1. Variable
A variable can be defined as an entity that has the following six properties:
- Name
- Address
- Type
- Value
- Scope
- Lifetime
Variable Scope
Variables (and constants) have a scope, which indicates where in the program the variable is
recognized or visible to the code, that is, where it can be referred to in code.
Local variables: block-level and procedure-level scope
If a variable is declared inside a code block (a set of statements that is terminated by an End...,Loop , or Next statement), then the variable has block-level scope ; that is, it is visible only within that block.
For example, consider the following code:
If x <> 0 Then
Dim rec As Integer
rec = 1/x
End If
MsgBox CStr(rec)
In this code, the variable rec is not recognized outside the block in which it is defined, so the final statement produces an error.
Module-level and project-level scope
We first note that a standard module itself can be declared using one of the access modifiers Public, Friend, or Private (this is the default). Using such a modifier simply restricts the individual members to that level of access at most. Thus, for instance, a Public variable declared in a Friend module has only Friend scope.
Private access
A variable declared in the Declarations section of a standard module using the Private access modifier has module-level scope; that is, it is visible in the entire module, but nowhere else. Using the Dim keyword also gives the variable module-level scope, but its use is not as clear and should be avoided for readability sake.
Friend access
A variable declared in the Declarations section of a standard module using the Friend access modifier is visible in the entire project and thus has project-level scope. However, it is not visible to other projects.
Public access
A variable declared in the Declarations section of a Public standard module using the Public access modifier is visible not only to the project in which it is declared, but also to any external project that holds a reference to the project. For instance, consider the following module declared in Project1:
Public Module Module1
Public iModulePublic As Integer
Friend iModuleFriend As Integer
End Module
If Project2 has a reference to Project1, then we can write:
Project1.Module1.iModulePublic = 100
However, the code:
Project1.Module1.iModuleFriend = 100
generates a "not accessible" syntax error.
Declaring Variables and Constants
A variable declaration is an association of a variable name with a data type. In and of itself, this does not imply variable creation. However, for nonobject variables, a variable declaration does create a variable. A declaration such as:
Dim x As Integer
creates an Integer variable named x. We can also write:
Dim x As Integer = New Integer( )
which emphasizes the role of the constructor function for the Integer data type. (The constructor is the function that VB .NET uses to create the variable.)
When multiple variables are declared on the same line, if a variable is not declared with an explicit type declaration, then its type is that of the next variable with an explicit type declaration. Thus, in the line:
Dim x As Long, i, j, k As Integer, s As String
the variables i, j, and k have type Integer. (In VB 6, the variables i and j would have type Variant, which is VB 6's default data type.)
VB .NET permits the initialization of variables in the same line as their declaration (at long last!). Thus, we may write:
Dim x As Integer = 5
to declare an Integer variable and initialize it to 5. Similarly, we can declare and initialize more than one variable on a single line:
Dim x As Integer = 6, y As Integer = 9
Note that in this case, each variable that you declare must explicitly be assigned a data type. You cannot assign each variable an explicit value without explicitly declaring the data type of each variable.
Data Types
Value and Reference Types
The types defined in the CTS fall into three categories:
- Value types
- Reference types
- Pointer types
However, pointer types are not implemented in VB, so we will not discuss these types. The difference between value and reference types is how variables of the corresponding type represent that type. When a value-type variable is defined, as in:
Dim int As Integer = 5
a memory location is set aside to hold the actual data (in this case the number 5). In contrast, when a reference-type variable is defined, as in:
Dim obj As New CEmployee
the VB compiler creates the object in memory, but then sets the variable obj to a 4-byte memory location that contains the address of the object.
In short, value-type variables contain the data, whereas reference-type variables point to the data. The following lists the data types supported by VB .NET, along with their underlying .NET type, storage requirements, and range of values:
Simple Data Types in Visual Basic
- Numeric data type
- Integer data type
- Floating-point data type
- Boolean data type
- Character data type
2. Arithmetic Operator
Operator
|
Example
|
Description
|
+
|
Net + Disc
|
Adds two values
|
-
|
Price - 4.00
|
Subtracts one value from another value
|
*
|
Total * Fact
|
Multiplies two values
|
/
|
Tax / Adjust
|
Divides one value by another value
|
^
|
Adjust ^ 3
|
Raises a value to a power
|
& (or +)
|
Name1 & Name2
|
Concatenates two strings
|
3. Logic Operator
Operator
|
Usage
|
Description
|
And
|
If (A > B) And (C <>
|
Produces True if both sides of the And are true. Therefore, A must be greater than B and C must be less than D. Otherwise, the expression produces a false result.
