Visual Basic 2008 Tutorial

Lesson 6: Managing VB2008 Data

　

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There are many types of data that we come across in our daily life. For example, we need to handle data such as names, addresses, money, date, stock quotes, statistics and etc everyday. Similarly in Visual Basic 2008, we have to deal with all sorts of  of data, some can be mathematically calculated while some are in the form of text or other forms. VB2008 divides data into different types so that it is easier to manage when we need to write the code involving those data. 6.1 Visual Basic 2008 Data Types Visual Basic 2008 classifies the information mentioned above into two major data types, they are the numeric data types and the non-numeric data types. 6.1.1 Numeric Data Types 　 　 Numeric data types are types of data that consist of numbers, which can be computed mathematically with various standard operators such as add, minus, multiply, divide and so on. Examples of numeric data types are your examination marks, your height, your weight, the number of students in a class, share values, price of goods, monthly bills, fees and etc. In Visual Basic 2008, numeric data are divided into 7 types, depending on the range of values they can store. Calculations that only involve round figures or data that don't need precision 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 requires to perform calculations that involve a many decimal points, we can use the decimal data types. These data types summarized in Table 6.1 Table 6.1: Numeric Data Types 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). 　        　   6.1.2 Non-numeric Data Types Nonnumeric data types are data that cannot be manipulated mathematically using standard arithmetic operators. The non-numeric data comprises  text or string data types, the Date data types, the Boolean data types that store only two values (true or false), Object data type and Variant data type .They are summarized in Table 6.2  Table 6.2: Nonnumeric Data Types  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 True or False 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

　

6.1.3 Suffixes for Literals

Literals are values that you assign to a data. In some cases, we need to add a suffix behind a literal so that VB2008 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 6.3.

Table 6.3

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#

　

6.2 Managing Variables

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 VB2008, 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 2008, you have to follow a set of rules.

6.2.1 Variable Names The following are the rules when naming the variables in Visual Basic 2008 It must be less than 255 characters No spacing is allowed It must not  begin with a number Period is not permitted Examples of valid and invalid variable names are displayed in Table 6.4

                                                  Table 6.4

6.2.2 Declaring Variables

In Visual Basic 2008, one needs to declare the variables before using them by assigning names and data types. If you fail to do so, the program will show an error. They are normally declared in the general section of the codes' windows using the Dim statement.
The format  is as follows:

Dim Variable Name As Data Type

Example 6.1

Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load

Dim password As String
Dim yourName As String
Dim firstnum As Integer
Dim secondnum As Integer
Dim total As Integer
Dim doDate As Date

End Sub

You may also combine them in one line , separating each variable with a comma, as follows:

Dim password As String,  yourName As String, firstnum As Integer,.............

For string declaration, there are two possible formats, 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 6.1 above. However, for the fixed-length string, you have to use the format as shown below:

Dim VariableName as String * n, where n defines the number of characters the string can hold.

Example 6.2:

Dim yourName as String * 10

yourName can holds no more than 10 Characters.  

6.2.3 Assigning Values to Variables

After declaring various variables using the Dim statements, we can assign values to those variables. The general format of an assignment is

Variable=Expression

The variable can be a declared variable or a control property value. The expression could be a mathematical expression, a number, a string, a Boolean value (true or false) and etc. The following are some examples:  firstNumber=100 secondNumber=firstNumber-99 userName="John Lyan" userpass.Text = password Label1.Visible = True Command1.Visible = false Label4.Caption = textbox1.Text ThirdNumber = Val(usernum1.Text) total = firstNumber + secondNumber+ThirdNumber

　

6.3 Constants

Constants are different from variables in the sense that their values do not change during the running of the program.

6.3.1 Declaring a Constant

The format to declare a constant is

Const  Constant Name  As Data Type = Value

Example 6.3

Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load

Const Pi As Single=3.142

Const Temp As Single=37

Const Score As Single=100

End Sub

　

　

 

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