Programming with Microsoft Visual Basic 2012 Chapter 3: Using Variables and Constants.

Programming with Microsoft Visual Basic 2012

Chapter 3: Using Variables and Constants

Previewing the Modified Play It Again Movies Application

• Play It Again Movies application– New screen for salesclerk’s name– Sales tax added to order

Programming with Microsoft Visual Basic 2012 2

Figure 3-2 Completed sales receipt

Figure 3-1 Name Entry dialog box

Lesson A Objectives

After studying Lesson A, you should be able to:• Declare variables and named constants• Assign data to an existing variable• Convert string data to a numeric data type using the

TryParse method• Convert numeric data to a different data type using the

Convert class methods• Explain the scope and lifetime of variables and named

constants• Explain the purpose of Option Explicit, Option Infer, and

Option StrictProgramming with Microsoft Visual Basic 2012 3

• Controls and variables temporarily store data• Variable

– A temporary storage location in main memory– Specified by data type, name, scope, and lifetime

• Reasons to use variables – To hold information that is not stored in a control on the

form– To allow for more precise treatment of numeric data– To enable code to run more efficiently


Using Variables to Store Information

Using Variables to Store Information (cont.)


Selecting a Data Type for a Variable• Data type

– Specifies the type of data a variable can store– Provides a class template for creating variables

• Unicode– A universal coding scheme for characters– Assigns a unique numeric value to each character in the

written languages of the world



Figure 3-3 Basic data types in Visual Basic

• The textbook uses:– The Integer data type for all integers– Either the Decimal or Double data type for numbers

containing decimal places or numbers used in calculations

– The String data type for text or numbers not used in calculations

– The Boolean data type for Boolean values





Selecting a Name for a Variable– Names must begin

with a letter or underscore

– Names can contain only letters, numbers, or underscores• No punctuation, special characters, or spaces are allowed

– The recommended length for a name variable is 32 characters

– Variable names cannot be reserved words (such as Sub or Double)

Figure 3-4 Three-character IDs and examples


• Valid names:– intFeb_Income– decSales2014– dblEastRegion– strName

• Invalid Names:– 4thQuarter

• The name must begin with a letter or underscore– dblWest Region

• The name cannot contain a space– strFirst.Name

• The name cannot contain punctuation– decSales$East

• The name cannot contain a special character



Declaring a Variable• Declaration statement

– Used to declare (create) a variable and reserve space in memory for it

• If no initial value is given to a variable when declaring it, the computer stores a default value– Numeric variables are set to 0– Boolean variables are set to False– Object and String variables are set to Nothing– Date variables are set to 1/1/0001 12:00:00AM




Figure 3-6 Syntax and examples of a variable declaration statement

Assigning Data to an Existing Variable


• Assignment statement– Assigns a value to a variable at run time– Syntax: variablename = expression– An expression may include literal constants, object

properties, variables, keywords, and arithmetic operators• Literal constant

– A data item whose value does not change while the application is running

– Example: The string “Mary” • Literal type character

– Forces a literal constant to change its data type

Assigning Data to an Existing Variable (cont.)


Figure 3-7 Syntax and examples of assigning a value to a variable during run time



The TryParse Method• Converts a string to a number• Is preferred over Val

– Allows the programmer to specify the data type– Val only returns a Double number

• Arguments– dataType: A numeric data type, such as Integer– String: A string to be converted– Variable: A variable that receives the numeric value



Figure 3-9 Basic syntax and examples of the TryParse method



Figure 3-10 Results of the TryParse method for the Double, Decimal and Integer data types



The Convert Class• Can be used to convert a number from one type to

another• Methods include ToDecimal, ToDouble, ToInt32, and

ToString• TryParse is recommended for converting strings to

numeric data types– Will not produce an error if the conversion fails



Figure 3-11 Syntax and examples of the Convert class methods

The Scope and Lifetime of a Variable


• Scope – Indicates where a variable can be used

• Lifetime – How long a variable remains in memory

• Scope and lifetime are determined by where a variable is declared: either the General Declarations section or the form’s Declaration section

• Three types of scope:– Class: The variable can be used by all procedures in a form – Procedure: The variable can be used within a procedure– Block: The variable can be used within a specific code block

The Scope and Lifetime of a Variable (cont.)


