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 Visual Basic type Common language runtime type structure Nominal storage allocation Value range Boolean Depends on implementing platform True or False Byte 1 byte0 through 255 (unsigned) Char (single character) Char2 bytes0 through (unsigned) Date DateTime8 bytes0:00:00 (midnight) on January 1, 0001 through 11:59:59 PM on December 31, 9999

Data Types Decimal 16 bytes0 through +/- 79,228,162,514,264,337,593,543,950,33 5 (+/ E+28) † with no decimal point; 0 through +/ with 28 places to the right of the decimal; smallest nonzero number is +/ (+/- 1E-28) † Double (double- precision floating- point) Double8 bytes E+308 through E-324 † for negative values; E-324 through E+308 † for positive values IntegerInt324 bytes-2,147,483,648 through 2,147,483,647 (signed)

Data Types Long (long integer) Int648 bytes-9,223,372,036,854,775,808 through 9,223,372,036,854,775,807 (9.2...E+18 † ) (signed) ObjectObject (class) 4 bytes on 32-bit platform Any type can be stored in a variable of type Object 8 bytes on 64-bit platform SByte 1 byte-128 through 127 (signed) Short (short integer) Int162 bytes-32,768 through 32,767 (signed) Single (single- precision floating- point) Single4 bytes E+38 through E-45 † for negative values; E-45 through E+38 † for positive values

Data Types String (variable- length) String (class)Depends on implementing platform 0 to approximately 2 billion Unicode characters UIntegerUInt324 bytes0 through 4,294,967,295 (unsigned) ULongUInt648 bytes0 through 18,446,744,073,709,551,615 (1.8...E+19 † ) (unsigned) User-Defined (structure) (inherits from ValueType) Depends on implementing platform Each member of the structure has a range determined by its data type and independent of the ranges of the other members UShortUInt162 bytes0 through 65,535 (unsigned) † 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

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 1.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.Dim Statement (Visual Basic) 2.You can refer to the variable only from within the block. To make a variable visible only within a procedure 1.Place the Dim statement for the variable inside the procedure but outside any block (such as a With...End With block). 2.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 1.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 1.Place the Dim statement for the variable inside the module, class, or structure, but outside any procedure. 2.Include the Private (Visual Basic) keyword in the Dim statement.Private (Visual Basic) 3.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 1.Place the Dim statement for the variable inside the module, class, or structure, but outside any procedure. 2.Include the Friend (Visual Basic) or Public (Visual Basic) keyword in the Dim statement.Friend (Visual Basic)Public (Visual Basic) 3.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