To Lecture Notes

IT 231 -- 8/13/12

Takehome Midterm

• Go over the Takehome Midterm Exam.

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. For example:
  

  # Create Range object with short method.
  r = 1..5
  
  # Create Range object with long method.
  s = Range.new(1, 5)
  
  print s.begin, "\n"
  # Returns 1.
  
  print s.end, "\n"
  # Returns 5.
  
  print s.includes?(3), "\n"
  # Returns true.
  
  print s.includes?(8), "\n"
  # Returns false.
  
  print s == r, "\n"
  # Returns true.
  
  print s == (2..8), "\n"
  # Returns false.
  

• 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
  

Review Questions

Pay particular attention to Questions 8 and 9. Similar questions will be on the final exam.

1. Write a Ruby statement to create an empty array. Ans:
   

   a = [ ]
   or
   a = Array.new
   

2. What is the output?
   

   a = ["a", "bc", "efg", "hijk"]
   print "1#{a[1]}2#{a[3]}3#{a[5]}\n"
   
   Ans:
   
   1bc2hijk3
   

3. What is the output in each case? What does the Ruby code do?
   

   b = [56, 43, 11, 26, 98, 30]
   
   # Problem 14a.
   s = 0.0
   b.each do |x|
     s += x
   end
   print s / b.length
   Ans: It computes the average of the array items: 264.
   
   # Problem 14b.
   b.each do |y|
     if y > 50
        print y, " "
     end
   end
   Ans: It prints all the values greater than 50:
   56
   98
   
   # Problem 14c.
   m = -1000
   b.each do |n|
     if n > m
       m = n
     end
   end
   print m
   
   Ans: m is the running maximum.  Each loop of the for statement checks if n
   is greater than the current running max m.  If it is, m is updated.
   m contains the maximum value at the end of the for loop:
   98
   
   # Problem 14d.
   c = Array.new 
   b.each to |x|
     c << x * 5280
   end
   print c
   
   Ans: It creates a new array with the values in the array a converted 
   from miles to feet:
   [295680, 227040, 58080, 137280, 517440, 158400]
   

4. Write a Rails view that displays 1000 lines on a page like this:

   Line 1

   Line 2

   ...

   Line 999

   Line 1000

   Use the range 1..1000 with a for loop.
   

   # Ans: here is the view code:
   
   <Display 1000 Lines>
   
   <% for i in 1..1000 %>
   <p>Line <%= i %></p>
   <% end %>
   
   

5. What does each of these Ruby code fragments do?

   # Problem 7a.
   (1..3).each do |n|
      (1..4).each do |m|
         print "*"
      end
   end
   print "\n"
   
   Ans:
   ************
   
   # Problem 7b.
   (1..3).each do |n|
      (1..4).each do |m|
         print "*"
         print "\n"
      end
   end
   
   Ans:
   *
   *
   *
   *
   *
   *
   *
   *
   *
   *
   *
   *
   
   # Problem 7c.
   (1..3).each do |n|
      (1..4).each do |m|
         print "*"
      end
      print "\n"
   end
   
   Ans:
   ****
   ****
   ****
   

6. What does this Ruby code do? Replace the for statement with one using the Array each method:
   

   s = ["cat", "dog", "mouse"]
   t = ""
   s.each do |x|
     t = x + t
   end
   print t, "\n"
   
   Ans:
   mousedogcat
   

7. What does the <pre> tag do?
    
   Ans: Anything between begin and end pre tags preserves whitespace. Newline characters (denoted as "\n" in Ruby) are not eliminated as they are in the browser if the pre tags are not present.
    
8. * For a Student model with fields first_name, gender, year, gpa, write controller and view statements that will display these sentences with each ?? replaced by the correct number.
    
   
   1. The total number of students is ??. Ans:
      

      <p>The total number of students is 
      <%= Student.count %>.</p>
      

   2. The number of women students is ??.
      

      <p>The number of woman students is 
      <%= Student.find_all_by_gender('F').count %>.</p>
      

   3. The number of sophomore students is ??.
      

      <p>The number of sophomores is 
      <%= Student.find_all_by_gender(2).count %>.</p>
      

9. * In a scaffold-based project with model Order and fields customer_name, credit_card_number, credit_card_type, and order_date, the file _form.html.erb contains this ERB code for inputing credit_card_type:
   

   <%= f.label :credit_card_type %><br />
   <%= f.text_field :credit_card_type %><br />
   

   1. Replace the textfield with a select control (dropdown menu) having options Discover, Master Card, and Visa. Ans:
      

      <%= f.label :credit_card_type %><br />
      <%= f.select :credit_card_type,
                [['Discover', 'D'],
                [['MasterCard', 'M'],
                [['Visa', 'V']]          %><br />
      

   2. Replace the textfield with three radio button controls that correspond to Discover, Master Card, and Visa.
      

      <%= f.label :credit_card_type %><br />
      Discover 
      <%= f.radio_button :credit_card_type, 'D', :checked => false %>
      MasterCard
      <%= f.radio_button :credit_card_type, 'M', :checked => false %>
      Visa 
      <%= f.radio_button :credit_card_type, 'D', :checked => true %>
      

Review for Final Exam

• The final will be held on Wednesday, August 15.
   
• Final Exam review materials are on the Exam Info Page.