take care when assigning keys to Hashes. If you use the same key twice in a Hash, you will end up over-writing the original value. This is just like assigning a value twice to the same index in an array. Consider this example:

h2['treasure1'] = 'Silver ring'

h2['treasure2'] = 'Gold ring'

h2['treasure3'] = 'Ruby ring'

h2['treasure1'] = 'Sapphire ring'

Here the key ‘treasure1’ has been used twice. As a consequence, the original value, ‘Silver ring’ has been replaced by ‘Sapphire ring’, resulting in this Hash:

{"treasure1"=>"Sapphire ring", "treasure2"=>"Gold ring",

"treasure3"=>"Ruby ring"}

Indexing Into A Hash

To access a value, place its key between square brackets:

puts(h1['room2'])

#=> ‘The Throne Room’

If you specify a key that does not exist, the default value is returned. Recall that we have not specified a default value for h1 but we have for h2: p(h1['unknown_room'])

#=> nil

p(h2['unknown_treasure'])

#=> 'Some kind of ring'

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Use the default method to get the default value and the default= method to set it (see Chapter 4 for more information on get and set methods): p(h1.default)

h1.default = 'A mysterious place'

Hash Operations

hash2.rb

The keys and values methods of Hash each return an array so you can use various Array methods to manipulate them. Here are a few simple examples (note, the data shown in comments beginning #=> show the values returned when each piece of code is run) :

h1 = {'key1'=>'val1', 'key2'=>'val2', 'key3'=>'val3', 'key4'=>'val4'}

h2 = {'key1'=>'val1', 'KEY_TWO'=>'val2', 'key3'=>'VALUE_3',

'key4'=>'val4'}

p( h1.keys & h2.keys )

# set intersection (keys)

#=> ["key1", "key3", "key4"]

p( h1.values & h2.values )

# set intersection (values)

#=> ["val1", "val2", "val4"]

p( h1.keys+h2.keys )

# concatenation

#=> [ "key1", "key2", "key3", "key4", "key1", "key3", "key4", "KEY_TWO"]

p( h1.values-h2.values )

# difference

#=> ["val3"]

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p( (h1.keys << h2.keys) )

# append

#=> ["key1", "key2", "key3", "key4", ["key1", "key3", "key4",

"KEY_TWO"] ]

p( (h1.keys << h2.keys).flatten.reverse ) # ‘un-nest’ arrays and reverse

#=> ["KEY_TWO", "key4", "key3", "key1", "key4", "key3", "key2", "key1"]

Be careful to note the difference between concatenating using + to add the values from the second array to the first array and appending using << to add the second array itself as the final element of the first array: a =[1,2,3]

b =[4,5,6]

c = a + b

#=> c=[1, 2, 3, 4, 5, 6] a=[1, 2, 3]

a << b

#=> a=[1, 2, 3, [4, 5, 6]]

In addition << modifies the first (the ‘receiver’) array whereas + returns a new array but leaves the receiver array unchanged. If, after appending an array with << you decide that you’d like to add the elements from the appended array to the receiver array rather than have the appended array itself ‘nested’

inside the receiver, you can do this using the flatten method: a=[1, 2, 3, [4, 5, 6]]

a.flatten

#=> [1, 2, 3, 4, 5, 6]

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Chapter Seven

LOOPS AND ITERATORS…

Much of programming is concerned with repetition. You may want a program to beep 10 times, read lines from a file just so long as there are more lines to read or display a warning until the user presses a key. Ruby provides a number of ways of performing this kind of repetition.

For Loops

In many programming languages, when you want to run a bit of code a certain number of times you can just put it inside a for loop. In most languages, you have to give a for loop a variable initialized with a starting value which is incremented by 1 on each turn through the loop until it meets some specific ending value. When the ending value is met, the for loop stops running. Here’s a version of this traditional type of for loop written in Pascal:

# This is Pascal code, not Ruby!

for i := 1 to 3 do

writeln( i );

for_loop.rb

You may recall from Chapter Five (arrays) that Ruby’s for loop doesn’t work like this at all! Instead of giving it a starting and ending value, we give the for loop a list of items and it iterates over them, one by one, assigning each value in turn to a loop variable until it gets to the end of the list.

