Imperative vs. object- oriented paradigms 1 11/14/17 Imperative - - PDF document

11/14/17 1

CSCI-2320 Object-Oriented Paradigm: Ruby

Mohammad T . Irfan

Imperative vs. object-

• riented paradigms

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Imperative vs. object-oriented

u Imperative

u Procedural decomposition u Procedures are all powerful u Data is helpless, at the mercy of procedures

u Object-oriented (OO)

u Data-centric: data governs the decomposition u Classes – templates/patterns, abstracts away data u Objects – actual things/instantiations of classes

u Advantages of OO paradigm

u Collaboration u Debugging u Reuse

Roadmap

u OOP principles u Learn the basics of Ruby u Investigate Ruby’s object-oriented design

principles

u Ruby on Rails for web programming

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OOP Principles

Examples: Java

OOP principles

• 1. Encapsulation
• 2. Inheritance
• 3. Polymorphism
• 4. Abstraction

These principles typically interact with one another.

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• 1. Encapsulation

u Hide internal representation of data by

providing methods (e.g., getter and setter methods; other methods that work on data)

u Benefits: decoupling data and functionality,

protection of data, hiding unnecessary details on representation

Encapsulation example: Book class

u Want a class for representing certain

information about a book

u Note: each object is one single book u Multiple objects à many books

• 1. What are the attributes or properties of a

book?

• 2. What are the actions or behaviors that you

can apply on book data?

u Encapsulation: bind the above two together u Sample code:

u http://www.javaworld.com/article/2979739/

learn-java/java-101-classes-and-objects-in- java.html

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A tale of two cities 1859 Moby Dick 1851 Unknown

• 1

book1 book2 book3 Shared class variable: count = 3 Methods are also shared

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Encapsulation question

u Build on the Book class to include author

• names. How would you represent multiple

authors?

• 2. Inheritance

u Allows one class to "inherit" the methods

(functionalities) and attributes (variables) of another class

u Subclass (AKA derived class)

extends (or inherits) Superclass (AKA base class)

u Java's keyword: extends

11/14/17 8 Subclass of subclass variables and methods

Inheritance in picture

u Hierarchical organization

Subclass variables and methods

Superclass variables and methods

Inheritance in Java

u ✗ u ✓

Subclass Multiple superclasses Superclass Multiple subclasses Chain of inheritance

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Chain of inheritance (Multilevel inheritance)

u New class for shipping a box, inherits

WeightedBox

Demo

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• 3. Polymorphism

u The ability of an object to take many forms

u Compare with method overloading

u This is achieved via inheritance

u Note the inter-relation

u Superclass can refer to subclass object u Concept: method overriding

Method overriding

u Subclass re-defines a superclass method with

the same method signature

u Next few slides

u PlainBox class models a simple 3D box u WeightedBox class extends PlainBox class

u Adds weight variable u Overrides the multiply method

u BoxDemo class gives a demo

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Superclass' multiply method is hidden from the subclass unless the subclass explicitly calls it using super

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• 4. Abstraction

u Generalization u Examples

u C (not OOP): qsort works with different data types u C++: STL u Java: Abstract class allows generalization by hiding

implementation details

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Encapsulation vs abstraction

u Debates on orthogonality of concepts u http://www.tonymarston.co.uk/php-mysql/

abstraction.txt

u Roughly– information/representation hiding

(capsule) vs implementation hiding (generalization)

Ruby

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Ruby Installation

u Several ways to install, as described here:

https://www.ruby-lang.org/en/downloads/

u Mac/Linux: Use RVM (https://rvm.io/rvm/install)

u Command line$ \curl -sSL https://get.rvm.io | bash -s

stable –ruby

u Then, follow the instruction given in terminal u To test, commands are:

u ruby –v u rvm –v

u If you get errors, run the following commands (assuming

2.1.4 is the latest version—look up rvm website for it)

