1. Variable Review & IO
User
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2. Introduction Review variable typing and dimension statement format
Data type conversion
File IO
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3. Variable Initialization
Numeric variables are automatically initialized to 0:
Dim varName As Double
To specify a nonzero initial value
Dim varName As Double = 50
declares a variable named varName to be of type Double. Actually, the Dim statement
causes the computer to set aside a location in memory with the name varName. Since
varName is a numeric variable, the Dim statement also places the number zero in that
memory location. (We say that zero is the initial value or default value of the variable.)
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4. Multiple Declarations
multiple-declaration statements :
Dim a, b As Double
Dim a As Double, b As Integer
Dim c As Double = 2, b As Integer = 5
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5. Data Conversion Type-casting functions
Converts data from one type to another
such as CDbl(n), Cint(n), CStr(n)
Previously we used intrinsic functions to convert numeric to string and string to numeric Val(n), Str(n)
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6. Data Conversion numVar = CDbl(txtBox.Text) txtBox.Text = CStr(numVar)
Converts a String to a Double
Converts a number to a string
CType(numVar,string) ‘Alternative conversion methods
Convert.ToString(numVar)
numVar.ToString Str(numVar)
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7. CType() CType(expression, typename)
expression – any variable or string literal expression
typename – any VB datatype, object, class or structure allowed with in the VB framework
Data Conversion
numVar = CDbl(txtBox.Text)
txtBox.Text = CStr(numVar)
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8. ANSI Character Set If n is between 32 and 255, str
Chr(n):returns the string consisting of the
character with ANSI value n
Chr(65) A
Asc(str):returns the ANSI value of the first
character of str
Asc(“Apple”)
32 & Chr(176) & “Fahrenheit ° Fahrenheit
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9. Strings String literal
a sequence of characters treated as a single item, surrounded by “”
String variable
a name used to refer to a string
Dim varName As String
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10. String Properties and Methods
A string is an object, like controls, has both properties and methods.
Length
ToUpper
Trim
ToLower
IndexOf
Substring
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11. String Properties str.Length: the number of characters in str
Where str is any string literal or attribute
the number of characters in str
“Tacoma".Length is 5
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12. String methods str.ToUpper format str.ToLower
with all letters of str capitalized
format
“Tacoma".ToUpper is TACOMA.
str.ToLower
with all letters of str in lowercase format
“TCC1926".ToLower is tcc1926
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13. Concatenation
Combining two strings into a new string.Concatenation is represented by “&”
Dim str1 As String = “My GPA is A "
Dim Str2 As String = “in Winter 2002"
txtOutput.Text = str1 & str2
displays
My GPA is A in Winter 2002
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14. Concatenation Combining strings with numbers into a string
Dim str1 As String = “My grade is “
Dim grade As Double = 89
Dim str2 As String = “in Winter 2002"
txtOutput.Text = str1 & grade & str2
displays
My GPA is 89 in Winter 2002
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15. String Methods str.Substring(m,n) str.Substring(m)
substring consisting of n characters
beginning with the character in position m in
str
“Washington”.Substring(0,4) is “Wash”
str.Substring(m)
substring beginning with the character in
position m in str until the end of str
“Washington”.Substring(2) is “shington”
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16. The Empty String zero-length string ""
lstBox.Items.Add("") skips a line
txtBox.Text = "“ or
txtBox.Clear() clear the text box
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17. Initial Value of a String
A string should be assigned a value before being used
By default the initial value is Nothing
Strings can be given a different initial value as follows:
Dim today As String = "Monday“
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18. Option Strict
The Option Strict statement must appear before any other code
Restricts implicit data type conversions to only widening conversions
Provides compile-time notification of data lost conversions
Generates an error for any undeclared variable
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19. Option Strict The default is Off Option Strict { On | Off } On:
Optional. Enables Option Strict checking
Off :
Optional. Disables Option Strict checking
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20. Option Strict Example Option Strict On Dim MyVar As Integer
Dim Obj As Object
MyVar = 1000
MyVar = ‘ Data lost
MyInt = 10 ' Undeclared variable
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21. Using Text Boxes for Input and Output
The contents of a text box is a string
The Property “Text” can only be assigned to a string variable type
Assigning input
strVar = txtBox.Text
When using the textbox for output
Assigning Output set(ReadOnly = true)
txtBox.Text = strVar
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22. Data Files and IO Streams
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23. Creating a Sequential File
Choose a filename – may contain up to 215 characters
Select the path for the folder to contain this file
Execute a statement like the following:
Dim sw As IO.StreamWriter = IO.File.CreateText(filespec)
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24. Open Modes OpenText Format of the IO statement to read data:
Dim sr As IO.StreamReader = IO.File.OpenText(“Colleges.txt")
The argument “sr” is used here to indicate a read string
CreateText
Format of the IO statement to write data:
Dim sw As IO.StreamWriter = IO.File.CreateText(“Colleges.txt")
The argument “sw” is used here to indicate a write string
AppendText
Format of the IO statement to Add data:
Dim sw As IO.StreamWriter = IO.File.AppendText(“Colleges.txt")
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25. Open Modes
CreateText = Opens the file and clears the file, it writes only new data to the file
OpenText = Opens the file so that the records can be read
AppendText = Opens the file and writes new records to the end of the file
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26. Adding Items to a Sequential File
Execute the statement
Dim sw As IO.StreamWriter = IO.File.AppendText(filespec)
where sw is a variable name used to store the data in memory and filespec identifies the file.
Place data into the file with the WriteLine method.