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For example, here is a for loop that iterates over the items in an array, displaying each in turn:

# This is Ruby code…

for i in [1,2,3] do

puts( i )

end

The for loop is more like the ‘for each’ iterator provided by some other programming languages. Indeed, the author of Ruby describes for as “syntax sugar” for the each method which is implemented by Ruby's collection types such as Arrays, Sets, Hashes and Strings (a String being, in effect, a collection of characters).

For the sake of comparison, this is the for loop shown above rewritten using the each method:

each_loop.rb

[1,2,3].each do |i|

puts( i )

end

As you can see, there isn’t really all that much difference.

To convert the for loop to an each iterator, all I’ve had to do is delete for and in and append .each to the array. Then I’ve put the iterator variable, i, between a pair of upright bars after do.

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Compare these other examples to see just how similar for loops are to each iterators:

for_each.rb

# --- Example 1 ---

# i) for

for s in ['one','two','three'] do

puts( s )

end

# ii) each

['one','two','three'].each do |s|

puts( s )

end

# --- Example 2 ---

# i) for

for x in [1, "two", [3,4,5] ] do puts( x ) end

# ii) each

[1, "two", [3,4,5] ].each do |x| puts( x ) end

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Note, incidentally, that the do keyword is optional in a for loop that spans multiple lines but it is obligatory when it is written on a single line:

# Here the ‘do’ keyword can be omitted

for s in ['one','two','three']

puts( s )

end

# But here it is required

for s in ['one','two','three'] do puts( s ) end

for_to.rb

How to write a ‘normal’ for loop…

If you miss the traditional type of for loop, you can always ‘fake’ it in Ruby by using a for loop to iterate over the values in a range. For example, this is how to use a for loop variable to count up from 1 to 10, displaying its value at each turn through the loop:

for i in (1..10) do

puts( i )

end

Which can be rewritten using each:

(1..10).each do |i|

puts(i)

end

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Note, incidentally, that a range expression such as 1..3 must be enclosed between round brackets when used with the each method, otherwise Ruby assumes that you are attempting to use each as a method of the final integer (a FixNum) rather than of the entire expression (a Range). The brackets are optional when a range is used in a for loop.

When iterating over items using each the block of code between do and end is called (predictably, perhaps?) an ‘iterator block’.

Block Parameters: In Ruby any variables declared between upright bars at the top of a block are called ‘block parameters’. In a way, a block works like a function and the block parameters work like a function’s argument list. The each method runs the code inside the block and passes to it the arguments supplied by a collection (for example, an array).

Blocks

block_syntax.rb

Ruby has an alternative syntax for delimiting blocks. Instead of using do. end, like this...

# do. end

[[1,2,3],[3,4,5],[6,7,8]].each do

|a,b,c|

puts( "#{a}, #{b}, #{c}" )

end

...you can use curly braces {. } like this:

The Little Book Of Ruby :: Chapter Seven :: www.sapphiresteel.com :: page 58

# curly braces {. }

[[1,2,3],[3,4,5],[6,7,8]].each{

|a,b,c|

puts( "#{a}, #{b}, #{c}" )

}

No matter which block delimiters you use, you must ensure that the opening delimiter, ‘{‘ or ‘do’, is placed on the same line as the each method. Inserting a line break between each and the opening block delimiter is a syntax error.

While Loops

Ruby has a few other loop constructs too. This is how to do a while loop: while tired

sleep

end

Or, to put it another way:

sleep while tired

Even though the syntax of these two examples is different they perform the same function. In the first example, the code between while and end (here a call to a method named sleep) executes just as long as the Boolean condition (which, in this case, is the value returned by a method called tired) evaluates to true. As in for loops the keyword do may optionally be placed between the test condition and the code to be executed when these appear on separate lines; the do keyword is obligatory when the test condition and the code to be executed appear on the same line.

