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# Variable, Expressions, Statements and Operators - PowerPoint PPT Presentation

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.

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### Variable, Expressions, Statements and Operators

By: Engr. Faisal ur Rehman

CE-105 Fall 2007

• Variables and its types

• Scope, lifetime and access level of variables

• Constant

• Expression

• Statement

• Procedure

• Sub Procedure

• Functions

• 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

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

† 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.

• 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

• 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)

• Use:

• Integer.parse

• Double.parse

• Cdbl

• Cint

• Val

• Variable.tostring

• 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”

• 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.

• 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.

• 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 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

• 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

• A single line of code which is compiled successfully is called statement

• Statements are packed in to procedures

• Procedure is a group statements or block of code

• Sub Procedure

• Function

• 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 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

• Define:

• Variable

• Constant

• Expression

• Statement

• Scope of variable

• Procedure

• Sub Procedure

• Function