Ruby: Introduction (WIT-WSP)

part 1: basics

• created by Yukihiro Matsumoto (Matz) in 1993.
• popularised by Rails web framework (created by DHH) in 2005.
• many inspirations: Smalltalk, Lisp, Python....
• pure oo language (even the literals are objects).
  • 'hello'.contains('e'); 5.next();
• used for: web apps, text processing, ai and math research.
• open-source.

part 2: object-oriented

• what is object? an instance of a class :P;.
  • object has a uniq identifier #object_id
  • object allocated with the .new method
  • object instantiated from the #class.
• variables are objects? yes!
  • dynamic typing - variable's type is inferred.
  • instance variable holds object's state, store in object.
  • instance variable name start with @
  • class variable shared by all instances of a class
  • class variable name start with @@
• methods are objects? maybe yes~
  • the #method can convert symbol to Method object.
  • methods define the object behaviour, store in class.
  • instance methods define within a class, can access obj's state directly.
  • class methods define with class's singleton class, invoked by class.
  • inheritance symbol is visualisable and funny < also means less than..
  • use comparison operators to check the inheritance: < <= == >= >.
  • module methods for sharing methods across multiple classes.
• everything is object.muuu~
  • in java: Int num = Math.abs(-123)
  • in ruby: num = -123.abs

part 3: ruby scripts

• do not require class definition.
• no main() method needed.
• execute with $ ruby test.rb

part 4: naming conventions

• global variables are prefixed with a dollar sign ($)
• instance variables are begin with (@)
• class variables are begin with (@@)
• local variables, method name should start with lowercase letter.
• class names, module names, and constants must start with uppercase letter.
• method name end with ? : method return a boolean.
• method name end with ! : method do something danger.
• method name end with = : related to an assignment statement.

part 5: strings

• single quoted vs double quoted.
• single quoted is like java string.
• double quoted from supports expression interpolation.
• expression inside #{...} is evaluated and the result inserted into string,.
• arbitrarily complex expressions are allowed inside #{...}

part 6: collections

• most common collections: array, hash
• array and hashes are indexed collections.
• both store collections of objects. accessible using a key/index.
• array index(key) is an sequential integer start with 0.
• hash index(key) can be any objects.
• both array and hash grow as needed.
• both array and hash can hold objects of diff types.

part 7: array

• you can create and initialize a new array using []
• you can also use Array.new method which may support more options.
• lots and lots of built-in methods such as: reverse, reverse!...
• special shortcut for creating an array of words: %w

part 8: hash

• hashes are indexed using the square bracket notation.
• hash return nil by default when indexed a key which doesn't exist.
• you can set default value with: Hash.new(0) or hash.default = 0;

part 9: nil

• nil is an object of NilClass
• always means false when used in condition expressions.
• all other ruby values are true (except the value false of course)

part 10: symbols

• in programming you often need to create a name for something significant,
  • e.g. referring to compass points by name
    • NORTH=1; SOUTH=2; player.run(NORTH);
• most of the time, the actual numeric values of these constants are irrelevant.
• in java we use the global constant as the solution.
• in ruby we use the new feature call symbol to solve it.
  • simply constant names; no declaration needed; uniqueness is guarantee.
  • a symbol literal starts with a colon, followed by some kind of name;
    • ....player.run(:east); car.turn(:south);......
  • no need to assign a value to a symbol/
  • guarantees the particular symbol has the same meaning,.
  • special syntax in hash: {a: 'a', b: 'b'} same as {:a => 'a', :b => 'b'}

part 11: control structure

• usual control structures - if, while, until
• keyword end is used instead of braces as the delimiter.
• statement modifiers: single line control flow
  • puts 'danger!' if radiation > 3000

part 12: code blocks

• in ruby, method's parameters can be data or block
• code block behaviours as well as state can be passed around.
• syntax: code blocks are enclosed in braces or do/end keywords.
• convention: use braces for single-line block, otherwise use do/end style.
• a block must be the last argument of the method.
• a method invoke its associated block one or more times with 'yield'.
• blocks are used throughout the ruby library to implement iterators.
  • pass each element to the block for processing.

part 13: dynamic typing

• a variable's type does not need to be declared explicitly.
• ruby is dynamically typed. its runtime determines a variable's type
  • based on its initialisation and how it is used(duck typing)
  • duck typing: if it walks like a duck, it is a duck!. # respond?
• static versus dynamic typing
  • static: like java, better runtime optimisation, but slower development time.
  • dynamic: like ruby, greater developer productivity, but more testing required.

part 14: class

• attr_reader(getters), attr_writer(setters)
• attr_accessor(both getters and setters)
• these are ruby library methods for create getters and setters.