Computer Programming Concepts and Visual Basic David I. Schneider

Object Property Setting
frm3_4_7 Caption, ANSI Values
lblPress Caption Press any key
txtCharacter Text (blank)
picOutput
Private Sub txtCharacter_KeyPress(KeyAscii As Integer)
txtCharacter.Text = “”
picOutput.Cls
picOutput.Print Chr(KeyAscii);
“ has ANSI value”; KeyAscii
End Sub
[Run, and then press a key. For instance, if A is pressed, the following is displayed in the picture box.]
A has ANSI value 65
The KeyPress event procedure can alter the character typed into the text box. For
instance, if the statement
KeyAscii = 65
is placed in a KeyPress event procedure, the letter A is displayed when any standard key is
pressed. In Section 4, we use a decision structure to prevent the user from typing unwanted keys.
For instance, if we want the user to enter a number into a text box, we can intercept and discard
any key presses that are not digits. The statement
KeyAscii = 0
placed in a KeyPress event procedure discards the key pressed. Finally, a program can be made
more friendly by letting the Enter key (ANSI value 13) move the focus in the same way that
the Tab key moves the focus. This requires having a KeyPress event procedure for each object
that is to respond to the Enter key and then setting the focus to the next object when the value
of KeyAscii is 13.
COMMENTS
1. The string “” contains no characters and is called the null string or the empty
string. It is different than the string containing a single space (“ ”). String variables
that have not been assigned values initially have “” as their default values.
(Numeric variables have default value 0.)
2. The statement picBox.Print, with no string or number, simply skips a line in the
picture box.
3. Assigning a string value to a numeric variable can result in the error message
“Type mismatch.”
4. In Visual Basic 6.0, the maximum allowable number of characters in a string is
approximately 2 billion.
5. The quotation-mark character (“) can be placed into a string constant by using
Chr(34). For example, after the statement
txtBox.Text = “George ” & Chr(34) & “Babe” & Chr(34) & “ Ruth”
is executed, the text box contains
George “Babe” Ruth
Strings 47