Transcript Input and Output

Input and Output
• Reading Data From Files
Data files can be created with Windows NotePad,
MSWord, or any another text editor. It is important to
save the files as an “text only”,or “text only with line
breaks”.
A file can have either one item per line or many items
(separated by commas) can be listed on the same
line.
• Example: the following file contains payroll
information. Each line contains the name of a person,
that person’s hourly wage, and the number of ours
that person worked during the week.
______________________________________
“ Mike Jones”, 7.35, 35
“John Smith”, 6.75, 33
________________________________
•
Data stored in a file can be read in order ( that is,
sequentially) and assigned to variables with the
following steps:
1. Choose a number from 1 to 255 to be the reference
number for the file
2. Execute the statement
Open “filename”For Input As #n
Where n is the reference number
The above procedure is referred to as opening a file
for input. It establishes a communication link
between the computer and the disk drive for
reading data from the disk.
3.
Read items of data in order, one at item, from the
file with input # statements. The statement
Input #n, var
Causes the program to look in the file for the next
available item of data and assign it to the variable
var.
In the file individual items are separated by commas or
line breaks. The variable in the Input # statement
must be the same type as the data to be assigned
to it from the file.
4. After the desired items have been read from the file,
close the file with the statement
Close #n
• Note: A statement of the form
Input #n, var1, var2, …, varj
Has the same effect as the sequence of staments
Input #n, var1
Input #n, var2
……….
Input #n, varj
Example: The following program takes the average
annual amounts of money spent by single-person
households for several categories and converts
these amounts to percentages.
The form design for this example consists of a
command button and a picture box.
_______________________________
“Transportation”, 3887
“Housing”, 7643
“Food”, 3017
“Other”, 7804
_______________________________
Private Sub cmdCompute_Click()
Dim total As Single, category As string, amount As Single
Open “COSTS.TXT” For Input As #1
PicPercent.Cls
total=0
Input #1, category, amount
total= total+amount
Input #1, category, amount
total= total+amount
Input #1, category, amount
total= total+amount
Input #1, category, amount
total= total+amount
Close #1
Open “COSTS.TXT” For Input As #1
Input #1, category, amount
PicPercent.Print category; amount/total
Input #1, category; amount
PicPercent.Print category; amount/total
Input #1, category; amount
PicPercent.Print category; amount/total
Input #1, category; amount
PicPercent.Print category; amount/total
Close #1
End Sub
• Run and then click the command button. The
following is displayed in the picture box:
Transportation
0.1739072
Housing
0.3419534
Food
0.1349828
Other
0.3491566
Input from a Input Box
Normally, a text box is used to obtain input described by
a label. However, in some cases it is not practical to
have a text box and label on the screen permanently.
When a statement of the form
stringVar=InputBox(prompt, title)
Is executed, an input box like this
Pops up on the screen. After user types a response
into the text box at the bottom of screen and press
Enter (or clicks OK), the response is assigned to the
string variable stringVar.
• The title argument is optional and gives the caption
to appear in title bar. The prompt argument is a string
that ells the user what information to type into the text
box.
• Example:In this example the file name is provided by
the user in an input box.
• Run, and then click the command button. An input
box appears on the screen. Type Data.Txt into the
input box and click OK.
________________________
1600
“Pennsylvania Ave.”
________________________
Data.Txt
Private Sub cmdDisplay_Click()
Dim fileName as String, prompt As String, title As String
Dim houseNumber As Single, street As String
picAddress.Cls
prompt=“Enter the name of the file containing the information.”
Title= “Name of the File”
fileName=InputBox(prompt, title)
Open file Name For Input As #1
Input #1, houseNumber
Input #1, street
picAddress.Print “ The white House is located at”; houseNumber; street
Close #1
End Sub
• The White House is located at 1600 Pennsylvania Ave.
• Using a Message Box for Output
• Some times you want to grab the user’s attention with
a brief message such as”correct”. You want this
message only to appear on the screen until the user
has read it.
• When a statement of the form
MsgBox prompt, , title
Is executed, where prompt and title are strings, a
message box with prompt displayed and the title bar
caption title appears, and stay on the screen until the
users presses Enter or clicks OK. Example
MsgBox "Enter the name of a file", , "hi!“
produces
Example
• The following program is going to use input boxes to
allow the user to enter their name and age. It will do a
simple calculation and print the results to a label.
