Variable, Expressions, Statements and Operators

Variable, Expressions, Statements and Operators By: Engr. Faisal ur Rehman CE-105 Fall 2007

Goal • Variables and its types • Scope, lifetime and access level of variables • Constant • Expression • Statement • Procedure • Sub Procedure • Functions

Variable • It is the name of memory location where we put our data. • Declaring variable • Use Dim keyword • Assigning values • (sometimes declared with default value assigned) • Using variable • For data manipulation / mathematical calculation

Variables Dim x as integer = 2 Dim L as double = 10 Dim p as double = 5 Dim R1 as double, R2 as double Sub calcReac() R2 = P*x / L R1 = P – R2 End sub

Data Types

Data Types † In scientific notation, "E" refers to a power of 10. So 3.56E+2 signifies 3.56 x 102 or 356, and 3.56E-2 signifies 3.56 / 102 or 0.0356.

Data Types • Integers are used for whole numbers • Single is used for floating point number • Double is used for floating point number with greater accuracy w.r.t. Single • Long is used for financial calculation • String and char is used for Text variable • Object is used for reference of classes / controls • Date / time is clear from its name

Notes for Variables • VB.Net variables are not case sensitive • Conversion of variable from one form to other can be implicit or explicit • Example of implicit conversion: Dim intA as integer = 0 Dim doubA as double = 2.17 intA = doubA • Example of explicit conversion: Dim intA as integer = 0 Dim doubA as double = 2.17 intA = integer.parse(doubA)

Data Conversion • Use: • Integer.parse • Double.parse • Cdbl • Cint • Val • Variable.tostring

Example of object / control variable • Dim mybtn as button • Assignning a button to a form control btn1 = New Button btn1.Top = 38 btn1.Left = 12 btn1.Size = New Size(45, 33) btn1.Text = “Test btn” AddHandler btn1.Click, AddressOf btn1 _Click Me.Controls.Add(btn1)

Example of object / control variable • Event handler code: Private Sub mybtn_Click(ByVal sender As Object, ByVal e As EventArgs) ‘Write some code End Sub

Scope of variable (Assign) • Normally, a variable is in scope, or visible for reference, throughout the region in which you declare it. • In some cases, the variable's access level can influence its scope.

Scope of variable • To make a variable visible only within a block • Place the Dim Statement (Visual Basic) for the variable between the initiating and terminating declaration statements of that block, for example between the For and Next statements of a For loop. • You can refer to the variable only from within the block. To make a variable visible only within a procedure • Place the Dim statement for the variable inside the procedure but outside any block (such as a With...End With block). • You can refer to the variable only from within the procedure, including inside any block contained in the procedure. Scope at Module or Namespace Level • For convenience, the single term module level applies equally to modules, classes, and structures. The access level of a module level variable determines its scope. The namespace that contains the module, class, or structure also influences the scope.

Scope of variable • To make a variable visible throughout a module, class, or structure • Place the Dim statement for the variable inside the module, class, or structure, but outside any procedure. • Include the Private (Visual Basic) keyword in the Dim statement. • You can refer to the variable from anywhere within the module, class, or structure, but not from outside it. • To make a variable visible throughout a namespace • Place the Dim statement for the variable inside the module, class, or structure, but outside any procedure. • Include the Friend (Visual Basic) or Public (Visual Basic) keyword in the Dim statement. • You can refer to the variable from anywhere within the namespace containing the module, class, or structure.

Life time of variables (Assign) • The lifetime of a declared element is the period of time during which it is available for use. • Variables are the only elements that have lifetime. • For this purpose, the compiler treats procedure parameters and function returns as special cases of variables. • The lifetime of a variable represents the period of time during which it can hold a value. Its value can change over its lifetime, but it always holds some value.

Scope of variable (Assign) • Different life times • Beginning • End • Extension

Access Level (Assign) • The access level of a declared element is the extent of the ability to access it, that is, what code has permission to read it or write to it. • This is determined not only by how you declare the element itself, but also by the access level of the element's container. • Dim, Private, Protected, Friend, Protected Friend or Public.

Constant • Constant is the name of memory location with fixed value. • Public Const E As double = 200E9 • Private Const E As Double = 200E9 • Protected Friend Const E As String = 200E9

Expression • Constants and variables combined with algebraic / logical operator is called expression • Can be algebraic or logical If x > 3 then Do something End if If 3*x = 4 then Do something End if

Statement • A single line of code which is compiled successfully is called statement • Statements are packed in to procedures

Procedures • Procedure is a group statements or block of code • Sub Procedure • Function

Sub Procedure • Sub-Procedure is a block of code which do operation and do not report any result or • Any code within sub / end sub is called sub procedure • Sub are within block of sub / end sub • Statements within sub / end sub should be limited to a dozen line of code • Sub may or may not accept arguments

Function • Function is a procedure which do required operation and reports the result as well • Code within Function / End Function is called a functions • Function always need arguments • These are also called mini-programs

Q & A

Define: • Variable • Constant • Expression • Statement • Scope of variable • Procedure • Sub Procedure • Function

Thanks

Variable, Expressions, Statements and Operators