To Lecture Notes

IT 231 -- 10/2/12

Review Questions

1. Explain the difference between a class and an object.
   Ans: An object has instance variables that contain data and methods that can be executed. A class defines the behavior of the objects. Here is the miller analogy:
   class : object :: cookie cutter : cookie
    
2. What is the dot operator?
   Ans: The dot operator executes a method that belongs to the class or object on its left.
    
3. What is the difference between a class method and an object method? Give an example of each.
   Ans: A class method must be called using the class name; an object method must be called by an object. For example, in t = Time.now, now is an object method called by the class Time. In t.day, day is an object method that tells the current day for the object t.
    
4. Test these String class methods:
   

   reverse  to_f  index  chomp 
   
   Ans:
   s = "abcde"
   print s.reverse, "\n"
   t = "234.32"
   print t.to_f + 1, "\n"
   u = "elephant"
   print u.index(3), "\n"
   v = "abcde\n"
   print v, "\n"
   print v.chomp, "\n"
   

5. Choose four Fixnum class methods and write a Ruby script to test them.
   

   abs   size   zero?    to_s
   
   Ans:
   n = -34537
   m = 2000000000000000
   k = 0
   
   print n.abs, " ", n.size, " ", n.zero?, "\n"
   print m.abs, " ", m.size, " ", m.zero?, "\n"
   print n.abs, " ", k.size, " ", k.zero?, "\n"
   print "*" + n.to_s + "*"
   

Project 3

• Discuss the Dice Example. Write code to test the Dice class and draw a UML diagram for this class.
   
• Discuss problems 2 and 3 of Project 3.

Some ActiveRecord Method Calls

• Create a Rails project named School2. Then create a model named Student having fields first_name, year, and gpa. Add three students to the database using ActiveRecord method calls.
   
• Here are some ActiveRecord method calls for the model Student, which has fields stud_id, first_name, year, gpa:
  

  # Create a new Student object.
  s = Student.new
  
  # Save a Student object in the table.
  Student.save
  
  # Return the first record in the table.
  Student.first
  
  # Return the last record in the table.
  Student.last
  
  # Return number of rows in table.
  Student.count
  
  # Get student with id=5
  s = Student.find(5)
  
  # Get student with stud_id=4534, using dynamic finder method
  s = Student.find_by_stud_id(4534)
  
  # Get student named Bob
  s = Student.find_by_first_name("Bob")
  
  # Create and save new student
  s = Student.new
  s.first_name = "Patricia"
  s.year = 2
  s.gpa = 3.45
  s.save
  

• Later we will see how to retrieve and process an array of objects from the database table.

Project 3

• Discuss Problem 4 of Project 3.

Modifying Ruby Code for IO

• To modify command prompt Ruby code to accept user input: change
  

  c = 20
  f = 9 * c / 5 + 32
  puts f
  

  to

  input = STDIN.gets
  c = input.chomp.to_i
  f = 9 * c / 5 + 32
  puts f
  

Control Structures

• Some Ruby control structures are if..else statements and while loops.
   
• A Ruby/JavaScript Control Structures Dictionary
   
• Look at Examples 15 to 22

 
• The mod operator % returns the remainder from integer division.
   
• For example, consider the integer division problem 17 / 3.
   
• 3 goes into 17 5 times, with a remainder of 2. Therefore the Ruby script
  

  r = 17 % 3
  print r, "\n"
  

  outputs 2.
   
• The mod operator is used in Example 20.
   
• Fix up Examples 17, 18, and 20 so that they accept input from the keyboard.
   
• Fix up Examples 19 and 20 (ConsultantBSGenerator and ChineseZodiac Examples) so that they display their output on a Rails view instead of at the command prompt.