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Lecture Summary for IT 236 on 6/27/06

Review Questions

1. What is wrong with these two lines?
   y = a + b + c + d +_
       e + f + g + h
   Ans: The underscore must have a space before it to be a new line character.

2. Why does ReadLine( ) on a line by itself work to hold the window for the user to read in a console application?
   Ans: Because when the ReadLine method is called, the application must wait for the user to enter the information at the keyboard before continuing. The ReadLine at the end of the application tricks the program into waiting for the user to press the Enter key, then throws away the information and terminates.

3. Write a line that declares a form-wide global variable named count, initalized to 0.
   Ans: Private count As Integer = 0

4. Write code to show a message box with message "Your lucky number is #." (replace # with the value of n), caption "Lucky Number", OK button only, and display the information icon. Ans:
   MessageBox.Show("Your lucky number is " & n, _
       "Lucky Number", MessageBoxButtons.OK, _
       MessageBoxIcon.Exclamation)

5. True or false. All of the items in an array must be of the same type.
   Ans: True. An array is declared to contain objects of a specific type, although this type can be Object, which essentially means that the items can be of any type.

6. Explain the difference between declaration and instantiation of an object.
   Ans: Declaration means to create a reference variable that will refer to the object like this
   Dim a As StreamReader
   Instantiation means create the actual object like this:
   a = New StreamReader("a.txt")
   These two steps can be combined as Dim a As New StreamReader("a.txt")
   or Dim a As ArrayList = New StreamReader("a.txt")

7. What is the difference between a class and a module?
   Ans: A class is a template to create objects but objects cannot be created from a module.

8. What is the index lower bound for items in an array?
   Ans: 0.

9. Write a statement that prints the upper bound of the array a to the console. Ans:
   WriteLine(a.GetUpperBound(0))

10. Write VB.NET code that creates an array containing the odd numbers from 1 to 1000. Ans:
    Dim a(449) As Integer
    For i = 0 To 449
        a(i) = 2 * i + 1
    Next

11. Which Imports statement should be used to read from or write to a text file? Ans:
    Imports System.IO

12. Explain the difference between an absolute and a relative path for a file.
    Ans: An absolute path starts from the root folder on a specific disk drive like this:
    picDisplay.Image = Image.FromFile("c:\a.jpg")
    A relative path is specified with respect to the folder that contains the .exe. file for the application. This is the bin folder for VS 2003 and bin\Debug for VB 2005:
    picDisplay.Image = Image.FromFile("a.jpg")

13. Write the header for the Click event handler of the button btnSubmit for memory.
    Public Sub btnSubmit_Click(ByVal sender As Object, _
        ByVal e As EventArgs) Handles btnSubmit.Click

    End Sub

Alternative Ways to Create an Array

1. Dim a(3) As Integer

2. Dim a() As Integer = {3, 5, 7, 9}

3. Dim a() As Integer = New Integer(3) {}

4. Dim a() As Integer = New Integer() {3, 5, 7, 9}

5. Dim a() As Integer = New Integer(3) {3, 5, 7, 9}

6. Dim a() As Integer
   a = New Integer(3) {}

7. Dim a() As Integer
   a = New Integer() {3, 5, 7, 9}

8. Dim a() As Integer
   a = New Integer(3) {3, 5, 7, 9}

• Methods 5 and 8 are not recommended. Why?

String Objects

• Creating an object involves two steps:
  1. declaring the reference variable that will contain the address of the object,

  2. instantiating the object by calling a constructor with the New operator.

• Here are four ways to create a string object.
  Declaration and instantiation (long version) in separate lines:
  Dim s As String
  s = New String("cat")
  The New operator is used explicitly.

  Declaration and instantiation (long version) in the same line:
  Dim s As New String("cat")
  The New operator is used explicitly.

  Declaration and instantiation (short version) in separate lines:
  Dim s As String
  s = "cat"
  The New operator is used implicity.

  Declaration and instantiation (short version) in the same line:
  Dim s As String = "cat"
  The New operator is used implicity.

• String objects are immutable. This means that a string object cannot be changed after it has been instantiated. You can, however, reassign the String reference variable to a different string object.
  Dim s As String
  s = "cat"
  s = "dog"
• String properties to know for the midterm:
  Length.

