Classes, Objects, And Variables Sudheer Gupta Bysani

Class definition & instantiation Use keyword “class” Class definition ends by keyword “end” Instantiation : OneSong = Song.new #class definition example class Song # methods goes here # …. end

Constructors in ruby “initialize” method is used Member variables start with @ Eg: @name class Song def initialize(name, artist, duration) @name     = name      @artist   = artist      @duration = duration    end end

Attributes : Readable & Writable Readable : return values of instance variables Writable : assign values to instance variables class Song attr_reader :name, :artist, :duration attr_writer :duration # note : no “@” for attributes def initialize(name,artist,duration) # …. end aSong = Song.new("Bicylops", "Fleck", 260) aSong.duration >> 260 aSong.duration = 257 aSong.duration >> 257

Virtual Attributes Wrappers around an object’s instance variables Eg: to access duration in minutes class Song def durationInMinutes @duration/60.0   # force floating point end def durationInMinutes=(value) @duration = (value*60).to_i

Inheritance Use “<“ to derive from base class Use “super” to call the base call initializer class KaraokeSong < Song def initialize(name, artist, duration, lyrics) super(name, artist, duration) @lyrics = lyrics end

Class variables & Class Method Just like static variables in C++ Starts with “@@” Eg : @@num_songs By default, private to the class accessor method to be written explicitly to be accessible to outside world. Accessor method could be instance method or class method Class Methods definition preceded by class name eg: def Song.class_method # .. end

Access Control Two ways to specify access levels of methods 1st Method: Similar to C++ class MyClass def method1 # default is public end Private #subsequent methods are private def method2 Protected #subsequent methods are protected def method3

Access Control ..continued 2nd Method: class MyClass def method1 end # other methods public : method1, :method2 private : method3 protected : method4

Protected Access Used when objects need to access internal state of other objects class Account    attr_reader :balance       # accessor method 'balance'    protected :balance         # and make it protected # need a reader attribute and still not accessible to outside world    def greaterBalanceThan(other)      return @balance > other.balance    end

Container, Blocks and Iterators Sudheer Gupta Bysani

Arrays Holds a collection of object references 2 ways to create arrays 1st Method: a = [3.14, “pie”, 99 ] a.Class  Array a.Length  3 A[0]  3.14 2nd Method: b = Array.new b[0] = 3.14 b[1] = “pie”

Handy ways of using an Array a[-1]  returns last element a[-2]  returns last but one element a[1 , 3]  [start, count] a[1..3]  [start, end] Concatenation : [1, 2,3] + [4,5]  [1,2,3,4,5] Difference : [1,1,1,2,3,2,4,5] – [1,2]  [3,4,5] Removes all the occurrences Append : [1,2] << “c” << “d” << [4,5]  [1,2,”c”,”d”,4,5] Equality : [“a”, “b”] == [“a”,”c”]  false Clear : a.clear # removes all the elements of the array Each : a.each { |x| puts x }

Hashes Associate arrays/maps/dictionaries h = { “dog” => “canine, “cat” => “feline” } h.length  2 h[“dog”]  “canine” h[“cow”] = “bovine” h.each { |key, value| puts “#{key} is #{value}” } Calls block once for each key, passing key and value as parameters h.each_key { |key| puts key } Calls block once for each key Merge, has_key, has_value, invert methods available

Blocks and Iterators Iterator : a method that invokes block of code repeatedly eg: def fibUpTo(max)    i1, i2 = 1, 1         # parallel assignment    while i1 <= max      yield i1      i1, i2 = i2, i1+i2    end fibUpTo(1000) { |f| print f, " " } def threeTimes    yield    yield end threeTimes { puts "Hello" }

Find method .... Using iterator A block may also return a value to the method class Array    def find      for i in 0...size        value = self[i]        return value if yield(value)      end      return nil    end [1, 3, 5, 7, 9].find {|v| v*v > 30 }  7 # Block returns true if v*v > 30

Blocks as Closures Code block as Proc Object Use Proc#call method to invoke the block Proc object carries all the context in which block was defined. In below case, its on method nTimes def nTimes(aThing)    return lambda { |n| aThing * n } end p1 = nTimes(23) p1.call(3)  69 p1.call(4)  92 p2 = nTimes("Hello ") p2.call(3)  Hello Hello Hello "