1. Chapter 3.5 Input and Output

2. Input and Output Input Output Reading Data from Files
Getting Input from an Input Dialog Box
Formatting Output with Format Functions
Formatting Output with Zones
Using a Message Dialog Box for Output
Writing new Data to files
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Input and Output

3. Handling Files Two objects used to handle files Four step process
StreamReader used to read files
StreamWriter used to write files
Four step process
Declare an object variable to indicate object type
Create the object
Use object’s methods to perform operations
Close the object
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Input and Output

4. Declaring Object Variables
Two step process
Declare variable name and type
Dim PhoneFileI as System.IO.StreamReader
System.IO referred to as namespace
Create the object and associate file with variable
PhoneFileI = New System.IO.StreamReader(filename)
New keyword creates object and associates variable with file name
Can be combined into one step
Dim PhoneFileI as New System.IO.StreamReader(filename)
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Input and Output

5. Reading Data From a File
StreamReader object used to read from a file
StreamReader provides several methods
Peek returns the next character, but does not advance
Read reads specified number of characters and advances
ReadLine reads one line of data starting from current position
ReadToEnd reads remainder of file starting from current position
Close dissociates object from physical file
Closing the object after you have finished reading is crucial
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6. Testing for End of File Eventually, you get to the end of the file
If you try to read more data, you get a runtime error
Use Peek method to test for end of file
Peek returns the next character in the file
If there is no more data, Peek will return -1
Read more data if Peek method returns something other than -1
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Input and Output

7. Input

8. System.IO.StreamReader
IO.StreamReader (INPUT)
Reads data from sequential text files
File mode for StreamReader is OpenText
[System Keyword can be omitted]
Declare StreamReader Object
Dim sr as IO.StreamReader
SR is the variable that you will use to reference the file in your code.
Initialize the sr variable
sr = IO.File.OpenText(filespec)
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9. File Specification filespec – the path to find the file.
File specification consist of four parts
Drive:\Folder\Name.Extension
It should be assigned to a variable or enclosed in “”
“C:\data\Payroll.txt”
strFileName = “C:\data\Payroll.txt”
If the file location is the bin folder of the application only the name.extension are necessary. Example: “Payroll.txt”
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Input and Output

10. Input data from a file
Establish a communication link between computer and the disk drive for reading data from disk
Dim readerVar As IO.StreamReader = _
IO.File.OpenText(filespec)
DIM sr as IO.StreamReader = IO.File.OpenText(“Payroll.txt”)
Read data in order, one at a time, from the file with the ReadLine method assuming the file contains one item of data per line.
strVar = readerVar.ReadLine ‘readline method
terminate the communications link
readerVar.Close()’close method
1. Execute a statement of the form
Dim readerVar As IO.StreamReader
A StreamReader is an object from the Input/Output class that can read a stream of
characters coming from a disk or coming over the Internet. The Dim statement
declares the variable readerVar to be of type StreamReader.
2. Execute a statement of the form
readerVar = IO.File.OpenText(filespec)
where filespec identifies the file to be read. This statement establishes a communi-cations
link between the computer and the disk drive for reading data from the disk.
Data then can be input from the specified file and assigned to variables in the pro-gram.
This assignment statement is said to “open the file for input.”
Just as with other variables, the declaration and assignment statements in Steps
2 and 3 can be combined into the single statement
Dim readerVar As IO.StreamReader = IO.File.OpenText(filespec)
3. Read items of data in order, one at a time, from the file with the ReadLine method.
Each datum is retrieved as a string. A statement of the form
strVar = readerVar.ReadLine
causes the program to look in the file for the next unread line of data and assign it
to the variable strVar. The data can be assigned to a numeric variable if it is first
converted to a numeric type with a statement such as
numVar = CDbl(readerVar.ReadLine)
Note: If all the data in a file have been read by ReadLine statements and another item
is requested by a ReadLine statement, the item retrieved will have the value Nothing.
4. After the desired items have been read from the file, terminate the communications
link set in Step 3 with the statement
readerVar.Close()
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Input and Output

11. Input Data From A File filespec – the path to find the file.
A:\Grades.txt, C:\Grades.txt
Default folder is bin folder
Each datum (record) is retrieved as a String
The item retrieved will have value Nothing if it is the end of the file (Peek) ‘Peek Method
The data can be assigned to a numeric variable if converted into a numeric data type
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Input and Output

12. Code
Our Program is designed to read three records from a sequential data file.
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13. Code ' 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
lstDisplay.Items.Clear()
lstDisplay.Items.Add(sr.ReadLine()) ‘read line 1
lstDisplay.Items.Add(sr.ReadLine()) ‘read line 2
lstDisplay.Items.Add(sr.ReadLine()) ‘read line 3
' Terminate the communication link
sr.Close()
End Sub
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14. Input Dialog Box strVar = InputBox(prompt, title) 12/30/2018
Input and Output

15. Code 12/30/2018 Input and Output
' 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
' 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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Input and Output

16. FILE IO OUTPUT

17. System.IO.StreamWriter
IO.StreamWriter (OUTPUT)
Writes New data to a sequential file
File modes for StreamWriter
AppendText – adds records to end of file
CreateText – creates a new instance of file
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18. StreamWriter Methods Write method writes the data to the file
WriteLine method also writes data to file
Adds an “end of line” marker at end
Close method dissociates file from variable
Closing the object after you have written all your data is crucial
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Input and Output

19. Code ' Write data to the end of a data file
Private Sub btnWrite_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnWrite.Click
' Establish a communication link
Dim sw As IO.StreamWriter = IO.File.AppendText("Grades.txt")
Dim strGrade as string
' Write data line by line
strGrade = txtGrade1.text
sw.WriteLine(strGrade) ‘write line 1
strGrade = txtGrade2.text
strGrade = txtGrade3.text
' Terminate the communication link
sw.Close()
End Sub
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Input and Output

20. Formatting Report Output

21. 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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Input and Output

22. 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%
The Default # of Decimals is 2
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23. Code
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24. Code ' 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(FormatNumber( , 1))
lstDisplay.Items.Add(FormatNumber(1 + 2))
lstDisplay.Items.Add(FormatCurrency( ,2)) lstDisplay.Items.Add(FormatCurrency(-100)) lstDisplay.Items.Add(FormatPercent(0.185,2)) lstDisplay.Items.Add(FormatPercent("0.07"))
End Sub
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Input and Output

25. Formatting Output with Landing Zones
Line up items in columns
Use a fixed-width font Courier New
Divide the characters into zones with a format string.
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Input and Output

26. 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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Input and Output

27. Formatting output with zones
A colon and formatting symbol after width to specially format numeric data
Zone Format
Term
Number to be formatted
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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Input and Output

28. Formatting Output 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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Input and Output

29. Code
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30. Code 12/30/2018 Input and Output
' 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 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(String.Format(fmtStr, "First Grade", "Second Grade",
"Third Grade"))
lstDisplay.Items.Add(String.Format(fmtStr, sr.ReadLine(), sr.ReadLine(),
sr.ReadLine()))
' Terminate the communication link
sr.Close()
End Sub
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Input and Output

31. 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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32. Code 'Message are sent before clearing the items in the 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
Primarily use to display Information or Error Messages
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Input and Output