The Little Book Of Ruby :: Chapter Seven :: www.sapphiresteel.com :: page 59

While Modifiers

In the second version of the loop (sleep while tired), the code to be executed (sleep) precedes the test condition (while tired). This syntax is called a ‘while modifier’. When you want to execute several expressions using this syntax, you can put them between the begin and end keywords:

begin

sleep

snore

end while tired

while.rb

This is an example showing the various alternative syntaxes: $hours_asleep = 0

def tired

if $hours_asleep >= 8 then

$hours_asleep = 0

return false

else

$hours_asleep += 1

return true

end

end

def snore

puts('snore. . ')

end

The Little Book Of Ruby :: Chapter Seven :: www.sapphiresteel.com :: page 60

def sleep

puts("z" * $hours_asleep )

end

while tired do sleep end

# a single-line while loop

while tired

# a multi-line while loop

sleep

end

sleep while tired

# single-line while modifier

begin

# multi-line while modifier

sleep

snore

end while tired

The last example above (the multi-line while modifier) needs close consideration as it introduces some important new behaviour. When a block of code delimited by begin and end precedes the while test, that code always executes at least once. In the other types of while loop, the code may never execute at all if the Boolean condition initially evaluates to true.

The Little Book Of Ruby :: Chapter Seven :: www.sapphiresteel.com :: page 61

while2.rb

Ensuring a Loop Executes At Least Once

Usually a while loops executes 0 or more times since the Boolean test is evaluated before the loop executes; if the test returns false at the outset, the code inside the loop never runs.

However, when the while test follows a block of code enclosed between begin and end, the loop executes 1 or more times as the Boolean expression is evaluated after the code inside the loop executes.

To appreciate the differences in behaviour of these two types of while loop, run while2.rb. These examples should help to clarify: x = 100

# The code in this loop never runs

while (x < 100) do puts('x < 100') end

# The code in this loop never runs

puts('x < 100') while (x < 100)

# But the code in loop runs once

begin puts('x < 100') end while (x < 100)

The Little Book Of Ruby :: Chapter Seven :: www.sapphiresteel.com :: page 62

Until Loops

Ruby also has an until loop which can be thought of as a ‘while not’ loop. Its syntax and options are the same as those applying to while – that is, the test condition and the code to be executed can be placed on a single line (in which case the do keyword is obligatory) or they can be placed on separate lines (in which case do is optional). There is also an until modifier which lets you put the code before the test condition; and there is the option of enclosing the code between begin and end in order to ensure that the code block is run at least once.

until.rb

Here are some simple examples of until loops:

i = 10

until i == 10 do puts(i) end

# never executes

until i == 10

# never executes

puts(i)

i += 1

end

puts(i) until i == 10

# never executes

begin

# executes once

puts(i)

end until i == 10

The Little Book Of Ruby :: Chapter Seven :: www.sapphiresteel.com :: page 63

Both while and until loops can, just like a for loop, be used to iterate over arrays and other collections. For example, this is how to iterate over all the elements in an array:

while i < arr.length

puts(arr[i])

i += 1

end

until i == arr.length

puts(arr[i])

i +=1

end

The Little Book Of Ruby :: Chapter Eight :: www.sapphiresteel.com :: page 64

Chapter Eight

CONDITIONAL STATEMENTS…

Computer programs, like Life Itself, are full of difficult decisions waiting to be made. Things like: If I stay in bed I will get more sleep, else I will have to go to work; if I go to work I will earn some money, else I will lose my job - and so on…

We’ve already performed a number of if tests in previous programs. To take a simple example, this is from the Tax calculator in chapter one: if (subtotal < 0.0) then

subtotal = 0.0

end

In this program, the user was prompted to enter a value, subtotal, which was then used in order to calculate the tax due on it. The little test above ensures that subtotal is never a minus figure. If the user, in a fit of madness, enters a value less than 0, the if test spots this since the condition (subtotal < 0.0) evaluates to true, which causes the body of the code between the if test and the end keyword to be executed; here, this sets the value of subtotal to 0.

