Variable and constants in Vb.NET

The slides helps to understand how to declare variable, constants and enumeration in VB.NET

1. VB.NET:- Variable and Constants BCA -501

2. 2 Content Variable Variable Declarations Variable Declaration Example Variable Initialisations Getting Values from user Lvalues & Rvalues Constants Enumeration Scope of Variable

3. 3 Variable A  variable  is used to hold the value that can be used further in the programming . A variable is a simple name used to store the value of a specific data type in computer memory . In VB.NET, each variable has a particular data type that determines the size, range, and fixed space in computer memory . With the help of variable, we can perform several operations and manipulate data values in any programming language.

4. 4 Variable Declarations The  Dim  statement is used for variable declaration and storage allocation for one or more variables. The Dim statement is used at module, class, structure, procedure or block level. . Each variable in the variable list has the following syntax and parts − variablename [ ( [ boundslist ] ) ] [ As [ New ] datatype ] [ = initializer ] Where, variablename − is the name of the variable boundslist − optional. It provides list of bounds of each dimension of an array variable. New − optional. It creates a new instance of the class when the Dim statement runs. datatype − Required if Option Strict is On. It specifies the data type of the variable. i nitializer − Optional if New is not specified. Expression that is evaluated and assigned to the variable when it is created.

5. 5 Variable Declarations Syntax Syntax: Dim [ Variable_Name ] As [Defined Data Type] Name Descriptions Dim It is used to declare and allocate the space for one or more variables in memory. Variable_Name It defines the name of the variable to store the values. As It is a keyword that allows you to define the data type in the declaration statement. Data Type It defines a data type that allows variables to store data types such as Char, String, Integer, Decimal, Long, etc. Value Assign a value to the variable.

6. 6 Variable Declarations Example Dim  Roll_no  As Integer   Dim  Emp_name  As String   Dim Salary As Double   Dim  Emp_id ,  Stud_id  As Integer   Dim  result_status  As Boolean Note: I f we want to  declare more than one variable  in the same line, we must separate each variable with a comma. Dim  Variable_name1 As DataType1, variable_name2 As DataType2, Variable_name3 As DataType3  

7. 7 Variable Initialization After the declaration of a variable, we must assign a value to the variable. The following syntax describes the initialization of a variable: Syntax: Variable_name  = value   Dim  Roll_no  As Integer  'declaration of  Roll_no      Roll_no  = 101  'initialization of  Roll_no       Initialize the  Emp_name     Dim  Emp_name  As String   Emp_name  =  “Jaya"  'Here  Emp_name  variable assigned a value of  Jaya       Initialize a Boolean variable    Dim status As Boolean  'Boolean value can be True or False.    status = True  'Initialize status value to True    Initialzation at the time of declaration: Dim  Roll_no  As Integer = 101   Dim  Emp_name  As String = " Stephen Robert  "

8. 8 Getting Values from the User: In VB.NET, the Console class provides the Readline () function in the System namespace. It is used to take input from the user and assign a value to a variable Dim name As String   name =  Console.ReadLine ()   Or name =  Console.ReadLine    Imports System   Module  User_Data        Sub Main()           Dim  num  As Integer           Dim age As Double           Dim name As String            Console.WriteLine ("Enter your favourite number")            '  Console.ReadLine  or  Console.ReadLine () takes value from the user            num  =  Console.ReadLine             Console.WriteLine (" Enter Your Good name")            'Read string data from the user            name =  Console.ReadLine             Console.WriteLine (" Enter your Age")           age =  Console.ReadLine             Console.WriteLine (" You have entered {0}",  num )            Console.WriteLine (" You have entered {0}", name)            Console.WriteLine (" You have entered {0}", age)            Console.ReadKey ()         End Sub   End Module   Note: Console.Read () and Console.ReadKey () function is used to read a single character from the user.

9. 9 Lvalues & Rvalues Lvalue :  It is an lvalue expression that refers to a memory location for storing the address of a variable. An lvalue is a variable that can appear to the left or right of the assignment operator to hold values. Furthermore, in comparison to or swapping the variables' values, we can also define the variable on both sides (left or right-side) of the assignment operator . Dim  num  As Integer   Num  = 5    Or    Dim  num  As Integer = 5    But when we write the following statement, it generates a compile-time error because it is not a valid statement. Dim x As Integer   10 = x   Rvalue :  It is an rvalue expression that is used to store a value in some address of memory. An rvalue can appear only on the right- hand side because it is a value of the variable that defines on the right-hand side. Dim  college_name  As String   college_name  =  “ISM"  //  rvalue  define at right side of the assignment operator.  