• Variables with Procedure Scope– Can be used only by that procedure– Declared at the beginning of the procedure– Removed from memory when the procedure ends– Declared using the Dim keyword

• Most variables used in this course will be procedure-level variables

Figure 3-12 User Interface for the Discount Calculator application



Figure 3-13 Click event procedures using procedure-level variables



Figure 3-14 Code Editor window for the Discount Calculator application



• Variables with Class Scope– Can be used by all procedures in the form– Declared in the form’s Declarations section– Will remain in memory until the application ends– Declared using the Private keyword

Figure 3-16 User Interface for the Total Scores application



Figure 3-17 Total Scores application code using a class-level variable

Static Variables


• Static variable– A procedure-level variable with an extended lifetime

• Remains in memory between procedure calls• Retains its value even when the procedure ends

– Static keyword• Used to declare a static variable

– Static variables act like class-level variables but have narrower scope • They can only be used by the procedure in which they are

declared

Static Variables (cont.)


Figure 3-19 Total Scores application’s code using a static variable

Named Constants


• Named constant– A memory location inside the computer whose contents

cannot be changed at run time • Const statement

– Creates named constant– Stores value of expression in a named constant– expression: Can be a literal constant, another named

constant, or an arithmetic operator– Cannot contain a variable or method

Named Constants (cont.)


Figure 3-20 Syntax and examples of the Const statement

Named Constants (cont.)


Figure 3-21 User interface for the Area Calculator application

Figure 3-22 Calculate Area button’s Click event procedure

Option Statements


Option Explicit and Option Infer• Prevent you from using undeclared variables Option Strict• Implicit type conversion

– Converts the right-side value to the data type on the left side• Promotion

– Data is converted to a greater precision number (e.g., Integer to Decimal)• Demotion

– Data is truncated (e.g., Decimal to Integer)– Data loss can occur when demotion occurs

• Infer – Ensures that every variable is declared with a data type

Option Statements (cont.)


Option Strict (cont.)– Disallows implicit conversions – Type conversion rules are applied when this option is on

Figure 3-24 Rules and examples of type conversions

Option Statements (cont.)


Figure 3-25 Option statements entered in the General Declarations section


Lesson A Summary

• Declare a variable using {Dim | Private | Static}• An assignment statement assigns a value to a variable• Three levels of scope: block, procedure, class• The TryParse method converts strings to numeric data• Use Const to declare a named constant• Avoid programming errors by using Option Explicit On,

Option Infer Off, and Option Strict On

Lesson B Objectives

After studying Lesson B, you should be able to:• Include procedure-level and class-level variables 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• Format numbers using the ToString method



Modifying the Play It Again Movies Application

Figure 3-26 Revised TOE chart for the Play It Again Movies application

• Modifications needed– Calculate and display

the sales tax – Display salesperson’s

name• Revise the TOE chart to

reflect the new tasks• You must modify btnCalc

button’s Click event and the form’s Load event

Modifying the Play It Again Movies Application (cont.)


Figure 3-27 Modified user interface for the Play It Again Movies application


Modifying the Calculate Button’s Code

• General strategy– Remove the existing code from the Click event procedure– Recode the procedure using variables in equations

• Use the Option Explicit On statement– Enforces full variable declaration

• Use the Option Infer Off statement– Ensures that variables are declared with data types

• Use the Option Strict On statement– Suppresses implicit type conversions

Modifying the Calculate Button’s Code (cont.)


Figure 3-29 Lines to delete from the procedure

Figure 3-28 A jagged blue line indicates a syntax error



Figure 3-30 Revised pseudocode and flowchart for the btnCalc control’s Click event procedure



Figure 3-31 List of named constants and variables

Figure 3-32 Const and Dim statements entered in the procedure



Figure 3-33 Code entered in the btnCalc control’s Click event procedure

Figure 3-34 Calculated amounts shown in the interface


Concatenating Strings

• Concatenate– To connect strings together

• Concatenation operator– The ampersand (&)– Include a space before and after the ampersand

• Numeric values used with the & operator are converted to strings

Figure 3-35 Examples of string concatenation


Concatenating Strings (cont.)