u brew update && brew upgrade u rvm reinstall 2.1.4 --disable-binary

u Windows: Install Ruby 2.0.0 from RubyInstaller.org

http://rubyinstaller.org/downloads/

u Recommended IDE

u Aptana Studio http://www.aptana.com/

Ruby resources

u Learning

u English translation of the creator’s user guide (by

Mark Slagell)

u http://www.rubyist.net/~slagell/ruby/index.html

u Go to reference

u Documentation: http://ruby-doc.org/core-2.0.0/ u http://www.tutorialspoint.com/ruby/

u Interactive tutorial using only your web-browser

u http://tryruby.org

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Origin

u Designed by Yukihiro Matsumoto (Matz) in

early 1990s

u Inspired by Perl and Python

u Less scripting than Perl u More object-oriented than Python u Happy experience!

Quotes

u Bruce Stewart (2001): Did you have a guiding philosophy

when designing Ruby?

u Matz: Yes, it's called the "principle of least surprise." I believe

people want to express themselves when they program. They don't want to fight with the language. Programming languages must feel natural to programmers. I tried to make people enjoy programming and concentrate on the fun and creative part of programming when they use Ruby.

(http://www.linuxdevcenter.com/pub/a/linux/2001/11/29/ruby.html)

u Bill Venners (2003): In an introductory article on Ruby, you

wrote, "For me the purpose of life is partly to have joy. Programmers often feel joy when they can concentrate on the creative side of programming, So Ruby is designed to make programmers happy." How can Ruby make programmers happy?

u Matz: You want to enjoy life, don't you? If you get your job done

quickly and your job is fun, that's good isn't it? That's the purpose of life, partly. Your life is better. I want to solve problems I meet in the daily life by using computers, so I need to write programs. By using Ruby, I want to concentrate the things I do, not the magical rules of the language, like starting with public void something something something to say, "print hello world." I just want to say, "print this!" I don't want all the surrounding magic

• keywords. I just want to concentrate on the task. That's the basic
• idea. So I have tried to make Ruby code concise and succinct.

(http://www.artima.com/intv/ruby.html)

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Interview of Matz

u http://vimeo.com/52954702

Features

u Purely object oriented

u Every data value is an object – no primitive type u Every subroutine is a method u Inheritance can be applied to any class

u Both classes and objects are dynamic!

u Can add methods to classes and objects dynamically u Different objects of the same class can behave

differently

u Dynamically typed u Static scoping u 37 reasons to love Ruby!

u http://rubyhacker.com/ruby37.html

You should be able to explain these!

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Let’s code in Ruby

Before we start

u If you want to quickly check something without

writing a program

u Use the irb command in Terminal

u Examples

u x = 10

if x % 2 == 0 puts “Even” else puts “Odd” end

u What does nil mean in the output? In Ruby, there is

no statement. Everything is an expression returning a value, whether you explicitly say return or not.

u x = [“NFL”, “NBA”, 100]

x.class x.class.methods x.include? “NBA” x.include? 200

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Variables

u Type is implicit u Type can be changed dynamically u Naming: u Examples (in irb)

u x = 10.99

x.class #prints Float x = “Hello Ruby!” x.class #prints String

u Very rich String class

u Examples: http://ruby-doc.org/core-2.0.0/String.html

$ Global variable @ Instance variable [a-z] or _ Local variable [A-Z] Constant

Arrays (mutable)

u Creation, insertion, deletion

u myArray = [“NFL”, “NBA”, 100] u myString = myArray.join(“ ”) #outputs “NFL NBA 100” u left = myArray.shift #left has value “NFL” u myArray #myArray is now [“NBA”, 100] u myArray.push(“MLS”) #myArray is now [“NBA”, 100,

“MLS”]

u myArray.unshift(“NFL”)

#myArray is now [“NFL”, “NBA”, 100, “MLS”]

u delete(obj), delete_at(index), delete_if { |item|

block }

u Accessing elements

u myArray[0] #“NFL” u myArray[0..-1] #everything in the array u myArray.each {|item| puts item} #iterate through items u myArray.each_index {|i| print i, “->”, myArray[i], “\n”}