Equals once = or equals twice == ?

In common with many other programming languages, Ruby uses one equals sign to assign a value = and two to test a value ==.

The Little Book Of Ruby :: Chapter Eight :: www.sapphiresteel.com :: page 65

If..Then..Else

if_else.rb

A simple if test has only one of two possible results. Either a bit of code is run or it isn’t, depending on whether the test evaluates to true or not.

Often, you will need to have more than two possible outcomes. Let’s suppose, for example, that your program needs to follow one course of action if the day is a weekday and a different course of action if it is a weekend. You can test these conditions by adding an else section after the if section, like this: if aDay == 'Saturday' or aDay == 'Sunday'

daytype = 'weekend'

else

daytype = 'weekday'

end

The if condition here is straightforward. It tests two possible conditions: 1) if the value of the variable, aDay is equal to the string ‘Saturday’ or..

2) if the value of aDay is equal to the string ‘Sunday’.

If either of those conditions is true then the next line of code executes: daytype = 'weekend'

In all other cases, the code after else executes:

daytype = 'weekday'.

The Little Book Of Ruby :: Chapter Eight :: www.sapphiresteel.com :: page 66

if_then.rb

When an if test and the code to be executed are placed on separate lines, the then keyword is optional. When the test and the code are placed on a single line, the then keyword (or, if you prefer really terse code, a colon character) is obligatory:

if x == 1 then puts( 'ok' ) end

# with 'then'

if x == 1 : puts( 'ok' ) end

# with colon

if x == 1 puts( 'ok' ) end

# syntax error!

An if test isn’t restricted to evaluating just two conditions. Let’s suppose, for example, that your code needs to work out whether a certain day is a working day or a holiday. All weekdays are working days; all Saturdays are holidays but Sundays are only holidays when you are not working overtime.

This is my first attempt to write a test to evaluate all these conditions: working_overtime = true

if aDay == 'Saturday' or aDay == 'Sunday' and not working_overtime daytype = 'holiday'

puts( "Hurrah!" )

else

daytype = 'working day'

end

The Little Book Of Ruby :: Chapter Eight :: www.sapphiresteel.com :: page 67

and_or.rb

Unfortunately, this doesn’t have quite the effect intended. Remember that Saturday is always a holiday. But this code insists that ‘Saturday’ is a working day. This is because Ruby takes the test to mean: “If the day is Saturday and I am not working overtime, or if the day is Sunday and I am not working overtime”

whereas what I really meant was “If the day is Saturday; or if the day is Sunday and I am not working overtime” .

The easiest way to resolve this ambiguity is to put brackets around any code to be evaluated as a single unit, like this:

if aDay == 'Saturday' or (aDay == 'Sunday' and not working_overtime) And. Or. Not

Incidentally, Ruby has two different syntaxes for testing Boolean (true/false) conditions.

In the above example, I’ve used the English-language style operators: and, or and not. If you prefer you could use alternative operators similar to those used in many other programming languages, namely: && (and), || (or) and ! (not).

Be careful, though, the two sets of operators aren’t completely interchangeable.

For one thing, they have different precedence which means that when multiple operators are used in a single test, the parts of the test may be evaluate in different orders depending on which operators you use.