• String methods to know for the midterm:
  IndexOf, IndexOfAny, Insert, Remove, Replace, Substring, ToLower, ToUpper, Trim.

• See the Strings (W) example.

• What is the output?
  s = "bcdefghijklmnop"
  v = "aeiou"
  n = s.IndexOfAny(v)
  WriteLine(n)

Practice Problems

1. Write lines of code that print Yes if the string s begins with "a" and No otherwise. Ans:
   If s.Substring(0, 1) = "a" Then
       WriteLine("Yes")
   Else
       WriteLine("No")
   End If

2. Write lines of code that print Yes if the string s ends with "a" and No otherwise. Ans:
   indexOfLast = s.Length - 1
   If s.Substring(indexOfLast, 1) = "a" Then
       WriteLine("Yes")
   Else
       WriteLine("No")
   End If

3. Write a console application that inputs a string and outputs the string reversed. Ans:
   Dim t As Index = 0
   For i = 1 To a.Length - 1
       t = s.Substring(i, 1) & t
   Next
   WriteLine(t)

Subroutines vs. Functions

• Both subroutines and functions are types of methods.

• A subroutine does not return a value.

• A function does return a value.

• Examples of subroutine invocations:
  Me.Close()

  MessageBox.Show("Hello!", "Greeting", MessageBoxButtons.OK, _
       MessageBoxIcon.Information)

• Both subroutines and functions can accept arguments, but they are not required to have arguments.

• A subroutine is invoked by placing its name on a line by itself with its arguments in parentheses.

• Even if there are no arguments in the subroutine invocation, a pair of empty parenthesis must be supplied as in Me.Close( )

• Examples of function invocations:
  x = 2 * Math.Sqrt(2) + 1

  s = String.Format("***{0}***", 5)

• A function is invoked by placing its name with its arguments into an expression. The function's return value is returned into the expression in which it was called.

Three Types of Methods

1. Event Handlers
   Event handler methods are written by the programmer and called by the operating system. For example, the event handler btnSubmit_Click is called by the operating system when when the user clicks on the button btnSubmit.

2. Builtin Methods
   Builtin methods are supplied by the system and invoked (called) by the programmer.

   Examples of builtin methods:
   System.Console.WriteLine, System.Console.Write, System.Windows.Forms.MessageBox.Show, Microsoft.VisualBasic.InputBox, System.String.Format, System.Math.Abs, System.Math.Ceiling, System.Math.Floor, System.Math.Max, System.Math, System.Math.Min, System.Math.Round.

   The ToString method is sometimes builtin but can also be user defined.

3. User Defined Methods
   User defined methods are defined by the programmer and invoked by the programmer.

User Defined Subroutines

A subroutine definition contains a header similar to this:
Public Sub F(ByVal parameter1 As Integer, _
    ByVal parameter2 As String)
    ' Subroutine body.
End Sub
• The corresponding subroutine invocation:
  F(argument1, argument2)

• Each parameter in the subroutine definition corresponds to an argument in the subroutine invocation.

• See the Subroutine (C) and OldMcDonald1 (W) examples.

User Defined Functions

• A function definition contains a header similar to this:
  Public Function F(ByVal p1 As Integer, ByVal p2 As String) As Integer
      ' Function body.
  End Function

• The corresponding function invocation:
  v = F(a1, a2)

• See the Function (C) example.

Practice Problems

• Write user defined subroutines according to the following specifications. Invoke each function in the Console Application Main method.
  1. Write a subroutine that take a name an an input string and outputs "Hello, ????." five times. Replace ???? by the input name.

  2. Write a subroutine that prints a grid of symbols like this:
     %%%%%%%
     %%%%%%%
     %%%%%%%
     %%%%%%%
     %%%%%%%

     The parameters are width, height, and symbol.

     Ans: See the SymbolGrid.

Reading from and Writing to Text Files

• Review these examples:
  AveWordLength (C)   ShowWords (W)   PhoneBook1 (C)   PhoneBook2 (W)
  PhoneBook-NoArrays (W).

More PhoneBook Examples

• PhoneBook3 (W)   PhoneBook4 (W)

The FlightInfo1 Project

• Discuss the FlightInfo1 Project.