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Sample program: factorial

u Save it as source.rb u Ways to run

u 1. Add this line at the end of source.rb and click on

run

u puts fact(10)

u 2. ruby -I ./ -r source.rb -e "puts fact(10)” u Command line arguments are also supported

def fact(n) if n == 0 1 else n * fact(n-1) end end

Problem: Collatz Conjecture

u From Wikipedia

http://en.wikipedia.org/wiki/Collatz_conjecture

u Take any integer n > 0 as input. The conjecture is

that no matter what n is, you will always eventually reach 1 if you follow this procedure:

u If n is even, assign n = n/2. If n is odd, assign n =

3n + 1. Repeat the process until you reach n = 1 (conditional statements and loops)

u (Extra job) Print all these numbers to a file

u The # of steps is called the cycle length of n

u Output the cycle length (to standard output) u (Extra job) Also write it to the file

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# of steps (y) vs. input number (x)

Solution

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Review: What’s new in Ruby? (vs. Java/C++)

u Purely object oriented u Classes and objects are dynamic u Class can be defined later, dynamically

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Control structure

u Conditional

u if – elsif – else – end u ---- if condition

u Iteration

u Usual while loops u arrayName.each do |item|

... end

u arrayName.each { |item| ...} u Other ways: for loop

for i in 0..4 ... end

Cool stuff: Reading a website

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More fun: Can we “crawl” the web?

• 1. Extract all links from a web page
• 2. Do recursion [Assignment—later]

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Check out: rubular.com

“Gem” for crawling the web

u Example: anemone http://anemone.rubyforge.org/

u Uses another gem called nokogiri for parsing web pages

u Command line: $ gem install anemone u Ruby Code: require 'anemone' Anemone.crawl("http://www.Bowdoin.edu/") do |anemone| anemone.on_every_page do |page| puts page.url end end

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Object-oriented features

Open class

u Can add a method to an existing class

class Array def summarize self.each do |x| print x, " " end #iterator print "\n" end #def end #class

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In Matz’s words... [Artima]

u Bill Venners: In Ruby, I can add methods and

variables to objects at runtime. ... But in Java, for example, once a class is loaded or an object is instantiated, its interface stays the same. Allowing the interface to change at runtime seems a bit scary to me. ... What's the benefit of being able to add methods at runtime?

u Yukihiro Matsumoto: First of all, you don't have

to use that feature. The most useful application

• f dynamic features, such as adding methods to
• bjects, is meta-programming. Such features

allow you to create a library that adapts to the environment, but they are not for casual uses.

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Naming rules

Starts with Category of variable $ Global variable @ Instance variable @@ Class variable [a-z] or _ Local variable [A-Z] Constant

Next: Classes in Ruby – the usual stuff

Website .rb

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Classes in Ruby: surprise!

Yes, classes are

• bjects of

Class

What does it mean?

u We can create classes

dynamically (just like any other object)

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Modifying a class

u Modify the Website class dynamically

Website.rb (After the previous code that defines the Website class and creates an object of it)

Modify a specific object dynamically! (Not the whole class)

u Singleton method

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Inheritance

Inheritance: the usual stuff

Website .rb

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No multiple inheritance

u Matz: Single inheritance is good because the

whole class inheritance structure forms a single tree with a single root, named Object, and that is very easy to understand. In languages with multiple inheritance, the classes form a network, which is harder to understand.

Inheritance: cool stuff!

u Mix-in: multiple inheritance in some sense

u Share the behavior, not data

u Building block: module

u Collection of methods and constants u Unlike a class, modules cannot be instantiated u Example: Math

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Mix-in example

module A PI = 3.14 E = 2.718 def printPI puts PI end def printE puts E end end module B PI = 3.14159 def printPI puts PI end end class C #mix-in class include A include B end c = C.new c.printPI #=> 3.14159 c.printE #=> 2.718

Collision! Mixin.rb

Matz on Mix-ins

u Matz: “[...] approach of plugging in modules

is called Mix-ins. Mix-ins originally started in LISP culture as a usage of multiple

• inheritance. In fact, a mix-in is actually a

strict way of using multiple inheritance. So in LISP , it's a style of using multiple inheritance. In Ruby, we force you to use mix-ins by supporting classes and modules.”