• The form looks like this:
•
Build the form according to the following
specification:
Form
Command Button
Command Button
Command Button
Label
Name
Caption
Name
Caption
Name
Caption
Name
Caption
Name
Border Style
Caption
frmAge
Age
cmdExit
E&xit
cmdDetails
Enter&Details
cmdResults
Print &Results
lblResults
Fixed Single
none
•
Notice that the Caption property for each of the command
buttons has an ampersand - a '&' character - in it and this has
the effect of underlining the letter that follows it. When you run
the program, users now have a choice of using the mouse to
click on a button or using a key combination. Let's see how it
works...
1. Enter the code for the Exit button in the usual way
2. Run the program
3. Hold the ALT key and press the 'x' key
•
So holding the ALT key and pressing the underlined letter has
the same effect as clicking the button with a mouse. Clearly
you must make sure that each button has a different letter
that's underlined - you can't have two buttons with the same
key combination.
• we have two variables. The user's name is a string,
or text, variable and we will call it UserName. The
person's age is an integer and we will call it Age.
• If you've ever used a typewriter - well, we did say
history - you'll know that when you want to start a
new line, you have to do a line feed and a carriage
return. The first of these moves the paper up a line
and the second moves you to the start of the line.
• The upshot is that we really need to declare another
variable called LFCR (Line Feed, Carriage Return)
which we can use to do the job for us. You'll see how
it works a little later but for now we need to declare all
three variables in the General Declarations section of
the project.
• Declare the variables like this:
• Here's the code for the cmdDetails Click event:
Private Sub cmdDetails_Click()
UserName = InputBox("Please enter your name:")
Age = InputBox("please enter your age:")
End Sub
• Type in the code
• Run the program
• Let's move now to the code for the Click event of the
cmdResults button.
Private Sub cmdResults_Click()
Dim Days As Integer
Dim TextOut As String
LFCR = Chr(10) + Chr(13)
Days = 365 * Age
'build output string
TextOut = "Hello " + UserName
TextOut = TextOut + LFCR
TextOut = TextOut + "You are at least "
TextOut = TextOut + Str(Days)
TextOut = TextOut + " days old!"
lblResults.Caption = TextOut
End Sub
• Enter the code and save your work
• Run the program and note what happens
• OK, let's go through this code, starting with these two
lines:
Dim Days As Integer
Dim TextOut As String
Nothing too mysterious here. We've declared Days as
an integer and this is what we use for the person's
age in days. We've also declared TextOut as a string
and this is what we're going to use as the message to
be printed out.
• The line
LFCR = Chr(10) + Chr(13)
needs some explanation. Chr(10) is the linefeed
character and Chr(13) is the carriage return
character and so LFCR is a combination of the two,
which we need when we print out the text in the label.
All it does is move any following text onto a new line,
just as if you'd pressed the ENTER key.
• Days = 365 * Age
This calculates the number of days assuming that there
are 365 days in a year
• The next line
'build output string
doesn't in fact do anything and is ignored by the
computer. It does however act as a comment to us
and it is considered good practice to use comments
in order to help other people understand your
programs and make them more readable.
And now to the following code:
TextOut = "Hello " + UserName
TextOut = TextOut + LFCR
TextOut = TextOut + "You are at least "
TextOut = TextOut + Str(Days)
TextOut = TextOut + " days old!"
• The comment said it all really. We are building up a string, or
piece of text, step by step. We start with the word "Hello " and
we add the variable UserName, which we know is another piece
of text
• . The line
TextOut = TextOut + LFCR
takes the string TextOut and adds to it our linefeed/carriage
return combination. We proceed to add to the TextOut string
as we go. There is a slight problem because, while UserName is
a string, Days is an integer, and it doesn't make sense to add a
number to a string - they are completely different data types.
The way round it, though, is to use a special function called Str
which Visual Basic has thoughtfully provided. It's job is to take a
numerical value and convert it into a string, which we can then
add to TextOut.
• Finally, the line
lblResults.Caption = TextOut
assigns our completed string to the Caption property of
the label, and that's what you see printed out.