© 2006 Lawrenceville Press Slide 1 Chapter 4 Variables  A variable is a name for a value stored in memory.  Variables are created using a declaration statement. For example: Dim intLength As Integer  Declaration statements include the keyword Dim, the variable name, and the data type.

© 2006 Lawrenceville Press Slide 2 Chapter 4 Variable Assignment A variable can store only one value at any time. Dim x As Integer; x = 5; x = 10; x 5 10

© 2006 Lawrenceville Press Slide 3 Chapter 4 Retrieving User Input  Input can be in the form of data typed by the user at run time.  A TextBox object allows users to enter values.  A text box usually displays a prompt to inform the user what type of data is expected.

© 2006 Lawrenceville Press Slide 4 Chapter 4 The TextBox Control  (Name) should begin with txt.  Text is what is displayed in the text box. At run time, this property can be used in an assignment statement to retrieve the data typed by the user.  TextAlign sets the alignment of text relative to the text box.  TextChange event is used with a TextBox object to clear a Label object. The event is fired when the user types in the text box.

© 2006 Lawrenceville Press Slide 5 Chapter 4 The Val() Function  A function performs a single, well-defined task and returns a value.  Val() is used to convert text box data to a numeric value.  Val() requires a string and then returns a number corresponding to the string. For example: Dim intHeight As Integer intHeight = Val("62 cm.") 'height = 62 intHeight = Val("33")'height = 33 intHeight = Val(Me.txtHeight.Text)

© 2006 Lawrenceville Press Slide 6 Chapter 4 Named Constants  A constant is a name for a memory location that stores a value that cannot be changed from its initial assignment.  Constants are created using a declaration statement and must have a value assigned in the declaration. For example: Const PI As Double = 3.14  Constant identifiers are typically all uppercase with an underscore ( _ ) separating words within the identifier name.

© 2006 Lawrenceville Press Slide 7 Chapter 4 Built-In Data Types TypeUsed For Integer whole numbers Double numbers with a decimal portion Decimal currency values Date dates Char individual characters String a set of characters Boolean true/false, on/off, yes/no values

© 2006 Lawrenceville Press Slide 8 Chapter 4 Type Conversion In an assignment statement, Visual Basic automatically converts data to match the type of the variable it is being assigned to. For example: Dim x As Integer x = 6.7'x assigned 7

© 2006 Lawrenceville Press Slide 9 Chapter 4 Variable Scope

© 2006 Lawrenceville Press Slide 10 Chapter 4 Integer Division Integer division ( \ ) truncates the decimal portion of the quotient. Only the integer portion of the quotient is returned:

© 2006 Lawrenceville Press Slide 11 Chapter 4 Modulus Division Modulus division returns the remainder of a division operation:

© 2006 Lawrenceville Press Slide 12 Chapter 4 Visual Basic Keywords AndDimFalseLongReturn BooleanDoFinallyMeSelect ByteDoubleForModShort CallEachFriendModuleSingle CaseElseFunctionNewStatic CatchElseIfGetNotStop ConstEndHandlesNothingString DateEnumIfObjectStructure DecimalEraseInOrSub DeclareErrorIntegerPrivateThen DefaultEventIsProtectedTo DelegateExitLikePublicTrue

© 2006 Lawrenceville Press Slide 13 Chapter 4 Code Conventions  Variable identifiers should begin with a lowercase letter and any word after the first within the identifier should begin with an uppercase letter.  Group variables together in the same declarations only when variables represent related items.  Use a blank line after a group of declarations to make it clear where declarations end.  Use a descriptive prompt next to a text box to tell the user what kind of input is expected.

© 2006 Lawrenceville Press Slide 14 Chapter 4 Programming Errors  Syntax errors violate the rules of Visual Basic.  Logic errors, also called semantic errors, occur in statements that are syntactically correct, but produce undesired or unexpected results.  Run-time errors, also called exceptions, halt program execution at the statement that cannot be executed.

© 2006 Lawrenceville Press Slide 15 Chapter 4 The Visual Basic Debugger

© 2006 Lawrenceville Press Slide 16 Chapter 4 Code Conventions (cont.)  Choose data types that appropriate for the type of data being represented.  Declare variables so that their scope is limited to where they are needed.  Constant identifiers should be all uppercase with underscore characters separating words.  Group constant declarations before variable declarations.