Computer Programming Concepts and Visual Basic David I. Schneider

136 Computer Programming Concepts and Visual Basic
■ EOF FUNCTION
Data to be processed are often retrieved from a file by a Do loop. Visual Basic has a useful
function, EOF, that tells us if we have reached the end of the file from which we are reading.
Suppose a file has been opened with reference number n. At any time, the condition
EOF(n)
will be true if the end of the file has been reached, and false otherwise.
One of the first programs I wrote when I got my computer stored a list of names and
phone numbers and printed a phone directory. I first had the program display the directory
on the screen and later changed the picNumbers.Print statements to Printer.Print statements
to produce a printed copy. I stored the names in a file so I could easily add, change, or delete
entries.
EXAMPLE 1
The following program displays the contents of a telephone directory. The names and phone numbers are
contained in the file PHONE.TXT. The loop will repeat as long as the end of the file is not reached.
PHONE.TXT contains the following four lines:
“Bert”, “123-4567”
”Ernie”, “987-6543”
”Grover”, “246-8321”
”Oscar”, “135-7900”
Object Property Setting
frmPhone Caption Directory
cmdDisplay Caption Display Phone Numbers
picNumbers
Private Sub cmdDisplay_Click()
Dim nom As String, phoneNum As String
picNumbers.Cls
Open “PHONE.TXT”For Input As #1
Do While Not EOF(1)
Input #1, nom, phoneNum
picNumbers.Print nom, phoneNum
Loop
Close #1
End Sub
[Run, and press the command button.]
The program in Example 1 illustrates the proper way to process a list of data contained in a
file. The Do loop should be tested at the top with an end-of-file condition. (If the file is empty,
no attempt is made to input data from the file.) The first set of data should be input after the
Do statement, and then the data should be processed. Figure 5-3 contains the pseudocode and
flowchart for this technique.