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Another example of modules: Singleton design pattern

u Use predefined Singleton module

require 'Singleton' class SingletonClass include Singleton #include module # ... end a = SingletonClass.instance b = SingletonClass.instance a == b #=> true c = SingletonClass.new #=> NoMethodError # new is private

load vs. include vs. require

u load 'open-uri.rb'

u Must mention .rb u Can load the same library files multiple times

u require 'open-uri'

u No .rb u Loads a library only once– prevents multiple

loading u include

u Used for including modules within a class u Like copying and pasting code (not file)—within a

class or module

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Mix-in: more cool stuff!

u Modules do not have states – why? u But... it can fake it!

u Example on the next slide: Personal website

subclasses Website and includes a module called PersonalInformation

WebMixin .rb

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Assignment (Due on Tue, Nov 21)

u Crawl the web beginning at www.bowdoin.edu in

an object-oriented way and detect broken links

u Define necessary classes (and modules if needed) u Recursion is the key

u Example: www.bowdoin.edu has a link to

www.bowdoin.edu/computer-science, which has a broken link www.bowdoin.edu/~who. To detect the broken link, you will have to recursively fetch web pages and check links therein.

u Confine your program to the Bowdoin domain. u Caution: it will take a really long time. So, do not re-

check the same link.

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Assignment

u Special cases

u http://www.bowdoin.edu/news u Currently there's some non-UTF8 characters u webpage.scan("...").flatten will generate

exception

u Handle that exception gracefully (that is, just ignore

the content of http://www.bowdoin.edu/news

u Make sure your program works on the testbed u It's a good idea to also test using

http://www.bowdoin.edu/computer-science

Ruby Gems

Twitter—Class Participation HW SQLite Database

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Installation

u First, install gems package management

system

• 1. Get zip file from http://rubygems.org/

pages/download

u Extract it u Go into the extracted folder where you can see

the "setup.rb" file

u Execute this command:

u ruby setup.rb

• 2. Update gem after installation

u

Execute this command: gem update --system

Installing new gems

u Command

u gem install twitter u gem install mail

u Other gems we will need later:

u gem install sqlite3 u gem install rails

u Useful commands:

gem uninstall ..., gem list ..., etc.

u http://guides.rubygems.org/command-reference/

SQLite DBMS Ruby on Rails Twitter API Email API

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Working with the Twitter gem

u Examples

u http://sferik.github.io/twitter/

u Full documentation

u http://rdoc.info/gems/twitter/index

Working with the Twitter gem

u Preparation

u Sign up for Twitter u Sign in with your Twitter account at developer site

u https://dev.twitter.com/apps

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Working with the Twitter gem

Copy and paste from your developer account

Common problem

u Rate limit exceeded! u Twitter’s rules:

u https://dev.twitter.com/rest/public/rate-limiting

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Store search results in database

Class Participation HW Do in groups

u Create a Twitter developer account and use

the Twitter API (see the previous slides) to search Twitter.

u New: need to enter your cell phone number

u Enter the search results into a database (the

previous slide).

u Submit a print-out of all the output of your

program (do not submit code—it contains personal information).

u Due: Next class

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Creating Sqlite3 database using Ruby

Ruby program that creates database Salary.db

Accessing Salary.db from terminal (outside of Ruby)

Path to the file Salary.db here

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A few other useful SQL commands

u delete from SalaryTable where name="Alice";

u Deletes only those rows where the name is Alice u You can specify complex conditions using and/or

u delete from SalaryTable;

u Deleles all rows from that table

u drop table SalaryTable;

u Deletes the SalaryTable itself u Both the schema and the contents will be gone

u Best tutorial

u http://www.w3schools.com/sql/default.asp

Coming up next

u Ruby on Rails

u Web programming platform built using Ruby u Model

u DB and constraints on data

u View

u Generates what users see

u Controller

u Takes user input u Consults with model u Directs the view