|
Or
|
If (A > B) Or (C <>
|
Produces True if either side of the Or is true. Therefore, A must be greater than B or C must be less than D. If both sides of the Or are false, the entire expression produces a false result.
|
Not
|
If Not(strAns = "Yes")
|
Produces the opposite true or false result. Therefore, if strAns holds "Yes", the Not turns the true result to false.
|
4. Selection
The If Statement
If makes decisions. If a comparison test is true, the body of the If statement executes. (In fact, the previous sentence is almost identical to Visual Basic's If statement!) Here is one format of If:
If comparisonTest Then
One or more Visual Basic statements
End If
Example :
If (intUnitsSold > 10000) Then
sngBonus = 50.00
End If
The If Statements Else Branch
If comparisonTest Then
One or more Visual Basic statements
Else
One or more Visual Basic statements
End If
Example :
If (intDivNum = 3) Or (intDivNum = 4) Then
curDivTotal = curDivSales3 + curDivSales4
curGrandDivCosts = (curDivCost3 * 1.2) + (curDivCost4 * 1.4)
Else
curDivTotal = curDivSales1 + curDivSales2
curGrandDivCosts = (curDivCost1 * 1.1) + (curDivCost5 * 1.9)
End If
Multiple Choice with Select Case
Select Case Expression
Case value
One or more Visual Basic statements
Case value
One or more Visual Basic statements
[Case value
One or more Visual Basic statements]
[Case Else
One or more Visual Basic statements]
End Select
Example :
Select Case intAge
Case 5: lblTitle = "Kindergarten"
Case 6: lblTitle = "1st Grade"
Case 7: lblTitle = "2nd Grade"
Case 8: lblTitle = "3rd Grade"
Case 9: lblTitle = "4th Grade"
Case 10: lblTitle = "5th Grade"
Case 11: lblTitle = "6th Grade"
Case Else: lblTitle = "Advanced"
End Select
Two Additional Select Case Formats
Select Case Expression
Case Is Relation:
One or more Visual Basic statements
Case Is Relation:
One or more Visual Basic statements
[Case Is Relation:
One or more Visual Basic statements]
[Case Else:
One or more Visual Basic statements]
End Select
Select Case Expression
Case expr1 To expr2:
One or more Visual Basic statements
Case expr1 To expr2:
One or more Visual Basic statements
[Case expr1 To expr2:
One or more Visual Basic statements]
[Case Else:
One or more Visual Basic statements]
End Select
Example :
Select Case intAge
Case Is <5: span=""> lblTitle.Text = "Too young"
Case 5: lblTitle.Text = "Kindergarten"
Case 6 To 11: lblTitle.Text = "Elementary"
lblSchool.Text = "Lincoln
Case 12 To 15: lblTitle.Text = "Intermediate"
lblSchool.Text = "Washington
Case 16 To 18: lblTitle.Text = "High School"
lblSchool.Text = "Betsy Ross"
Case Else: lblTitle.Text = "College"
lblSchool.Text = "University"
End Select
5. Procedure
Sub procedure_name(parameter)
statement
End sub
6. Function
Function function_name(parameter) as typedata
Statement
End sub
7. Passing Parameter by Value and Reference
Visual Basic lets you pass arguments two ways: by reference and by value. The way you use them determines whether the receiving procedure can change the arguments so that those changes remain in effect after the calling procedure regains control. If you pass and receive by reference (the default method), the calling procedure's passed local variables may be changed in the receiving procedure. If you pass and receive by value, the calling procedure can access and change its received arguments, but those changes don't retain their effects in the calling procedure
Sub Changes (N As Integer, S As Single)
` Receives arguments by reference
N = N * 2 ` Double both
S = S * 2 ` arguments
` When the calling routine regains control,
` its two local variables will now be twice
` as much as they were before calling this.
End Sub
Sub NoChanges (ByVal N As Integer, ByVal S As Single)
` Receives arguments by value
N = N * 2 ` Double both
S = S * 2 ` arguments
` When the calling routine regains control,
` its two local variables will not be
` changed from their original values
End Sub
8. Data Type Conversion
The process of converting a value of one data type to another is called conversion or casting. A cast operator can be applied to a literal value or to a variable of a given type. For instance, we have:
Dim lng As Long
Dim int As Integer = 6
' Cast an Integer variable to a Long
lng = CLng(Int)
' Cast a literal integer to a Long
lng = CLng(12)
where expression is an expression that is in the range of the target data type. Specifically, we have the following conversion functions:
- CBool
- CByte
- CChar
- CDate
- CDbl
- CDec
- CInt
- CLng
- CObj
- CShort
- CSng
- CStr
- CType
Members of the System.Convert class
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