To Lecture Notes

IT 231 -- 8/6/12

Review Questions

1. What is the value of this Ruby expression?
   

   27 % 8
   
   Ans: 3, because the remainder when you divide 27 by 8 is 3.
   

2. What is the output:
   

   a = "apple"
   if a.length < 4
      b = "pear"
      print "grapefruit\n"
   else
      b = "apricot"
      print "orange\n"
   end
   print "a #{a} b #{b.reverse}\n"
   

   Ans: Note than when an assignment is made, no output occurs until a print statement is executed.
   In line 1, a is assigned the value of "apple".
   Since the length of a is 5 characters, lines 3 and 4 are not executed because only the else statements (lines 6 and 7) are executed.
   This causes b to be assigned the value of "apricot" and the first output "orange" to be printed. Because of the \n, orange is on a line by itself.
   Now the final print statement in line 9 occurs: The literal values are printed, except when #{a} or #{b} occurs.
   #{a} and #{b} mean insert the values a and b into the expression. The output from lines 6 and 7 is
   

   "a" + value of a + "b" + value of b reversed, which is
   "a " + "apple" + " b " + "tocirpa" + "\n"
   

   The complete output is:
   

   orange
   a apple b tocirpa
   

3. What is the output?
   

   n = 45
   m = 31
   print m >= n, "\n"
   
   s = "dog"
   t = "cat"
   print s >= t, "\n"
   
   a = Time.now
   b = Time.new(2012, 1, 1, 0, 0, 0)
   print a >= b, "\n"
   
   Ans:
   false   (It is false that 31 >= n.)
   true    (It is true that "dog" comes after "cat" in alphabetic order.)
   true    (It is true that the time a comes after the time b.)
   

4. Create a Rails view that displays randomly one of the images 1.png, 2.png, 3.png, or 4.png. Ans:
   

   # Long version of controller code:
   number = 1 + rand 4
   if number == 1
      img = "1.jpg"
   elsif number == 2
      img = "2.jpg"
   elsif number == 3
      img = "3.jpg"
   else
      img = "4.jpg"
   end
   
   # Short version of controller code:
   number = 1 + rand(4)
   img = "#{number}.jpg"
   
   # Even shorter version:
   img = "#{1 + rand(4)}.jpg"
   
   # View code:
   <%= image_tag img, :class = "imgclass" %>
   
   # CSS code for image class:
   .imgclass { width: 200px; }
   

5. What does the String method chomp do?
   Ans: It removes a trailing \n character from a string.
    
6. Modify Example 21 (LongSong Example) so that it accepts input from the keyboard.
   Ans: Replace the line

   n = 10 + rand(90)
   

   with
   

   print "How many verses do you want?"
   n = STDIN.gets.chomp.to_i
   

7. Rewrite this Ruby source code to display the output in a view.
   Ans: Here are the controller and view code:
   

   #--- app/controllers/prediction_controller.rb
   class PredictionController < ApplicationController
   	
     def show
     	number = rand(6)
       if number == 0
         response = 'As I see it yes'
       elsif number == 1
         response = 'Most likely'
       elsif number == 2
         response = 'Reply hazy, try again'
       elsif number == 3
         response = 'Cannot predict now'
       elsif number == 4
         response = 'My sources say no'
       elsif number == 5
         response = 'Very doubtful'
       end
       @prediction = "My prediction is: #{response}.\n"
   
     end
   end
   
   #--- app/views/prediction/show.html.erb
   <h1>Show Crazy Eight Ball Prediction</h1>
   
   <p> <%= @prediction %< </p>
   

8. Create the Ruby arrays letters1 and letters2 containing the letters "a", "b", "c", "d" using the short method and the long method, respectively. Ans:
   

   letters1 = ["a", "b", "c", "d"] 
   letters2 = Array.new
   letters2 << "a"
   letters2 << "b"
   letters2 << "c"
   letters2 << "d"
   print letters1, "\n", letters3, "\n"
   

9. Give some examples of one-to-many relationships. Ans:
    
   

   Primary Model	Secondary Model	Foreign Key
   * Movie	Review	movie_id
   * Student	Owner	student_id
   * Pet	Pet	owner_id
   AirlineFlight	Passenger	airline_flight_id
   Store	StockItem	store_id
   SportsTeam	Player	sports_team_id

   
   * = do not use for your Project 5 because it was a class example.
    
10. What is a foreign key?
    Ans: The field that links a record in the secondary model to its owner in the primary model. Rails foreign keys always have the suffix "_id".
     
11. List the steps to setting up a one-to-many relationship.
    Ans: See the One-to-many Relationships Tutorial.
     
12. What does a one-to-many relationship do for you in a Rails scaffold project?
    Ans: If Student is the primary model, Award is the seconary model, s is a Student object, and a is an Award object,
    1. s.awards gives the array of all Award objects that belong to s;
    2. a.student gives the Student object that owns a.

The Array Method collect

• The Array method collect is used to build a new array out of an existing array. It collects all the outputs from the code block and collects them all together go build a new array.
   
• Examples:   What is the output of each of these Ruby code fragments:

  a = [2, 6, 4, 5, 1]
  
  # Example 1
  b = a.collect do |x|
     2 * x
  end
  print b, "\n"
  
  Ans: [4, 12, 8, 10, 2]
  
  # Example 2
  b = (0..5).to_a.collect do |x|
     2 * x + 1
  end
  print b, "\n"
  
  Ans: [1, 3, 5, 7, 9, 11]
  
  # Example 3
  b = (1..10).to_a.collect do |x|
     [x, x**2]
  end
  print b, "\n"
  
  Ans: [[1, 1], [2, 4], [3, 9], [4, 16], [5, 25],
        [6, 36], [7, 49], [8, 64], [9, 81], [10, 100]]
  

One-to-many Relationships

• Complete the the One-to-many Relationships Tutorial.