The Little Book Of Ruby :: Chapter Eight :: www.sapphiresteel.com :: page 68

If..Elsif

There will no doubt be occasions when you will need to take multiple different actions based on several alternative conditions. One way of doing this is by evaluating one if condition followed by a series of other test conditions placed after the keyword elsif. The whole lot must then be terminated using the end keyword.

if_elsif.rb

For example, here I am repeatedly taking input from a user inside a while loop; an if condition tests if the user enters ‘q’ (I’ve used the chomp() method to remove the carriage return from the input); if ‘q’ is not entered the first elsif condition tests if the integer value of the input (input.to_i) is greater than 800; if this test fails the next elsif condition tests if it is less than or equal to 800: while input != 'q' do

puts("Enter a number between 1 and 1000 (or 'q' to quit)") print("?- ")

input = gets().chomp()

if input == 'q'

puts( "Bye" )

elsif input.to_i > 800

puts( "That's a high rate of pay!" )

elsif input.to_i <= 800

puts( "We can afford that" )

end

end

This code has a bug. It asks for a number between 1 and 1000 but it accepts other numbers. See if you can rewrite the tests to fix this!

The Little Book Of Ruby :: Chapter Eight :: www.sapphiresteel.com :: page 69

if_else_alt.rb

Ruby also has a short-form notation for if..then. else in which a question mark ? replaces the if..then part and a colon : acts as else…

< Test Condition > ? <if true do this> : <else do this>

For example:

x == 10 ? puts("it's 10") : puts( "it's some other number" ) When the test condition is complex (if it uses ands and ors) you should enclose it in brackets.

If the tests and code span several lines the ? must be placed on the same line as the preceding condition and the : must be placed on the same line as the code immediately following the ?.

In other words, if you put a newline before the ? or the : you will generate a syntax error. This is an example of a valid multi-line code block:

(aDay == 'Saturday' or aDay == 'Sunday') ?

daytype = 'weekend' :

daytype = 'weekday'

The Little Book Of Ruby :: Chapter Eight :: www.sapphiresteel.com :: page 70

Unless

unless.rb

Ruby also can also perform unless tests, which are the opposite of if tests: unless aDay == 'Saturday' or aDay == 'Sunday'

daytype = 'weekday'

else

daytype = 'weekend'

end

Think of unless as being an alternative way of expressing ‘if not’. The following is equivalent to the code above:

if !(aDay == 'Saturday' or aDay == 'Sunday')

daytype = 'weekday'

else

daytype = 'weekend'

end

If and Unless Modifiers

You may recall the alternative syntax for while loops in Chapter 7. Instead of writing this…

while tired do sleep end

…we can write this:

sleep while tired

The Little Book Of Ruby :: Chapter Eight :: www.sapphiresteel.com :: page 71

This alternative syntax, in which the while keyword is placed between the code to execute and the test condition is called a ‘while modifier’. It turns out that Ruby has if and unless modifiers too. Here are a few examples: if_unless_mod.rb

sleep if tired

begin

sleep

snore

end if tired

sleep unless not tired

begin

sleep

snore

end unless not tired

The terseness of this syntax is useful when, for example, you repeatedly need to take some well-defined action if some condition is true.

This is how you might pepper your code with debugging output if a constant called DEBUG is true:

puts( "somevar = #{somevar}" ) if DEBUG

The Little Book Of Ruby :: Chapter Eight :: www.sapphiresteel.com :: page 72

Case Statements

When you need to take a variety of different actions based on the value of a single variable, multiple if..elsif tests are verbose and repetitive. A neater alternative is provided by a case statement. This begins with the word case followed by the variable name to test. Then comes a series of when sections, each of which specifies a ‘trigger’ value followed by some code. This code executes only when the test variable equals the trigger value: case.rb

case( i )

when 1 : puts("It's Monday" )

when 2 : puts("It's Tuesday" )

when 3 : puts("It's Wednesday" )

when 4 : puts("It's Thursday" )

when 5 : puts("It's Friday" )

when (6. 7) : puts( "Yippee! It's the weekend! " ) else puts( "That's not a real day!" )

end

In the example above, I’ve used colons to separate each when test from the code to execute. Alternatively, you could use the then keyword: when 1 then puts("It's Monday" )

The colon or then can be omitted if the test and the code to be executed are on separate lines. Unlike case statements in C-like languages, there is no need to enter a break keyword when a match is made in order to prevent execution trickling down through the remainder of the sections.

