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Dr Alexiei Dingli

Web Science Stream

Introducing Ruby

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• Originated in Japan in 1995 and it was created by Yakihiro Matsumoto

• High level programming language

• Scripting language which is interpreted

• Object Oriented

What is Ruby?

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• Code caching– Caching the output of a script for reuse rather

than executing the script every time

• Persistent interpreters– Loading the interpreter once and keeping it

running

• What about your performance when developing an application?

What about performance?

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• Program made of objects capable of communicating with other objects

• Each object can store data internally

• Objects with similar characteristics are instances of the same class

What about OOP?

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• The shell where we can input ruby commands

Note: In windows we won’t be using a standard DOS box but use the “Open Ruby Console Window” from the Instant Rails application

Interactive Ruby Shell

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• Open a Ruby Console Window• Type “irb”• And we’re ready to start ...

– Type “1”– Type “2”– Type “3”– What is the result?– Is it the same?

As easy as 1, 2, 3

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• The result might look the same as the input but– Its not the same number– The output is a Ruby object

• As a proof, type– 1.class– What’s the result?

In Ruby everything is an object!

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1.classÞ Fixnum

What if we try

Fixnum.class

More and more classes

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• 1 + 2 • 4 – 3 • 3 / 2 (Integers)• 3.0 / 2.0 (Floats)• 3 ** 2 (3 to the power of 2)• 5 % 2 (5 remainder 2)• 17_000_000_000_000_000_000

(What’s the effect of the underscore?)

• 1.7e19

The world is full of numbers ...

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1. What’s the result of

17_000_000_000_000_000_000 == 1.7e19

2. What happens when you write

googol = 10.0 ** 100

googolplex = 10.0 ** googol

Numbering exercises

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• Strings or numbers that appear directly in the code– String literal

Irb> “The dog ate a bone”

=> “The dog ate a bone”

Irb> “The dog ate a bone”.class

=> String

Irb> “The dog ate a bone”.length

=> 18

Literal objects

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• “Hello “ + “World”• “hi “ * 3• “1” + “2”• “1” * 2• “Hello”.capitalize• “Hello”.reverse• “Hello”.upcase• “Hello”.downcase• “Hello”.swapcase• “a”.next• “aa”.next

Even more strings ...

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• "hello".length + "world".length • "".empty? • "Zoo".include? "oo" • "cats".chop• How do you display your name

backwards?

String exercise

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• Convert anything to ...– .to_s String– .to_i Integer– .to_f Float

• What’s the result of ...– 2.to_s

Easy conversions ...

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• Name of an object– city = “Valletta”

• Variables always start with a lowercase letter

Variables

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• Name of an object– City = “Valletta”

• Constants always start with an uppercase letter• Constants should not change, if you try Ruby will send

a warning• Try

– City = “Valletta”– City = “Mdina”

Constants

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var = var + 2 var += 2 Add 2 to var

var = var - 3 var -= 3 Subtract 3 from var

var = var * 6 var *= 6 Multiply var by 6

var = var / 2 var /= 2 Divide var by 2

var = var** 3 var **=3 Cube var

var = var % 4 var %= 4 var modulo 4

Shortcuts

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Create a first.rb file and type the following ...

name = “Tom”

puts “Hello “ + name + “. How are you?”

no1 = 2

no2 = 4

no3 = no1 + no2

puts “The answer is “ + no3.to_s

Our first program

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• Please use meaningful names for variables ...– age vrs a

• Use the following approach with Multiwords– studentAge or student_age vrs studentage

• Don’t be afraid to use constants where values don’t change

• Use irb when you need to test small sections of code

• When you need help use ri XXXX– Eg ri String – Eg ri String#upcase

Some tips and conventions

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4.times do

puts “Hello”

end

ExerciseWhat is the sum of all the integers from 1 to 1000?

Loops

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name = gets

To remove any carriage returns or new lines use chomp

“Alexiei\n”.chomp

Getting user input

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• Write a small program which asks for your age, calculates the year you were born and displays:

You were born in 19XX

Input exercise

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if city == “Valletta"

licence = “V Licence”

else

licence = “normal”

end

= is an assignment

== is a boolean comparison

Conditions

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if city == “Valletta"

licence = “V Licence”

elsif city == “Mdina”

licence = “M Licence”

else

licence = “normal”

end

Note that only the first elsif that returns true gets executed

Conditions

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• == equal• != not equal to• > greater than• < less than• >= greater than or equal to• <= less than or equal to

Comparisons

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“9” < “D”

“a” < “b”

“h” == “h”

“H” == “h”

“Z” <= “b”

“j” != “r”

String comparison

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count = 0

while count < 10

count += 1

end

While loop

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• Use proper indentation

• Write comments when needed

# I’m a comment and can write whatever i want

More tips and conventions

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>> numbers = [ "zero", "one", "two", "three", "four" ]

=> ["zero", "one", "two", "three", "four"]

>> numbers.class

=> Array

>> numbers[0]

=> "zero"

Arrays

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names = [ "Melissa", "Daniel", "Samantha", "Jeffrey"]

What about ...

names.sort

names.reverse

names.length

names + [“Tom”]

names - [“Daniel”]

names * 2

puts names.to_s

Fun with Arrays

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names.each do |friend|

puts “I have a friend called “ + friend

end

What about using 4.times or ...

names.length.times do |i|

puts "I have a friend called " + names[i]

end

What if I want to print my friends in sorted order?

Let’s iterate

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addressBook = {

“Valletta" => “Tom",

“Sliema" => “Jack",

“Mdina" => “Ben”

}

What’s in a Hash?

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addressBook.each do |key, value|

puts key + " => " + value

end

There is also ...

addressBook.each_key do |key|

addressBook.each_value do |value|

Iterating Hashes

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• Not associated with any other object

def say_hi

puts "Hello, How are you?"

end

say_hi

Functions ...

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def say_hi(name)

puts "Hello " + name + ", How are you?"

end

say_hi("Daniel")

say_hi "Sandy"

Function parameters ...

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• The class keyword defines a class

• By defining a method inside this class, we are associating it with this class

• The initialize method is what actually constructs the data structure. Every class must contain an initialize method.

• The @ sign in front of variables distinguishes the variable as an object variable.

Classes

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class Address

def initialize(street)

@street = street

end

end

address = Address.new(“2 Republic Str")

Example class

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class Address

def initialize(street)

@street = street

end

def street

@street

end

end

>> address.street

=> " 2 Republic Str"

Example class with return

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class Address def

attr_reader: street

initialize(street)

@street = street

end

end

Shortcut to class with return

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class Address def

attr_reader: street

attr_writer: street

initialize(street)

@street = street

end

end

Shortcut to set a variable

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class Address def

attr_accessor: street

initialize(street)

@street = street

end

end

Shortcut to getting and setting a variable in one go

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class SomeClass

def method1 # default to public

...

end

private # subsequent methods are private.

def method2 # private method

...

end

def method3 # private method

...

end

public # Set back to public.

def method4 # public method

...

end

end

Private vrs Public classes

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• Save them in a className.rb file

• Make use of the following command

require “className“

• Just use the classes normally

Using classes

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• If you can't sumarize in one sentence what the function does, it's probably too complicated

• If you have to scroll to see the entire function, it is too long

• Studies suggest that a person can only keep track of at most 7 or so things at one time. If your function has more than 5 or 6 variables, it is probably too long.

Some final guidelines

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Questions?