The Range Class

• Look at the Range Class UML Diagram.
   
• Two ways to create a new Range object.
  

  # 1. Create a range that includes the end using new method.
  r = Range.new(1, 5)
  
  # 2. Shortcut to create a range that includes the end.
  r = 1..5
  

• Write code that tests some of the methods of the Range class in the UML diagram.
   
• Ruby code that tests the VendingMachine class using a nested for statement with ranges:
  

  vm = VendingMachine.new
  (1..6).each do |i|
     (1..3).each do |j|
        vm.deposit_quarter
     end
     vm.select_candy
  end
  

Sending Data from a View Back to the Controller

• To send data from a view to the controller, data is entered onto a form via HTML controls:
   
  
  Textfield:  
  

  <input type="text" name="txtName" size="20" />
  

  Dropdown Menu:   Discover Master Card Visa
  

  <select name="ddmCardType">
      <option value="1">Discover</option>
      <option value="2">Master Card</option>
      <option value="3">Visa</option>
  </select>
  

  Check Box:   Fulltime Student
  

  <input type="checkbox" name="txtFultimeStatus" /> Fulltime Student
  

  Radio Button:   Discover    Master Card    Visa
  

  <input type="radio" name="cardType" /> Discover &nbnsp;
  <input type="radio" name="cardType" /> Master Card &nbnsp;
  <input type="radio" name="cardType" checked="checked" /> Visa
  

• To send data back to the server, these controls must be placed onto a form:
  

  <form name="frmMain" action="summary.php" method="get">
      ... Controls go here.
  </form>
  

• See Examples 11 and 12 that show how to submit data back to the server running PHP using HTML.
   
• In Rails, we use FormTagHelper methods (for example form_tag, text_field_tag, and submit_tag) to generate the form and the controls.
   
1. Form
   

   <%= form_tag 'show' do %>
   ... Controls go here.
   <% end %>
   

   1. 'show' is the action, which is taken when the submit button is clicked.
       
   2. do means that everything until the end statement is included in the form.
       
   3. The equals sign is not used in the Rails delimiter for the end statement because end is a control statement.
   
   
2. Textfield
   

   <%= text_field_tag :cel, @c, :class => 'ctrl' %>
   

   1. :cel is the name of the textfield control.
       
   2. @c is the initial value of the textfield.
   
   
3. Submit
   

   <%= submit_tag 'Convert Temperature', :class => 'ctrl' %>
   

   1. 'Convert Temperature' is the text of the submit control.
       
   2. When the submit control is clicked, the action specified in the form_tag control is executed.
   
   
• See Example 23 (CToF2 Example).
   
• For Project 5, we will use a different style of helper methods called FormHelper methods that interface directly with the model.
   

Catching Data Sent from a View by the Controller

• Data that are sent back to the controller from a view are caught in a params statement.
   
• For example, Example 23 contains the following ERB code:
  

  <%= form_tag 'show' do %>             
  <%= text_field_tag :cel, @cel %>
  <%= submit_tag 'Convert Temperature' %>
  <% end %>
  

• The form tag shows that the show action is taken (goes to the show view), when the submit tag is clicked.
   
• This implies that the contents of each control on the form (between form_tag and end) is sent back to the controller.
   
• The piece of data sent back to the controller is identified by the label of the corresponding controller (in this case :cel).
   
• For example, the contents of the text field is identified by the symbol :cel.
   
• The leading colon shows that :cel is a symbol, not a variable or string.
   
• Symbols are used for control names in Rails because they are processed more efficiently than strings or variables.
   
• Here is the Example 23 controller code that catches the data from the text field (identified with the symbol :celsius) in a params statement and assigns it to the variable input:
  

  input = params[:cel]
  @c = input.to_i
  

  The String input is then converted to the integer @c.
   
• The file config/routes.rb contains the line
  

  get "/convert/show"
  

  It specifies the route with controller = convert, action = show, and method = get as a named route.
   
• Recall that the HTTP verb GET is used to retrieve a web page when the user types the URL into a browser, or as the target page to retrieve with a hyperlink.
   
• However, when Rails uses HTTP to send data back to the server to be processed by the controller, the POST verb is used.
   
• To allow the POST verb to be used for sending the contents of :celsius back to the controller temp_version using the action convert, add this line to config/routes.rb:
  

  post "/convert/show"
  

• Practice Problem:   Convert the TemperatureDescriptors Example (Example 18) to a Rails project that accepts the temperature in a textfield and then shows the descriptor on the view. Modify the Ruby code of Example 18 to work in the controller.

Hex Color Codes

• Discuss Hex Color Codes.
   
• A Hex to Decimal Conversion Table
   
• Match Color Game
   
• While most browsers accept color codes with decimal components like this:
  

  body { background-color: rgb(245,245,255) }
  

  such a color code is not strictly XHTML complient.
   
• To be complient, Use a hex color code instead:
  

  body { background-color: #f5f5ff; }
  

• The only XHTML complient methods for specifying a color are:
   
  
  1. use a hex color code,
      
  2. use one of the seventeen HTML named colors:
      
     
     aqua  black  blue  fuschia  gray  green  grey  lime  maroon  navy  olive  purple  red  silver  teal  white  yellow
     
     
• Even the extended HTML colors that are not one of the 17 HTML colors are not XHTML complient.
   
• In addition to being XHTML complient, hex color codes are standard for professional web designers.

The DisplayGradeData Example

• Create the Rails project as specified in Example 26 (DisplayGradeData Example).