The Little Book Of Ruby :: Chapter Eight :: www.sapphiresteel.com :: page 73

In Ruby, once a match is made the case statement exits:

case( i )

when 5 : puts("It's Friday" )

puts(". .nearly the weekend!")

when 6 : puts("It's Saturday!" )

# the following never executes

when 5 : puts( "It's Friday all over again!" )

end

You can include several lines of code between each when condition and you can include multiple values separated by commas to trigger a single when block, like this:

when 6, 7 : puts( "Yippee! It's the weekend! " )

The condition in a case statement is not obliged to be a simple variable; it can be an expression like this:

case( i + 1 )

You can also use non-integer types such as string.

If multiple trigger values are specified in a when section, they may be of varying types – for example, both string and integers:

when 1, 'Monday', 'Mon' : puts( "Yup, '#{i}' is Monday" ) The Little Book Of Ruby :: Chapter Eight :: www.sapphiresteel.com :: page 74

case2.rb

Here is a longer example, illustrating some of the syntactical elements mentioned earlier:

case( i )

when 1 : puts("It's Monday" )

when 2 : puts("It's Tuesday" )

when 3 : puts("It's Wednesday" )

when 4 : puts("It's Thursday" )

when 5 then puts("It's Friday" )

puts(". .nearly the weekend!")

when 6, 7

puts("It's Saturday!" ) if i == 6

puts("It's Sunday!" ) if i == 7

puts( "Yippee! It's the weekend! " )

# the following never executes

when 5 : puts( "It's Friday all over again!" )

else puts( "That's not a real day!" )

end

The Little Book Of Ruby :: Chapter Nine :: www.sapphiresteel.com :: page 75

Chapter Nine

MODULES AND MIXINS…

As mentioned in an earlier chapter, each Ruby class can only have one immediate ‘parent’, though each parent class may have many ‘children’.

By restricting class hierarchies to single line of descent, Ruby avoids some of the problems that may occur in those programming languages (such as C++) which permit multiple-lines of descent.

When classes have many parents as well as many children and their parents, and children, also have many other parents and children, you risk ending up with an impenetrable network (or ‘knotwork’?) rather than the neat, well-ordered hierarchy which you may have intended.

Nevertheless, there are occasions when it is useful for a class to be able to implement features which it has in common with more than one other pre-existing class.

For example, a Sword might be a type of Weapon but also a type of Treasure; a House might be a type of Building but also a type of Investment and so on.

A Module Is Like A Class…

Ruby’s solution to this problem is provided by Modules. At first sight, a module looks very similar to a class. Just like a class it can contain constants, methods and classes.

The Little Book Of Ruby :: Chapter Nine :: www.sapphiresteel.com :: page 76

Here’s a simple module:

module MyModule

GOODMOOD = "happy"

BADMOOD = "grumpy"

def greet

return "I'm #{GOODMOOD}. How are you?"

end

end

As you can see, this contains a constant, GOODMOOD and an ‘instance method’, greet. To turn this into a class you would only need to replace the word module in its definition with the word class.

Module Methods

In addition to instance methods a module may also have module methods which are preceded by the name of the module:

def MyModule.greet

return "I'm #{BADMOOD}. How are you?"

end

In spite of their similarities, there are two major features which classes possess but which modules do not: instances and inheritance. Classes can have instances (objects), superclasses (parents) and subclasses (children); modules can have none of these.

Which leads us to the next question: if you can’t create an object from a module, what are modules for?

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This is another question that can be answered in two words: namespaces and mixins. Ruby’s ‘mixins’ provide a way of dealing with the little problem of multiple inheritance which I mentioned earlier. We’ll come to mixins shortly.

First though, let’s look at namespaces.

