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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


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Goal

  • Variables and its types

  • Scope, lifetime and access level of variables

  • Constant

  • Expression

  • Statement

  • Procedure

  • Sub Procedure

  • Functions


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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


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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





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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.


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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


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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)


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Data Conversion

  • Use:

    • Integer.parse

    • Double.parse

    • Cdbl

    • Cint

    • Val

    • Variable.tostring


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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)


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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


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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.


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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.


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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.


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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.


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Scope of variable (Assign)

  • Different life times

  • Beginning

  • End

  • Extension


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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.


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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


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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


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Statement

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

  • Statements are packed in to procedures


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Procedures

  • Procedure is a group statements or block of code

    • Sub Procedure

    • Function


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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


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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



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  • Define:

  • Variable

  • Constant

  • Expression

  • Statement

  • Scope of variable

  • Procedure

  • Sub Procedure

  • Function