Variables
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| Tips >> Visual
Basic |
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TEMPORARY STORAGE:
Most programs require the temporary storage of data. The
data is stored in a variable, which is a temporary storage in the
computer's memory. The variable name is the "identifier",
used to identify the data. Variables names must begin with a letter
and comprise a series of alphanumeric and '_' up to 255 characters
in length. Visual Basic 6 does not require that a variable
be declared before use, but this is a dangerous practice and can
lead to logical errors that are difficult to detect. In order to
force variable declaration, specify Option Explicit in General Declarations.
VB.Net does require variables to be formally declared.
When a variable name is first used, Visual Basic automatically
creates it and assigns a default value of zero for numeric data
types, null string for a String variable, False for
Boolean, and Empty for a variant.
SCOPE AND DURATION:
The scope and duration of a variable is determined
by the keywords Dim, Redim, Static, Public or Private.
Whilst declarations can appear anywhere within the body of code,
it is good programming practice to declare all variables at the
beginning.
SCOPE OF VARIABLE:
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| Declaration Type |
Scope and Duration |
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Dim
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Used to declare variables in procedures. Each
time the procedure is called, the data stored in variables declared
with this keyword are reset (eg. an Integer data type will be
reset to zero). |
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Static
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Used to declare variables in procedures. Each
time the procedure is called, the data stored in variables declared
with this keyword are retained. |
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Public
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Used to declare variables within a module. The
variable is available to all procedures in the project. |
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Private
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Used to declare variables within a module. The
variable is only available to procedures within the project.
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ReDim
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The ReDim keyword is used to re-dimension an array.
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DATA TYPES OF VARIABLES:
The data type of a variable is determined by the "As"
keyword. If the As keyword is omitted, the data type is a "Variant".
The following illustrates the basic data types that may be used.
DATA TYPE:
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| Data Type |
Example |
Description |
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Byte
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Dim tinyNumber As Byte
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Stores an integer in the range 0 to 255 (8 bits).
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Boolean
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Dim finished As Boolean
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Stores a value that is either True or False. |
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Integer
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Dim counter As Integer
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Stores an integer (whole number) in the range
-32768 to + 32767 (16 bits).
In VB.Net, the Integer data type is 32 bits. A new data type,
"Short", has been introduced to represent 16-bit numbers.
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Long
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Dim longNumber As Integer
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Stores an integer in the range -2,147,483,648
to +2,147,483,647 (32 bits).
In VB.Net, the Long data type is 64 bits bits. |
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Currency
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Dim cost As Currency
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Stores a real number in the range -922,337,203,685,477.5808
to +922,337,203,685,477.5807. |
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Single
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Dim gradient As Single
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Stores a single-precision floating point number,
accurate to 8 significant digits (32 bits). |
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Double
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Dim atoms As Double
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Stores a double-precision floating point number,
accurate to 16 significant digits (64 bits). |
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Date
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Dim today As Date
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Stores a Date. |
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String
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Dim strSurname As String
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Stores a sequence of up to 63,000 characters.
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Variant
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Dim unkown As Variant
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The data type will change depending on what value
is assigned to the variable. If not data type is specified,
this is the data type that will be used.
In VB.Net, the Variant data type is no longer supported, and
should be replaced by the "Object" data type. |
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DECLARING VARIABLES:
If no data type is specified when declaring a variable, its default
type will be a Variant. If more that one variable is declared on
the same line, the data type must be provided for each variable.
In the following example, x has no data type so will
be declared as a Variant.
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The following declares the variables x and y as Integers.
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| Dim x As Integer, y As Integer |
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CONSTANTS:
A Constant value never changes. Visual Basic has some built
in constants, such as, vbCrLf, to insert a carriage return
/ line feed on an output stream. Constants are declared with
the Const keyword.
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| Const deadline As Date = #1/20/2002# |
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Once the constant has been assigned a value, the value can't be
changed. Attempting to re-assign a value will result in a run-time
error.
VISUAL BASIC CONVERSION FUNCTIONS:
Conversion functions may be used to convert data types to
another type.
CONVERSATION FUNCTIONS:
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| Function |
Example |
Description |
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CBool
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newValue = CBool(oldValue) |
Convert a data type to Boolean. |
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CByte
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newValue = CByte(oldValue) |
Convert a data type to Byte. |
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CCur
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newValue = CCur(oldValue) |
Convert a data type to Currency. |
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CDate
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newValue = CDate(oldValue) |
Convert a data type to Date. |
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CDbl
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newValue = CDbl(oldValue) |
Convert a data type to Double. |
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CInt
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newValue = CInt(oldValue) |
Convert a data type to Integer. |
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CSng
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newValue = CSng(oldValue) |
Convert a data type to Single. |
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CStr
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strNewValue = CStr(oldValue) |
Convert a data type to String. |
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