Computer Programming Concepts and Visual Basic David I. Schneider

44 Computer Programming Concepts and Visual Basic
■ DECLARING VARIABLE TYPES
So far, we have not distinguished between variables that hold strings and variables that hold
numbers. There are several advantages to specifying the type of values, string or numeric, that
can be assigned to a variable. A statement of the form
Dim variableName As String
specifies that only strings can be assigned to the named variable. A statement of the form
Dim variableName As Single
specifies that only numbers can be assigned to the named variable. The term Single derives from
single-precision real number. After you type the space after the word “As,” the editor displays
a list of all the possible next words. In this text we use only a few of the items from this list.
A Dim statement is said to declare a variable. From now on we will declare all variables.
However, all the programs will run correctly even if the Dim statements are omitted. Declaring
variables at the beginning of each event procedure is regarded as good programming
practice because it makes programs easier to read and helps prevent certain types of errors.
EXAMPLE 5
The following rewrite of Example 3 declares all variables.
Private Sub cmdCompute_Click()
Dim interestRate As Single
Dim principal As Single
Dim phrase As String
picBalance.Cls
interestRate = 0.0655
principal = 100
phrase = “The balance after a year is”
picBalance.Print phrase; (1 + interestRate) * principal
End Sub
Several Dim statements can be combined into one. For instance, the first three Dim
statements of Example 5 can be replaced by
Dim interestRate As Single, principal As Single, phrase As String
Visual Basic actually has several different types of numeric variables. So far, we have
used only single-precision numeric variables. Single-precision numeric variables can hold
numbers of magnitude from as small as 1.4 � 10 -45 to as large as 3.4 � 10 38 . Another type
of numeric variable, called Integer, can hold only whole numbers from –32768 to 32767.
Integer-type variables are declared with a statement of the form
Dim intVar As Integer
The Integer data type uses less memory than the Single data type and statements using the
Integer type execute faster. (This is only useful in programs with many calculations, such as
the programs in later sections that use For...Next loops.) Of course, Integer variables are limited
because they cannot hold decimals or large numbers. We will use Integer variables extensively
with For...Next loops in Section 6 and occasionally when the data clearly consist of
small whole numbers.
Other types of numeric variables are Long, Double, and Currency. We do not use them
in this text. If you want to learn about them, consult Appendix C. Whenever we refer to a
numeric variable without mentioning a type, we mean Single or Integer.