Figure 3-37 Concatenated strings displayed in the lblMessage control

Figure 3-36 String concatenation included in the assignment statement


The InputBox Function

InputBox function– Displays an input dialog box and retrieves user input

• Arguments– prompt: Contains the message to display inside the dialog

box– title: Controls the text that appears in the dialog box’s title

bar– defaultResponse: Controls the text that appears in the

input field• The returned value most often assigned to String variable• The syntax is shown in Figure 3-39 on the next slide


The InputBox Function (cont.)

Figure 3-38 Example of an input dialog box

Figure 3-39 Basic syntax and examples of the InputBox function


The InputBox Function (cont.)

Figure 3-41 frmMain Load event procedure

Figure 3-40 Class-level variable declared in the form’s Declarations section

Figure 3-42 Dialog box created by the InputBox function


The ControlChars.Newline Constant

ControlChars.NewLine constant– Advances the insertion point to the next line in a control– Also used to advance the insertion point in file or on the

printer• To use, type ControlChars.NewLine in the

appropriate location– Can be used with string concatenation

• Line continuation character (_)– Used to break up a long line of code into two or more

lines


The ControlChars.Newline Constant (cont.)

Figure 3-43 Modified assignment statement

Figure 3-45 Salesclerk’s name shown on the sales receipt


Designating a Default Button

Default button– Activated by pressing the Enter key – Not required to have the focus– Only one per form

• The default button should be the button used most often by the user– Except if button’s task is destructive and irreversible, such

as deleting data• Set the form’s AcceptButton property to the button

name


Using the ToString Method to Format Numbers

• Formatting– Specifying decimal places and special characters to

display• The ToString method replaces the Format function • Syntax: variablename.ToString(formatString)

– variablename: The name of a numeric variable– formatString: The string specifying the format you want

to use• formatString must take the form Axx, where A is the

format specifier and xx is the precision specifier


Using the ToString Method to Format Numbers (Cont.)

Figure 3-46 Syntax and examples of the ToString method



Figure 3-48 Play It Again Movies application’s code at the end of Lesson B (continues…)



Figure 3-48 Play It Again Movies application’s code at the end of Lesson B


Lesson B Summary

• The concatenation operator (&) is used to link strings• The InputBox function displays an interactive dialog box • Use ControlChars.NewLine to move the insertion point

to a new line • Set the default button in the form’s AcceptButton

property• The ToString method formats a number for string

output

Lesson C Objectives

After studying Lesson C, you should be able to:• Include a static variable in code• Code the TextChanged event procedure• Create a procedure that handles more than one event


Modifying the Load and Click Event Procedures


Figure 3-51 Revised TOE Chart for the Play It Again Movies application in Lesson C

Modifying the Load and Click Event Procedures (Cont.)


Figure 3-52 Revised pseudocode for the Calculate button in Lesson C

Coding the TextChanged Event Procedure

• TextChanged event– Occurs when the Text property value of a control changes

• Can occur when:– The user enters data into the control– The code assigns data to the control’s Text property

• Example: A change is made to the number of items ordered


Coding the TextChanged Event Procedure (Cont.)

Associating a Procedure with Different Objects and Events• Handles clause

– Appears in an event procedure’s header – Indicates the object and event associated with the

procedure• You can associate an event procedure with more than one

object and/or event– In the Handles section of procedure header, list each object

and event separated by commas– Procedure names are entered in Pascal case

• Capitalize the first letter in the name and the first letter of each subsequent word in the name


Coding the TextChanged Event Procedure (cont.)


Figure 3-53 TextChanged event procedure associated with the txtDvds control

Figure 3-54 Completed ClearLabels procedure


Lesson C Summary

• The TextChanged event procedure responds to a change in the value of a control’s Text Property

• The Handles clause determines which objects and events are associated with the event procedure

• To create a procedure for more than one object or event, list each object and event after the Handles keyword

Programming with Microsoft Visual Basic 2012 Chapter 3: Using Variables and Constants.

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