Chapter 3: Using Variables and Constants

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Chapter 3 Using Variables and Constants
Programming with Microsoft Visual Basic .NET,
Second Edition
2
Creating Variables and Named Constants Lesson A
Objectives

• Create a procedure-level and module-level
  variable
• Select an appropriate data type for a variable
• Select an appropriate name for a variable

3
Creating Variables and Named Constants Lesson A
Objectives (continued)

• Assign data to an existing variable
• Explain the scope and lifetime of a variable
• Create a named constant

4
Previewing the Completed Application

• To preview the completed Skate-Away Sales
  application
• Use the Run command on the Start menu to run the
  Skate (Skate.exe) file contained in the
  VBNET\Chap03 folder
• An order form similar to the one that you created
  in Chapter 2 appears on the screen

5
Using Variables to Store Information

• Besides storing data in the properties of
  controls, a programmer also can store data,
  temporarily, in memory locations inside the
  computer
• The memory locations are called variables,
  because the contents of the locations can change
  as the program is running

6
Using Variables to Store Information (continued)

• One use for a variable is to hold information
  that is not stored in a control on the user
  interface
• You can also store the data contained in a
  controls property in a variable

7
Selecting a Data Type for a Variable
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Selecting a Data Type for a Variable (continued)
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Selecting a Name for a Variable

• The naming convention used in this book
• The name indicates only the variables purpose
  and is entered using lowercase letters
• Use camel casing if a variables name contains
  two or more words, you capitalize the first
  letter in the second and subsequent words
• The name assigned to a variable must follow the
  rules listed in Figure 3-4

10
Selecting a Name for a Variable (continued)
Figure 3-4 Rules for variable names along with
examples of valid and invalid names
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Declaring a Variable

• You use a declaration statement to declare, or
  create, a variable
• Syntax Dim Private Static variablename As
  datatype initialvalue
• Examples
• Dim hoursWorked As Integer
• Dim dataOk As Boolean True
• Dim name As String, age As Integer

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Assigning Data to an Existing Variable

• You use an assignment statement to assign a value
  to a variable while an application is running
• Syntax variablename value
• Examples
• quantityOrdered 500
• firstName Mary
• state Me.uiStateTextBox.Text
• discountRate .03

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Assigning Data to an Existing Variable (continued)

• A literal constant is an item whose value does
  not change while the application is running
• String literal constants are enclosed in
  quotation marks, but numeric literal constants
  and variable names are not
• A literal type character forces a literal
  constant to assume a data type other than the one
  its form indicates

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Assigning Data to an Existing Variable (continued)
Figure 3-7 Literal type characters
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Assigning Data to an Existing Variable (continued)

• A variable can store only one item of data at any
  one time
• When you use an assignment statement to assign
  another item to the variable, the new data
  replaces the existing data
• After data is stored in a variable, you can use
  the data in calculations

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The Parse Method

• Every numeric data type in Visual Basic .NET has
  a Parse method that can be used to convert a
  string to that numeric data type
• Syntax numericDataType.Parse(string)
• Example Dim sales As Decimal sales
  Decimal.Parse(Me.uiSalesTextBox.Text)

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The Convert Class

• The Convert class contains methods to convert a
  numeric value to a specified data type
• Syntax Convert.method(value)
• Example Dim purchase As Double 500 Dim tax
  As Decimal tax Convert.ToDecimal(purchase)
  .03D

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The Convert Class (continued)
Figure 3-9 Most commonly used methods contained
in the Convert class
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The Scope and Lifetime of a Variable

• A variables scope indicates which procedures in
  an application can use the variable
• The scope is determined by where the Dim, Public
  or Private statement is entered
• When you declare a variable in a procedure, the
  variable is called a procedure-level variable and
  is said to have procedure scope

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The Scope and Lifetime of a Variable (continued)

• When you declare a variable in the forms
  Declarations section, it is called a module-level
  variable and is said to have module scope
• Block-level variables are declared within
  specific blocks of code, such as within
  If...Then...Else statements or For...Next
  statements

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The Scope and Lifetime of a Variable (continued)

• Creating a procedure-level variable
• Created with the Dim keyword
• The Dim statement is entered in an objects event
  procedure
• Only the procedure in which it is declared can
  use the variable
• Removed from memory when the procedure ends

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The Scope and Lifetime of a Variable (continued)

• Creating a module-level variable
• Created with the Private keyword
• Entered in a forms Declarations section
• Can be used by any of the procedures in the form
• Removed from memory when the application ends or
  the form is destroyed

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Named Constants

• A memory location whose contents cannot be
  changed while the program is running
• You create a named constant using the Const
  statement
• Syntax Const constantname As datatype
  expression
• Example
• Const PI As Double 3.141593

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Option Explicit and Option Strict

• Visual Basic .NET provides a way to prevent you
  from using undeclared variables in your code
• Enter the statement Option Explicit On in the
  General Declarations section of the Code Editor
  window

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Option Explicit and Option Strict (continued)

• To eliminate the problems that occur as a result
  of implicit type conversions
• Enter the Option Strict On statement in the
  General Declarations section of the Code Editor
  window

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Option Explicit and Option Strict (continued)