10. 10 Constants in VB.NET T he name constant refers to a fixed value that cannot be changed during the execution of a program. It is also known as  literals . These constants can be of any data type, such as Integer, Double, String, Decimal, Single, character, enum , etc . In VB.NET,  const   is a keyword that is used to declare a variable as constant. The Const statement can be used with module, structure, procedure, form, and class. Const   constname  As  datatype  = value  Item Name Descriptions Const It is a Const keyword to declare a variable as constant. Constname It defines the name of the constant variable to store the values. As It is a keyword that allows you to define the data type in the declaration statement. Data Type It defines a data type that allows variables to store data types such as Char, String, Integer, Decimal, Long, etc. Value Assign a value to the variable as constant.

11. 11 Print and Display Constant Const Name Descriptions vbCrLf Carriage return/linefeed character combination vbCr Carriage return character vbLf Linefeed character vbNewLine Newline character vbNullChar Null character vbNullString Not the same as a zero-length string (""); used for calling external procedures. vbObjectError Error number. User-defined error numbers should be greater than this value. For example: Err.Raise (Number) = vbObjectError + 1000 vbTab Tab character. vbBack Backspace character.

12. 12 Enumerations in VB.NET An enumerated type is declared using the  Enum  statement. The Enum statement declares an enumeration and defines the values of its members. The Enum statement can be used at the module, class, structure, procedure, or block level. Each member in the memberlist has the following syntax and parts: [< attribute list >] member name [ = initializer ] Where, name − specifies the name of the member. Required. initializer − value assigned to the enumeration member. Optional . Example: Enum Colors red = 1 orange = 2 yellow = 3 green = 4 azure = 5 blue = 6 violet = 7 End Enum

13. 13 Print and Display Constant Const Name Descriptions vbCrLf Carriage return/linefeed character combination vbCr Carriage return character vbLf Linefeed character vbNewLine Newline character vbNullChar Null character vbNullString Not the same as a zero-length string (""); used for calling external procedures. vbObjectError Error number. User-defined error numbers should be greater than this value. For example: Err.Raise (Number) = vbObjectError + 1000 vbTab Tab character. vbBack Backspace character.

14. 14 Scope of Variables The scope of a variable determines the accessible range of a defined variable at the time of declaration in any block, module, and class. It can be accessed, if the variable is in a particular region or scope in the same block. And if the variable goes beyond the region, its scope expires . The following are the methods to represent the scope of a variable in VB.NET. Procedure Scope Module Scope Public Scope

15. 15 Procedure Scope A  local variable  is a type of variable defined within a procedure scope, block, or function. It is available with a code inside the procedure, and it can be declared using the  Dim or static  statement. These variables are not accessible from outside of the local method. However, the local variable can be easily accessed by the nested programming function in the same method . Dim X As Integer    Local variables exist until the procedure in which they are declared is executed. Once a procedure is executed, the values of its local variables will be lost, and the resources used by these variables will be released. And when the block is executed again, all the local variables are rearranged.

16. 16 Module Scope All existing procedures can easily identify a variable that is declared inside a module sheet is called a  module-level variable . The defined module variable is  visible to all procedure s within that module only, but it is not available for other module's procedures. The   Dim or private statement  at the top of the first procedure declaration can be declared the module-level variables. It means that these variables cannot be declared inside any procedure block . Further , these variables are useful to share information between the procedures in the same module. And one more thing about the module-level variable is that these variables can remains existence as long as the module is executed.

17. 17 Globa l Scope As the name defines, a global variable is a variable that is used to access the variables  globally in a program . It means these variables can be accessed by all the  procedures or modules  available in a program. To access the variables globally in a program, we need to use  the friend or public keyword  with a variable in a module or class at the top of the first procedure function . Global scope is also known as the  Namespace scope . 'Global declaration of a variable        Public  str  As String =  "Hello, Programmer."        Public topic As String       Public  exp  As Integer