Introduction to Ruby CS 480/680 – Comparative Languages

Intro to Ruby2 Object Oriented Programming  Object: To facilitate implementation independent sharing of code, by providing well- behaved units of functional code For most languages, this unit is the class  Specify the behavior of an object, not its implementation

Intro to Ruby3 An Example From C++ class SimpleList { public: // Insert an integer at the start of the list virtual bool insertfront(int i) = 0; // Insert an integer at the end of the list virtual bool insertend(int i) = 0; // Get (and delete) the first value in the list virtual bool getfirst(int &val) = 0; // Get (and delete) the last value in the list virtual bool getlast(int &val) = 0; // Clear the list and free all storage virtual void clear() = 0; // Return the number of items in the list virtual int size() const = 0; };

Intro to Ruby4 Encapsulation  How is the SimpleList implemented? An array with dynamic resizing? An STL vector? A singly-linked list? A doubly-linked list?  You don’t need to know the implementation of the class, because it’s behavior has been specified for you.  This separation of behavior from implementation is called encapsulation, and is the key principle underlying object oriented programming.

Intro to Ruby5 Data Abstraction: Motivation  Focus on the meaning of the operations (behavior) rather than on the implementation  User: minimize irrelevant details for clarity  Implementer Restrict users from making false assumptions about behavior Reserve the ability to make changes later to improve performance

Intro to Ruby6 Using Classes  A class is a collection of data and functions (methods) for accessing the data.  An object is a specific instance of a class: SimpleList myList; class object

Intro to Ruby7 Relationships between classes  Inheritance – used when one class has all the properties of another class class Rectangle { private: int length, width; public: setSize(int newlength, int newwidth) { length = newlength; width = newwidth;} }; class coloredRectangle : public Rectangle { private: string color; public: setColor(string newcolor) {color = newcolor;} }; Base class members can be inherited or overridden by the derived class.

Intro to Ruby8 Relationships between classes (2)  Composition – one class containing another class: class Node { private: int value; // The integer value of this node. Node* next; // Pointer to the next node. public: Node(int newvalue = 0) {value = newvalue; next = NULL;} setNext(Node * newnext) {next = newnext;} }; class linkedList { private: Node* head; public: linkedList() {head = NULL); … }; Can be difficult to decide which to choose, since composition will work for any case where inheritance will work.

Intro to Ruby9 Polymorphism  Operators and member functions (methods) behave differently, depending on what the parameters are.  In C++, polymorphism is implemented using operator overloading  Should allow transparency for different data types: myObject = 7; myObject = 7.0; myObject = ”hello world”; myObject = yourObject;

Intro to Ruby10 Class variables and instance variables  Most data members are instance variables, each object gets its own independent copy of the variables.  Class variables (and constants) are shared by every object/instance of the class: class Student { private: static int total_students; string id, last_name, first_name; … } int Student::total_students = 0;

Intro to Ruby11 Ruby Basics  Ruby is probably the most object-oriented language around  Every built in type is an object with appropriate methods: "gin joint".length  9 "Rick".index("c")  abs  1942 sam.play(aSong)  "duh dum, da dum…"

Intro to Ruby12 Ruby Terminology  Class/instance – the usual definitions Instance variables – again, what you would expect Instance methods – have access to instance variables  Methods are invoked by sending messages to an object The object is called the receiver  All subroutines/functions are methods Global methods belong to the Kernel object

Intro to Ruby13 Code Structure  No semicolons. One statement per line. Use \ for line continuation  Methods are defined using the keyword def: def sayGoodnight(name) result = "Goodnight, " + name return result end # Time for bed... puts sayGoodnight("John-Boy") puts sayGoodnight("Mary-Ellen")

Intro to Ruby14 Code Structure (2)  Parens around method arguments are optional Generally included for clarity These are all equivalent: puts sayGoodnight "John-Boy" puts sayGoodnight("John-Boy") puts(sayGoodnight "John-Boy") puts(sayGoodnight("John-Boy"))

Intro to Ruby15 String Interpolation  Double quoted strings are interpolated  Single quoted strings are not name = ”John Kerry” puts(”Say goodnight, #{name}\n”)  Say goodnight John Kerry puts(’Say goodnight, #{name}\n’)  Say goodnight, #{name}\n

Intro to Ruby16 Variable Typing and Scope  Variables are untyped: var = 7; var *= 2.3; var = ”hello world”;  First character indicates scope and some meta- type information: Lower case letter (or _) – local variable, method parameter, or method name $ – global – instance variables – class variables Upper case letter – Class name, module name, const

Intro to Ruby17 Scope  Local variables only survive until the end of the current block while (var > 0) newvar = var * 2; // newvar created … end // now newvar is gone!

Intro to Ruby18 Ruby Operators  See operators.rb

Intro to Ruby19 Ruby Collections  Collections are special variables that can hold more than one object Collections can hold a mix of object types  Arrays – standard 0-based indexing Must be explicitly created a = [] a = Array.new a = [1, ’cat’, 3] puts a[2]  3

Intro to Ruby20 Collections (2)  Hash – like an array, but the index can be non- numeric Created with {}’s Access like arrays: [] student = { ’name’ => ’John Doe’, ’ID’ => ’ ’, ’year’ => ’sophomore’, ’age’ => 26 } puts student[’ID’] 

Intro to Ruby21 Collections (3)  Hashes and Arrays return the special value nil when you access a non-existent element  When you create a hash, you can specify a different default value: myhash = Hash.new(0)  This hash will return 0 when you access a non-existent member  We’ll see a lot more methods for arrays and hashes later

Intro to Ruby22 Hashes can be a very powerful tool  Suppose you wanted to read a file and… List all of the unique words in the file in alphabetical order List how many times each word is used  The answer is a hash words = Hash.new(0) while (line = gets) words[line] += 1 end words.each {|key, value| print "#{key} ==> #{value}\n" }

Intro to Ruby23 Control Structures  The basics (if, while, until, for) are all there: if (count > 10) puts "Try again“ elsif tries == 3 puts "You lose“ else puts "Enter a number“ end while (weight < 100 and numPallets <= 30) pallet = nextPallet() weight += pallet.weight numPallets += 1 end

Intro to Ruby24 Statement Modifiers  Single statement loops can be written efficiently with control modifiers: square = square*square while square < 1000 sum = sum * -1 if sum < 0

Intro to Ruby25 Reading and Writing  A key strength of interpreted languages is the ability to process text files very quickly I/O in Ruby (and Perl and Python) is extremely easy printf "Number: %5.2f, String: %s", 1.23, "hello“  Number: 1.23, String: hello while gets if /Ruby/ print end ARGF.each { |line| print line if line =~ /Ruby/ } We’ll talk more about regular expressions later. Blocks and iterators are very powerful in Ruby. More on this later, too.

Intro to Ruby26 Basic File I/O  Open a file by creating a new File object: infile = File.new(“name”, “mode”) StringModeStart Pos. “r”ReadBeginning “r+”Read/WriteBeginning “w”WriteTruncate/New “w+”Read/WriteTruncate/New “a”WriteEnd/New “a+”Read/WriteEnd/New infile = File.new(“testfile”, “r”) while (line = infile.gets) line.chomp! # do something with line end infile.close

Intro to Ruby27 Exercises  Write a Ruby program that: Reads a file and echoes it to standard out, removing any lines that start with #  Try also accounting for leading whitespace Reads a file and prints the lines in reverse order Reads a list of student records (name, ssn, grade1, grade2, grade3,…) and stores them in a hash.  Report min, max, and average grade on each assignment