• Type conversion rules used when the Option Strict
  On statement is used
• Strings will not be implicitly converted to
  numbers, and numbers will not be implicitly
  converted to strings
• Lower-ranking data types will be implicitly
  promoted to higher-ranking types
• Higher-ranking data types will not be implicitly
  demoted to lower-ranking data types rather, a
  syntax error will occur

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Modifying the Skate-Away Sales ApplicationLesson
B Objectives

• Include a procedure-level and module-level
  variable in an application
• Concatenate strings
• Get user input using the InputBox function
• Include the ControlChars.NewLine constant in code
• Designate the default button for a form

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Storing Information Using Variables

• You need to revise the Skate-Away Sales
  applications TOE chart and the pseudocode for
  the Calculate Order button
• The uiCalcButton controls Click event procedure
  now has two more tasks to perform
• It must calculate the sales tax
• It must display the message, sales tax, and
  salespersons name in the uiMessageLabel control

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Storing Information Using Variables (continued)

• Two additional objects (OrderForm and
  uiMessageLabel) are included in the revised TOE
  chart
• The OrderForms Load event procedure is
  responsible for getting the salespersons name
  when the application is started
• The uiMessageLabel control will display the
  message, sales tax, and salespersons name

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Storing Information Using Variables (continued)

• As the revised TOE chart indicates, you need to
• Change the code in the uiCalcButtons Click event
  procedure
• Code the forms Load event procedure

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Modifying the Calculate Order Buttons Code

• You will first remove the existing code from the
  Calculate Order buttons Click event procedure
• You then will recode the procedure using
  variables in the equations
• Figure 3-18 shows the revised pseudocode for the
  Calculate Order buttons Click event procedure
  (changes made to the original pseudocode are
  shaded in the figure)

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Modifying the Calculate Order Buttons Code
(continued)
Figure 3-18 Revised pseudocode for the Calculate
Order buttons Click event procedure
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Concatenating Strings

• Connecting strings together is called
  concatenating
• Use the concatenation operator, which is the
  ampersand (), to concatenate strings in Visual
  Basic .NET
• When concatenating strings, be sure to include a
  space before and after the concatenation operator

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Concatenating Strings (continued)
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The InputBox Function

• The InputBox function displays one of Visual
  Basic .NETs predefined dialog boxes
• Syntax InputBox(prompt, title,
  defaultResponse)
• Use sentence capitalization for the prompt, and
  book title capitalization for the title
• Has limitations cant control appearance and
  allows user to enter only one piece of data

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The InputBox Function (continued)
Figure 3-29 Example of a dialog box created by
the InputBox function
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The NewLine Character

• The NewLine character, which is Chr(13)
  Chr(10), instructs the computer to issue a
  carriage return followed by a line feed
• The ControlChars.NewLine constant advances the
  insertion point to the next line on the screen
• The ControlChars.NewLine constant is an intrinsic
  constant, which is a named constant that is built
  into Visual Basic .NET

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Designating a Default Button

• Can be selected by pressing the Enter key even
  when the button does not have the focus
• Set the forms AcceptButton property to the
  desired button
• If used, it is typically the first button
• If a buttons action is destructive and
  irreversible, then it should not be the default
  button

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Modifying the Skate-Away Sales Applications
CodeLesson C Objectives

• Include a static variable in code
• Code the TextChanged event procedure
• Create a procedure that handles more than one
  event

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Modifying the Code in the Load and uiCalcButton
ClickProcedures

• Mr. Cousard would like to have the order form ask
  for the salespersons name each time an order is
  calculated
• Before making modifications, you should review
  the applications documentation and revise the
  necessary documents
• Figure 3-44 shows the revised pseudocode for the
  Calculate Order buttons Click event procedure

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Modifying the Code in the Load and uiCalcButton
ClickProcedures (continued)
Figure 3-44 Revised pseudocode for the Calculate
Order button
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Static Variables

• A static variable is a local variable that
  retains its value when the procedure in which it
  is declared ends
• SyntaxStatic variablename As datatype
  initialvalue
• Removed from memory when application ends or form
  is removed from memory

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Coding the TextChanged Event Procedure

• A controls TextChanged event occurs when the
  contents of a controls Text property change
• This can happen as a result of either the user
  entering data into the control, or the
  applications code assigning data to the
  controls Text property

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Associating a Procedure with Different Objects
or Events

• The keyword Handles appears in a procedure header
  and indicates the object and event associated
  with the procedure
• You can associate a procedure with more than one
  object and event
• To do so list each object and event, separated
  by commas, in the Handles section of the
  procedure header

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Summary

• The syntax of a variable declaration statement
  isDim Private Static variablename As
  datatype initialvalue
• To create a procedure-level variable, enter the
  variable declaration statement in a procedure
• To create a module-level variable, enter the
  variable declaration statement in a forms
  Declarations section

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Summary (continued)

• To concatenate strings, use the concatenation
  operator the ampersand ()
• To display a dialog box containing a prompt, an
  input area, an OK button, and a Cancel button,
  use the InputBox function, whose syntax is
  InputBox(prompt, title, defaultResponse)
• To make a button the default button, set the
  forms AcceptButton property to the name of the
  button

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Summary (continued)

• To create a static variable, use a declaration
  statement that follows the syntax Static
  variablename As datatype initialvalue
• To process code when the contents of a control
  have changed, enter the code in the controls
  TextChanged event
• To create a procedure for more than one object or
  event, list each object and event after the
  Handles keyword in the procedure