Working with data

1
Working with data

• CS0004
• Lecture 11
• Chapter 6.2 (review) and 7.1

2
Administration

• Project 2 is posted
• A bit harder but similar to last project
• Use random number for dice roll
• Use functions to break tasks into smaller pieces
• Use loops to play like a computer
• Due next Wednesday (get working on it)

3
Reading Data From Files

• StreamReader
• Dim name As String
• Dim wage, hours As Double
• Dim sr As IO.StreamReader _
• IO.File.OpenText("PAYROLL.TXT")
• name sr.ReadLine
• wage CDbl(sr.ReadLine)
• hours CDbl(sr.ReadLine)

4
Lets Do Something

• Given a file containing phone numbers lets create
  a program that searches through the file for a
  given name and displays the phone number.

5
Recap

• Variables (Dim x As Integer)
• If statements
• Select Case Statements
• Procedures and Function
• Do While Loop
• Do Until Loop
• For Next
• StreamReader

6
Simple and Array Variables

• A variable (or simple variable) is a name to
  which Visual Basic can assign a single value.
• An array variable is a collection of simple
  variables of the same type to which Visual Basic
  can assign a list of values.

7
Example

• Suppose you wanted to evaluate exam scores for a
  class of 30 students.
• What would you need to store each of these values?

Dim score1 As DoubleDim score2 As DoubleDim
score3 As Double Dim score30 As Double
8
Using Arrays

• Instead of creating a simple variable for each
  students scores we can use an array to store a
  list of scores
• Think of an array as just a list of numbers

9
Using Arrays
Size of the array (ex 30 student scores)
Dim student_score(29) As Double
Array name
Data type

• 29 is actually an upper bound on the array
• Starts at 0 not 1 so size is actually 29 1 30

10
Using Arrays
student_store(0) 0.93
subscript
Read "student sub zero equals 0.93" Which means
that the double 0.93 is being stored at the first
location in the array called student_score
because all arrays begin counting at 0.
11
Array Terms

• Dim arrayName(n) As DataType
• 0 is the "lower bound" of the array
• n is the "upper bound" of the array the last
  available subscript in this array
• The number of elements, n 1, is the size of the
  array

12
Example
txtNumber
txtWinner
13
Example

• Private Sub btnWhoWon_Click(...) _
• Handles
  btnWhoWon.Click
• Dim teamName(3) As String
• Dim n As Integer
• 'Place Super Bowl Winners into the array
• teamName(0) "Packers"
• teamName(1) "Packers"
• teamName(2) "Jets"
• teamName(3) "Chiefs"
• 'Access array
• n CInt(txtNumber.Text)
• txtWinner.Text teamName(n - 1)
• End Sub

14
Example Output
15
Initializing Arrays

• Arrays may be initialized when they are created
• Dim arrayName() As varType value0,value1,value2
  , ..., valueN
• declares an array having upper bound N and
  assigns value0 to arrayName(0), value1 to
  arrayName(1), ..., and valueN to arrayName(N).

16
Getting the upper bound

• The value of
• arrayName.GetUpperBound(0)
• is the upper bound of arrayName().

17
Example

• Dim teamName() As String "Packers",
  "Packers", "Jets", "Chiefs"
• txtBox.Text CStr(teamName.GetUpperBound(0))
• Output 3

18
ReDim Command

• The size of an array may be changed after
• it has been created.
• ReDim arrayName(m)
• where arrayName is the name of the
• already declared array and m is an Integer
• literal, variable, or expression, changes the
• upper bound of the array to m.

19
Preserve Contents (ReDim)

• ReDim arrayName(m)
• resets all values to their default. This can be
  prevented with the keyword Preserve.
• ReDim Preserve arrayName(m)
• resizes the array and retains as many
• values as possible.

20
Counting Letters
Page 315 in book
21
Out of Bounds Error

• The following code references an array element
• that doesn't exist. This will cause an error.
• Dim trees() As String "Sequoia",
• "Redwood","Spruce"
• txtBox.Text trees(5)

22
Copying Arrays

• If arrayOne() and arrayTwo() have been declared
• with the same data type, then the statement
• arrayOne arrayTwo
• makes arrayOne() an exact duplicate of
• arrayTwo(). It will have the same size and
• contain the same information.

23
Erasing Arrays

• Arrays take up large amount of room
• Sometimes you want to delete them
• Erase arrayName