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Lecture Summary for IT 236 on 6/22/06

Review Questions

1. What is the precedence of these VB.Net operators:
   &   .   ( )   =   ^   <   *   +
   Ans: ( )   .   ^   +   *   &   = (comparison)   <   = (assignment)
   The comparison = and < have the same precedence.

2. Name two methods that can use the format string #0.000.
   Ans: Double.ToString and WriteLine. The String.Format method can also use the format string.

3. What is the difference between Public, Private and Dim?
   Ans: Public and Private variables are declared outside of any method, but inside of a class or module. A Public variable is accessible anywhere within the solution but a Private variable is accessible only within the class or module where it is defined. Dim refers to a local variable declared within a method. A local variable is only accessible within the method within which it is defined.

4. What is the difference between an If statement and a While statement?
   Ans: An If statement is executed zero or one time. A While statement is repeatedly executed zero or more times until the condition is false.

5. Which of these is correct?
   ElseIf     Else If     EndIf     End If
   Ans: ElseIf and End If are correct.

6. Convert the following statements to a Select statement:
   If x = 3 Then
       y = "a"
   ElseIf 4 <= x And x <= 7 Then
       y = "b"
   Else
       y = "c"
   End If

   Ans:
   Select Case x
       Case 3
          y = "a"
       Case 4 To 7
          y = "b"
       Case Else
          y = "c"
   End Select

7. Write an If..Else statement that writes Yes if CheckBox1 is checked and No otherwise.
   Ans:
   If CheckBox1.Checked Then
       WriteLine("Yes")
   Else
       WriteLine("No")
   End If

8. Write a statement that writes True if CheckBox1 is checked and False otherwise.
   Ans:
   WriteLine(CheckBox1.Checked)

9. What is the default event for the TrackBar control?
   Ans: Scroll

10. Name five properties of the TrackBar control.
    Ans: Value, Maximum, Minimum, SmallChange, LargeChange, TickFrequency.

11. Give two other ways of changing the location and size of a control besides setting its Left, Top, Width and Height properties separately.
    Ans: Represent these properties by x, y, w and h, respectively.
    
    • Using the SetBounds method:
      txtName.SetBounds(x, y, w, h)
      
    • Setting the Location and Size properties:
      txtName.Location = New Point(x, y)
      txtName.Size = New Size(w, h)
      

Boolean Expressions

• The comparison operators are
  <    <=    >    >=    <>    =

• Do not use != for not equals.

• The output of the comparison operators is always a Boolean value.

• The precedence of the comparison operators is lower than the arithmetic and concatenation operators, but higher than the assignment operators.

• If s and t are String variables, s < t means "precedes in alphabetic order."

• Examples: Suppose x and y are defined as
  Dim x = 3, y As Integer = 5
  Here are some expressions involving relational operators and their Boolean values:

  Expression	Value
  x >= y	False
  2 * x > y	True
  x <> x	False
  "cat" <= "dog"	True

• The logical operators
  And    Or    Not
  are defined as follows:

  True And True = True
  True And False = False
  False And True = False
  False And False = False

  True Or True = True
  True Or False = True
  False Or True = True
  False Or False = False

  Not True = False
  Not False = True

• The precidence of the logical operators And, Or and Not is lower than the relational operators, but higher than the assignment operators.

• Not has higher precidence than And.

• And has higher precidence than Or.

Practice Problem

1. Draw the variable trace table and predit the output.
   Dim x = 4, y As Integer = 9
   Dim f, g As Boolean
   f = (x + 5 >= y And Not y <> y) Or x + y - 3 = 9
   g = True And Not False
   WriteLine(f And Not g)

   Ans:
   f = (x + 5 >= y And Not y <> y) Or x + y - 3 = 9
     = (True And Not False) Or False
     = (True And True) Or False
     = True Or False = True

   g = True And Not False
     = True And True = True

   x	y	f	g
   4	9	True	True

   The output is
   f And Not g = True And Not True = True And False = False.

    

   Repetition

   • Do and While Loops

   • We will only use While loops in this class.

   • See the Doubling and RepSent1 examples.

   • For Loops

   • See the RepSent2 and Sum1To100 examples.

    

   Practice Problem

   1. Convert this While loop to a For loop:
      n = 5
      While n <= 20
          WriteLine(n & " " & n ^ 2)
          n = n + 3
      End While

      Answer:
      For n = 5 To 20 Step 3
          WriteLine(n & " " & n ^ 2)
      Next

    

   Arrays

   • An array is a set of variables all with the same name referenced by index.

   • Example: Defining and using an array.
     Dim a(3) As Integer
     a(0) = 3
     a(1) = 5
     a(2) = 8
     a(3) = 11
     WriteLine(a(2))

   • In the expression a(2), a is the name, 2 is the index and 8 is the value.

   • Use an array instead of a large number of related variables like
     a0, a1, ... , a999.

   • Use a For loop to print all the items in an array:
     For i = 0 To a.GetUpperBound(0)
         WriteLine(a(i))
     Next

   • See the Arrays and ArraySort examples.

   • To sort an array in reverse order, sort in normal order first, then call the Array.Reverse method:
     Array.Sort(a)
     Array.Reverse(a)

   • An alternative to the traditional For loop that is often easier to use is the For Each loop:
     For Each value In a
         WriteLine(value);
     Next

    

   Practice Problem

   1. Rewrite the Arrays Example using For Each loops instead of For loops.

      Ans: The loops look like this:
      For Each x As Integer In a
          WriteLine(x & " " )
      Next

      For Each x As String In b
          WriteLine(x & " " )
      Next

    

   The Split Function

   • The Split function extracts fields out of a delimited string.

   • The syntax for using Split is
     Dim fields() As String
     fields = Split(line, ",")

   • See these examples:
     SplitFunction (C)    ArraySearch (C) examples.

    

   Reading from a Text File

   • To read from the file "c:\file.txt", create a StreamReader object:
     reader = New StreamReader("c:\file.txt")

   • Use the ReadLine method of StreamReader to read a line from the file.

   • Use the Peek method of FileReader to check for end of file. Example:
     While reader.Peek() > -1
         word = reader.readLine()
         lengthSum += word.Length
         count += 1
     End While

   • The Peek method returns the ASCII code (which is always >= 0) of the next character in the input buffer. It returns -1 if there are no more characters in the input buffer.

   • Here is a Table of ASCII Codes (American System for Computer Information Interchange) Codes for printable characters.

   • See the FileIO Example AveWordLength (C).

   • To read from the file "c:\file.txt", create a StreamWriter object:
     writer = New StreamWriter("c:\file.txt")

   • To read from the file file.txt in the bin folder, create a StreamWriter object:
     writer = New StreamWriter("file.txt")

   • Use the Write or WriteLine methods of StreamWriter to write to an output file.

   • Close the StreamWriter object after writing to insure that the buffer is flushed:
     writer.Close( )

   • Look at this example:
     PhoneBook1 (C)

    

   The FlightInfo1 Project

   • Discuss the FlightInfo1 Project.

   • See the PhoneBook-NoArrays Example.