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

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CSCI-2325 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 Learn the basics of Ruby today u Investigate Ruby’s object-oriented design

principles

u Ruby on Rails for web programming

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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 3 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 get our hands dirty

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”, 2014]

x.class x.class.methods x.include? “NBA” x.include? “2010”

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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”, 2013] u myString = myArray.join(“ ”) #outputs “NFL NBA 2013” u left = myArray.shift #left has value “NFL” u myArray #myArray is now [“NBA”, 2013] u myArray.push(“MLS”) #myArray is now [“NBA”, 2013,

“MLS”]

u myArray.unshift(“NFL”)

#myArray is now [“NFL”, “NBA”, 2013, “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| print 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 natural number n. The conjecture is

that no matter what n is, you will always eventually reach 1.

u Take n as input from user.

u If n is even, divide it by 2 to get n/2. If n is odd,

multiply it by 3 and add 1 to obtain 3n + 1. Repeat the process until you reach n = 1. (conditional statements and loops)

u Print all these numbers to a file.

u The number of numbers is called the cycle length

• f n.

u Output the cycle length (to standard output)

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

Solution: Collatz.rb

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cycle_length = 0

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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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

• ther objects)

Website .rb

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Modify a class dynamically

u Modify the Website class dynamically

Website.rb (After the previous code that defines the Website class)

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

u Singleton method

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Modules and Inheritance

Singleton Design Pattern

u Use predefined Singleton module

u A module is a collection of methods, constants u Unlike a class, modules cannot be instantiated u Example: Math

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

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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 the code

Inheritance: the usual stuff

Inherita nce.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.

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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

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

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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.”

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

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WebMixin .rb

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Class Participation HW

u Create a Twitter developer account and use

the Twitter API to search Twitter.

u Submit the print-out of search results (not

code—contains personal information).

u Due: Next class (Thursday, Nov 13)

Assignment (Due on Wed, Nov 19)

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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Ruby Gems

Twitter SQLite Database

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

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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

Working with the Twitter gem

u Examples

u http://sferik.github.io/twitter/

u Full documentation

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

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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

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Common problem

u Rate limit exceeded! u Twitter’s rules:

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

Store search results in database

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Accessing database from outside Ruby

Ruby program that creates database Salary.db

Access Salary.db from terminal

Path to the file Salary.db here

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Coming up next

u Ruby on Rails

u Model

u DB and constraints on data

u View

u What users see

u Controller

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