Modules as Namespaces

You can think of a module as a sort of named ‘wrapper’ around a set of methods, constants and classes. The various bits of code inside the module share the same ‘namespace’ - which means that they are all visible to each other but are not visible to code outside the module.

The Ruby class library defines a number of modules such as Math and Kernel.

The Math module contains mathematical methods such as sqrt to return a square route and constants such as PI. The Kernel module contains many of the methods we’ve been using from the outset such as print, puts and gets.

Constants

Constants are like variables except their values do not (or should not!) change. In fact, it is (bizarrely!) possible to change the value of a constant in Ruby but this is certainly not encouraged and Ruby will warn you if you do so. Note that constants begin with a capital letter.

The Little Book Of Ruby :: Chapter Nine :: www.sapphiresteel.com :: page 78

modules1.rb

Let’s assume we have this module:

module MyModule

GOODMOOD = "happy"

BADMOOD = "grumpy"

def greet

return "I'm #{GOODMOOD}. How are you?"

end

def MyModule.greet

return "I'm #{BADMOOD}. How are you?"

end

end

We can access the constants using :: like this:

puts(MyModule::GOODMOOD)

We can similarly access module methods using dot notation – that is, specifying the module name followed by a full stop and the method name.

The following would print out “I'm grumpy. How are you?”:

puts( MyModule.greet )

The Little Book Of Ruby :: Chapter Nine :: www.sapphiresteel.com :: page 79

Module ‘Instance Methods’

This just leaves us with the problem of how to access the instance method, greet. As the module defines a closed namespace, code outside the module won’t be able to ‘see’ the greet method so this won’t work:

puts( greet )

If this were a class rather than a module we would, of course, create objects from the class using the new method – and each separate object (each ‘instance’

of the class), would have access to the instance methods. But, as I said earlier, you cannot create instances of modules. So how the heck can we use their instance methods? This is where those mysterious mixins enter the picture…

Included Modules or ‘Mixins’

modules2.rb

An object can access the instance methods of a module just by including that module using the include method. If you were to include MyModule into your program, everything inside that module would suddenly pop into existence within the current scope. So the greet method of MyModule will now be accessible:

include MyModule

puts( greet )

The process of including a module in a class is also called ‘mixing in’ the module – which explains why included modules are often called ‘mixins’.

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When you include objects into a class definition, any objects created from that class will be able to use the instance methods of the included module just as though they were defined in the class itself.

modules3.rb

class MyClass

include MyModule

def sayHi

puts( greet )

end

def sayHiAgain

puts( MyModule.greet )

end

end

Not only can the methods of this class access the greet method from MyModule, but so too can any objects created from the class, like this: ob = MyClass.new

ob.sayHi

ob.sayHiAgain

puts(ob.greet)

In short, then, modules can be used as a means of grouping together related methods, constants and classes within a named scope. In this respect, modules can be thought of as discreet code units which can simplify the creation of reusable code libraries.

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modules4.rb

On the other hand, you might be more interested in using modules as an alternative to multiple inheritance. Returning to an example which I mentioned at the start of this chapter, let’s assume that you have a Sword class which is not only a type of Weapon but also of Treasure. Maybe Sword is a descendant of the Weapon class (so inherits methods such as deadliness and power), but it also needs to have the methods of a Treasure (such as value and insurance_cost). If you define these methods inside a Treasure module rather than a Treasure class, the Sword class would be able to include the Treasure module in order to add (‘mix in’) the Treasure methods to the Sword class’s own methods.

mod_vars.rb

Note, incidentally, that any variables which are local to a module cannot be accessed from outside the module. This is the case even if a method inside the module tries to access a local variable and that method is invoked by code from outside the module – for example, when the module is mixed in through inclusion. The mod_vars.rb program illustrates this.

