We come across all kinds of data in our daily life. For example, we need to handle data such as names, addresses, money, date, stock quotes, statistics and more every day. Similarly, in Visual Basic, we have to deal with all sorts of data, some can be mathematically calculated while some are in the form text or other forms. VB divides data into different types so that they are easier to manage when we need to write the code involving those data. VB6 classifies the information mentioned above into two major data types, they are the numeric data types and the non-numeric data types.
Numeric data types are types of data that consist of numbers that can be computed mathematically with standard operators. Examples of numeric data types are height, weight, share values, the price of goods, monthly bills, fees and others. In Visual Basic, numeric data are divided into 7 types, depending on the range of values they can store.
Calculations that only involve round figures can use Integer or Long integer in the computation. Programs that require high precision calculation need to use Single and Double decision data types, they are also called floating point numbers. For currency calculation , you can use the currency data types. Lastly, if even more precision is required to perform calculations that involve many decimal points, we can use the decimal data types. These data types summarized in Table 5.1
| Type | Storage | Range of Values |
|---|---|---|
| Byte | 1 byte | 0 to 255 |
| Integer | 2 bytes | -32,768 to 32,767 |
| Long | 4 bytes | -2,147,483,648 to 2,147,483,648 |
| Single | 4 bytes | -3.402823E+38 to -1.401298E-45 for negative values 1.401298E-45 to 3.402823E+38 for positive values. |
| Double | 8 bytes | -1.79769313486232e+308 to -4.94065645841247E-324 for negative values 4.94065645841247E-324 to 1.79769313486232e+308 for positive values. |
| Currency | 8 bytes | -922,337,203,685,477.5808 to 922,337,203,685,477.5807 |
| Decimal | 12 bytes | +/- 79,228,162,514,264,337,593,543,950,335 if no decimal is use +/- 7.9228162514264337593543950335 (28 decimal places). |
Nonnumeric data types are data that cannot be manipulated mathematically. Non-numeric data comprises string data types, date data types, boolean data types that store only two values (true or false), object data type and Variant data type .They are summarized in Table 5.2
|
Data Type |
Storage |
Range |
|---|---|---|
|
String(fixed length) |
Length of string |
1 to 65,400 characters |
|
String(variable length) |
Length + 10 bytes |
0 to 2 billion characters |
|
Date |
8 bytes |
January 1, 100 to December 31, 9999 |
|
Boolean |
2 bytes |
|
|
Object |
4 bytes |
Any embedded object |
|
Variant(numeric) |
16 bytes |
Any value as large as Double |
|
Variant(text) |
Length+22 bytes |
Same as variable-length string |
Literals are values that you assign to data. In some cases, we need to add a suffix behind a literal so that VB can handle the calculation more accurately. For example, we can use num=1.3089# for a Double type data. Some of the suffixes are displayed in Table 5.3.
| Suffix | Data Type |
|---|---|
| & | Long |
| ! | Single |
| # | Double |
| @ | Currency |
In addition, we need to enclose string literals within two quotations and date and time literals within two # sign. Strings can contain any characters, including numbers. The following are few examples:
memberName="Turban, John."
TelNumber="1800-900-888-777"
LastDay=#31-Dec-00#
ExpTime=#12:00 am#
Variables are like mail boxes in the post office. The contents of the variables changes every now and then, just like the mail boxes. In term of VB, variables are areas allocated by the computer memory to hold data. Like the mail boxes, each variable must be given a name. To name a variable in Visual Basic, you have to follow a set of rules.
The following are the rules when naming the variables in Visual Basic
Examples of valid and invalid variable names are displayed in Table 5.4
| Valid Name | Invalid Name |
|---|---|
|
My_Car |
My.Car |
|
ThisYear |
1NewBoy |
|
Long_Name_Can_beUSE |
He&HisFather *& is not acceptable |
In Visual Basic, it is a good practice to declare the variables
before using them by assigning names and data types. They are normally declared
in the general section of the codes' windows using the Dim statement. You can use any variable to hold any data , but different types of variables are designed to work efficiently with different data types .
The syantax is as follows:
Dim VariableName As DataType
If you want to declare more variables, you can declare them in separate lines or you may also combine more in one line , separating each variable with a comma, as follows:
Dim VariableName1 As DataType1, VariableName2 As DataType2,VariableName3 As DataType3
Dim password As String Dim yourName As String Dim firstnum As Integer Dim secondnum As Integer Dim total As Integer Dim doDate As Date Dim password As String, yourName As String, firstnum As Integer
Unlike other programming languages, Visual Basic actually doesn't require you to specifically declare a variable before it's used. If a variable isn't declared, VB willautomatically declare the variable as a Variant. A variant is data type that can hold any type of data.
For string declaration, there are two possible types, one for the variable-length string and another for the fixed-length string. For the variable-length string, just use the same format as example 5.1 above. However, for the fixed-length string, you have to use the syntax as shown below:
Dim VariableName as String * n
where n defines the number of characters the string can hold.
For example,
Dim yourName as String * 10
*yourName can holds no more than 10 Characters.
Other than using the Dim keyword to declare the data, you can also use other keywords to declare the data. Three other keywords are private ,static and public. The forms are as shown below:
Private VariableName as Datatype Static VariableName as Datatype Public VariableName as Datatype
The above keywords indicate the scope of the declaration. Private declares a local variable or a variable that is local to a procedure or module. However, Private is rarely used, we normally use Dim to declare a local variable. The Static keyword declares a variable that is being used multiple times, even after a procedure has been terminated. Most variables created inside a procedure are discarded by Visual Basic when the procedure is finished, static keyword preserves the value of a variable even after the procedure is terminated. Public is the keyword that declares a global variable, which means it can be used by all the procedures and modules of the whole program.
Constants are different from variables in the sense that their values do not change during the running of the program.
The syntax to declare a constant is
Constant Name As Data Type = Value
In this example, we insert a Shape control and two command buttons. Set the shape value of the Shape control to 3 so that it becomes a circle. Rename one of the command buttons to CmdResize for changing the size of the circle. Rename the other command button as CmdArea for calculation of the area of the circle. In this program, we declare four variables and a constant in the General section. The varaible h is to store the value of height of the circle and the variable r is to store the value of the radius which is half of the height. In addtion, the variable a is to store the value of area in twip using the formula area of circle=πr2. Besides that, the constant Pi represents π which we fixed at 3.142. Finally, the variable area is to store the value in cm by multiplying a with 0.001763889. (1 twip =0.001763889 cm)
Dim h, r, a, rad, area As Single
Const Pi As Single = 3.142
Private Sub CmdArea_Click()
r = h / 2
rad = r * 0.001763889
a = Pi * rad ^ 2
area = Round(a, 2)
MsgBox ("The Area of the circle is " & area)
End Sub
Private Sub CmdResize_Click()
h = InputBox("Enter the value of height")
MyShape.Height = h
End Sub
Copyright©2008 Dr.Liew Voon Kiong. All rights reserved |Contact|Privacy Policy