After all the data have been recorded into the file, close the file with the statement sw.Close()
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27. Creating a Sequential File…
Place a line of data into the file with a statement of the form:
sw.WriteLine(attribute)
Close the file when all data has been added:
sw.Close()
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28. Example 1 Private Sub btnCreateFile_Click(...) _
Handles btnCreateFile.Click
Dim sw As IO.StreamWriter =
IO.File.CreateText(“Colleges.TXT")
With sw
.WriteLine(“Missouri Western")
.WriteLine(“5475")
.WriteLine(strCollege)
.WriteLine(strFaculty)
.Close()
End With
End Sub
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29. Caution
If an existing file is opened for output, the computer will erase the existing file and create a new one.
What open mode will clear the file?
What open mode will add to the file?
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30. Adding Items to a Sequential File
Execute the statement
Dim sw As IO.StreamWriter =
IO.File.AppendText(filespec)
where sw is a variable name to store the date in memory and filespec identifies the file.
Write to the file with the WriteLine method.
After all the data have been recorded close.
sw.Close()
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31. IO.File.AppendText Append adds to the end of the file
If a file does not exist, it will create one
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32. Sequential File Modes CreateText OpenText AppendText
A file should not be opened in two different modes at the same time.
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33. Code to Read from the file
' Read data from a file and display in a list box
Private Sub btnRead_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnRead.Click
' Establish a communication link
Dim sr As IO.StreamReader = IO.File.OpenText("Grades.txt")
' Read data line by line and display them in listbox
lstDisplay.Items.Clear()
lstDisplay.Items.Add(sr.ReadLine())
' Terminate the communication link
sr.Close()
End Sub
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34. Input Dialog Box
strVar = InputBox(prompt, title)
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35. Code for Input box
' Read data from a file ans display in a list box
Private Sub btnRead_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnRead.Click
Dim fileName As String ‘set an argument for the file name
Dim sr As IO.StreamReader ‘set an argument as the read string
' Get the file name from user
fileName = InputBox("Enter the file name", "Enter file name")
' establish a communication link
sr = IO.File.OpenText(fileName)
' Read data line by line, display them and the average
lstDisplay.Items.Clear()
lstDisplay.Items.Add(sr.ReadLine())
' Terminate the communication link
sr.Close()
End Sub
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36. Format Functions FormatNumber(n,r) FormatCurrency(n,r)
FormatPercent(n,r)
return a string value
n - a number, an numeric expression or a
string to be formatted
r - the number of decimal places
default value is 2
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37. Formatting Output with Format Functions
String Value
FormatNumber( ,1)
FormatNumber(1 + 2)
12,345.6
3.00
FormatCurrency( ,2)
FormatCurrency(-100)
$12,345.63
($100.00)
FormatPercent(0.185,2)
FormatPercent(“0.07”)
18.50%
7.00%
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38. Code the Output ' Format data using format functions
Private Sub btnFormat_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnFormat.Click
lstDisplay.Items.Add(strCollege)
lstDisplay.Items.Add(FormatNumber(lngFaculty))
lstDisplay.Items.Add(FormatNumber(lngStudents))
lstDisplay.Items.Add(FormatPercent(sngRatio,2))
End Sub
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39. Formatting Output with Zones
Line up items in columns
Use a fixed-width font such as Courier New
Divide the characters into zones with a format string.
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40. Formatting output with zones
Zone width
Dim fmtStr As String = "{0, 15}{1, 10}{2, 8}“
lstOutput.Items.Add(String.Format(fmtStr, _
data0, data1, data2))
Zone number
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41. Numbers with FMT-zones
A colon and formatting symbol after width to specially format numeric data
Zone Format Term
Data
Number displayed
{1,12:N3}
{1,12:N0}
34.6 34
{1,12:C1}
$1234.6
{1,-12:P}
0.569
56.90%
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42. Formatting with Zones Zone width
left adjusted if preceded with minus sign, right adjusted otherwise
Spaces between the successive pairs of brackets will be displayed in the corresponding zones in the output.
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43. Code Example
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44. Dim sr As IO.StreamReader Dim fmtStr As String = "{0,-15}{1,15}{2,15}"
' Read data from a file ans display in a list box
Private Sub btnRead_Click(ByVal sender As System.Object, ByVal e As
System.EventArgs) Handles btnRead.Click
Dim fileName As String
Dim sr As IO.StreamReader
Dim fmtStr As String = "{0,-15}{1,15}{2,15}"
' Get the file name from user
fileName = InputBox("Enter file name", “File name")
' Establish a communication link
sr = IO.File.OpenText(fileName)
' Read data line by line, display them and the average
lstDisplay.Items.Clear()
lstDisplay.Items.Add(String.Format(fmtStr, “City", “Population", “Jobs"))
lstDisplay.Items.Add(String.Format(fmtStr, sr.ReadLine(), sr.ReadLine(), sr.ReadLine()))
' Terminate the communication link
sr.Close()
End Sub
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45. Using a Message Dialog Box for Output
MsgBox(prompt,title)
MsgBox(prompt, , title)
is executed, where prompt and title are strings, a message dialog box appears with prompt
displayed and the title bar caption title and stays on the screen until the user presses
Enter, clicks on the box in the upper-right corner, or clicks OK. For instance, the state-ment
MsgBox("Nice try, but no cigar.", , "Consolation")
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46. Code 'Message sent before clearing the items in a list box
Private Sub Button1_Click(ByVal sender As System.Object,
ByVal e As System.EventArgs) Handles Button1.Click
MsgBox(" All items will be deleted", , "Delete items")
lstDisplay.Items.Clear()
End Sub
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