Including Modules From Files

requiremodule.rb

So far, we’ve mixed in modules which have all been defined within a single source file. Often it is more useful to define modules in separate files and include them as needed. The first thing you have to do in order to use code from another file is to load that file using the require method, like this: require( "testmod.rb" )

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The required file must be in the current directory, on the search path or in a folder listed in the predefined array variable $:. You can add a directory to this array variable using the usual array-append method, << in this way: $: << "C:/mydir"

The require method returns a true value if the specified file is successfully loaded; otherwise it returns false. If in doubt, you can simply display the result:

puts(require( "testmod.rb" ))

Pre-Defined Modules

The following modules are built in to the Ruby interpreter:

Comparable, Enumerable, FileTest, GC, Kernel, Math, ObjectSpace, Precision, Process, Signal

The most important of the pre-defined modules is Kernel which, as mentioned earlier, provides many of the ‘standard’ Ruby methods such as gets, puts, print and require. In common with much of the Ruby class library, Kernel is written in the C language. While Kernel is, in fact, ‘built into’ the Ruby interpreter, conceptually it can be regarded as a mixed-in module which, just like a normal Ruby mixin, makes its methods directly available to any class that requires it; since it is mixed in to the Object class, from which all other Ruby classes descend, the methods of Kernel are universally accessible.

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Chapter Ten

GOING FURTHER…

We’ve covered a lot of ground over the past ten chapters but, even so, we’ve only just begun to explore all the possibilities of programming with Ruby.

One of the areas we haven’t even touched upon is the development of web applications using the Rails framework (popularly known as ‘Ruby On Rails’).

The good news is that, developing with Rails will be much easier now that you have a fundamental understanding of programming in Ruby. While Rails has all kinds of tools to get a simple application up and running, trying to program in Rails without understanding Ruby would be like trying to write a novel without being able to speak the language!

We haven’t looked at the features that Ruby brings to specific operating systems either. There are, for example, several projects in development aimed at getting Ruby to run on the Microsoft .NET platform. There are also libraries and tools which can help you create graphic ‘front ends’ for your Ruby programs.

Saving Data

The time has now come to wrap up this Little Book Of Ruby. Let’s do that by looking at one more sample project – a little CD database which lets you create new objects (one for each disc in your CD collection), add them to an array and store them on disk.

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In order to save the data to disk I have used Ruby’s YAML library:

# saves data to disk in YAML format

def saveDB

File.open( $fn, 'w' ) {

|f|

f.write($cd_arr.to_yaml)

}

end

YAML

YAML describes a format for saving data as human-readable text. The data can be subsequently reloaded from disk in order to reconstruct the array of CD

objects in memory:

def loadDB

input_data = File.read( $fn )

$cd_arr = YAML::load( input_data )

end

Much of the coding in this little program should be familiar from our previous projects. A couple of things need to be highlighted, however.

First, variables beginning with a dollar $ are ‘global’ so are usable by all the code throughout the program (recall that instance variables, starting with @, are only usable within the confines of a specific object; while local variables, starting with a lowercase letter, are only usable within a well-defined ‘scope’

such as within a specific method).

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Files

Also notice that we use the File class to check if a File exists: if File.exist?( $fn )

Here, exist? is a ‘class method’ – that is, it ‘belongs to’ the File class rather than to an instance of the File class. That explains how we can invoke the method from File itself rather than having to invoke it from a new File object.

This may remind you of the module methods discussed in Chapter Nine –another example of the similarities between modules and classes.

Moving On..

The Ruby community is currently a very active one and new projects are constantly emerging. To keep up to date, we suggest that you visit the Sapphire In Steel site (www.sapphiresteel.com) to find links to some of the most useful resources for Ruby programmers. We shall also be adding more tutorials and sample projects to the site to continue our exploration of Ruby programming.

In conclusion, I hope you’ve enjoyed this little introduction to the Ruby language and that it may be just the start of many years of enjoyable and productive Ruby development.

Document Outline

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Table of Contents

Chapter One

Chapter Two

Chapter Three

Chapter Four

Chapter Five

Chapter Six

Chapter Seven

Chapter Eight

Chapter Nine

Chapter Ten