1. Classes, Objects, And Variables Sudheer Gupta Bysani

2. 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

3. 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

4. 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 end aSong = Song.new("Bicylops", "Fleck", 260) aSong.duration >> 260 aSong.duration = 257 aSong.duration >> 257

5. 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 end end

6. 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 end

7. 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

8. 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 • end • Protected #subsequent methods are protected • def method3 • end

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

10. 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 end

11. Container, Blocks and Iterators Sudheer Gupta Bysani

12. 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”

13. 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 }

14. 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

15. 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 end fibUpTo(1000) { |f| print f, " " } def threeTimes    yield    yield    yield end threeTimes { puts "Hello" }

16. 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 end [1, 3, 5, 7, 9].find {|v| v*v > 30 }  7 # Block returns true if v*v > 30

17. 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 "