To Lecture Notes

IT 231 -- 9/27/12

Review Questions

1. How do you change the port number to 3726 for the WEBrick server?
   

   > rails server --port=3726
   

2. What is the range of numbers for well-known ports?
   Ans: 1 to 1023.
    
3. What does \n mean in Ruby?
   Ans: It is the new line character.
    
4. Find the mistakes in this line of an .html.erb file:
   

   <p><% "The Fahrenheit temperature is ', f %></p>
   

Ans: Here are two possible corrected versions:


<!-- Version 1: -->
<p>The Fahrenheit temperature is <%= @f %></p>

<!-- Version 2: -->
<p><%= "The Fahrenheit temperature is " + @f.to_s %></p>

5. What do these Ruby statements do?

   x = 5 Ans: assigns the value 5 to the variable x.
   x.class Ans: returns the name of the class to which x belongs.
   

6. Write a Ruby script that prints "Hello, world."
   

   print "Hello, World.\n"
   

   
    
7. Write a Ruby statement that assigns 256 to the grand total.
   

   grand_total = 256
   

8. Write controller code with a view that converts 5 inches to centimeters and displays the answer. (1 inch = 2.54 cm). Ans:
   

   # Controller code:
   def display
      @in = 5
      @cm = 2.54 * @in
   end
   
   <!-- View Code -->
   <p>Number of centimeters is: <%= @cm %></p>
   

9. List six ways of running Ruby code. Ans:
   
   1. Interactive Ruby (IRB)
   2. Ruby Script
   3. Embedded Ruby (ERB) in a view
   4. In the Rails controller
   5. From the Rails Console
   6. Load from the Rails Console
   7. From a Rails seed file.
   
   
10. Which of these are legal Ruby variable names?.
    

    abc (legal)   
    
    b768 (legal)  
    
    4slots (illegal, starts with a digit)   
    
    First Name (illegal, can't have embedded space)   
    
    _n_ (legal)  
    
    hook&ladder (illegal, can't use &)  
    
    numberOfCustomers (legal, but not recommended Ruby syntax. Use underscore notation.)
    
    number-of-customers (illegal, can't use dash in variable name)
    
    PreferredCustomer (legal as a class name, which uses upper camel casing)  
    
    an_extremely_long_variable_name_if_you_ask_me (legal, but should be shorter)
    

11. Try out these Ruby statements in IRB. What do you think is happening?
    

    a = "Jeremy"
    b = 342
    print "My friend #{a} owes me $#{b}.\n"
    

    Ans: This is an example of variable interpolation. #{a} means insert the value of the variable a at this location in the string.  
12. Change the double quotes in Question 11 to single quotes. Explain.
    Ans: Variable interpolation and escape characters only work with double quotes, not single quotes.

Ruby Details

• Constants and Variables
   
• Operators
   
• Look at Examples 15 to 19.
   
• 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
  

• To modify command prompt Ruby code to display output on a Rails view: change
  

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

  to

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

  This code is placed into the controller method corresponding to the view that will contain the output. Then put this ERB code on the view page:
  

  <%= @f %>
  

Classes and Objects

• An object contains data values and methods for getting things done.
   
• Methods are also called functions or procedures.
   
• In a Ruby class, the data values are represented by instance variables, prefixed by @.
   
• A class is the design or blueprint for an object.
   
• Here is a Miller analogy:
  

  object : class :: cookie :: cookie-cutter
  

• See the Dice Example and Project 3 for examples of Ruby classes.
   
• Suggest plausable properties and methods for these classes:
   
  
  Quarterback   Windows Button   Dice   VendingMachine
  
  
• What do these Ruby statements do?

  x = 5
  x.class
  x.methods
  

UML Diagrams

• UML means Unified Modeling Language, which was a unification of the methodologies of James Rumbaugh's Object-modeling Technique (OMT) and Grade Booch's Object-oriented Design (OOD) in 1994.
   
• Software engineers use UML diagrams to represent the properties and methods of a class.
   
• Here are some examples of UML diagrams for Ruby classes that we will use in this course.
   
• There are three sections to a UML diagram: (1) the class name, (2) the instance variables, (3) the class methods.
   
• The instance variables are marked with - (minus sign) because they are private: they can only be used within the class where they are defined. This includes on an ERB view that is associated with the controller class where the instance variable is defined.
   
• The class methods are marked with + (plus sign) because they are public. They can be used in any Ruby project that includes the class that defines the public method.
   
• To write the UML diagram for a class like VendingMachine, for which you have the source code, search through the class to find all of the instance variables, noting their types. (Recall that an instance variable has the prefix @. Then find all the methods, marked with def.
   
• Write Ruby code to test this builtin Ruby class String.
  Ans: Here is the test code.

Project 3

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

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.

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. (We have already covered the ActiveRecord methods new, save, first, last, and count.
   
• 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.