Chapter 4 — Variables and Arithmetic Operations

CHAPTER FOUR
Variables and
Arithmetic Operations

Objectives
 Create, modify, and program a TextBox object
 Use code to place data in Text property on Label object
 Use the AcceptButton and CancelButton properties
 Understand and declare String and Numeric variables
 Use assignments statements to place data in variables
 Use literals and constants in coding statements
Chapter 4: Variables and Arithmetic Operations 2

Objectives
 Understand scope rules for variables
 Convert string and numeric data
 Understand & use arithmetic operators & arithmetic operations
 Format and display numeric data as a string
 Create a form load event
 Create a concatenated string
 Debug a program

Introduction

TextBox Objects
 With Visual Studio open and the frmDigitalDownloads.vb
[Design] tabbed page visible, point to the TextBox .NET
component in the Toolbox
 Drag the TextBox .NET component onto the Windows Form
object at the desired location

TextBox Objects
When the upper-left corner of the pointer is located where you want
the TextBox object’s upper-left corner, release the left mouse button

Sizing and Positioning a TextBox
Object
 Enter txtNumberOfDownload into the Name property of the TextBox
 Scroll to the Text property and click the right column of the Text property
 Type the maximum number of characters the user normally will enter into
the text box and then press the ENTER key. Use the digit 8 because it is
wider than other digits. In this example, the value 888 is entered because
three digits is the maximum number of digits the user normally will enter

Object
 Drag the right edge of the TextBox object to resize the TextBox
object so it is slightly wider than an 888 entry
 Scroll to the Font property to change font to Century Gothic and
the font size to 12.
 To horizontally align the text in the label and the text in the text
box, drag the text box up until a red snap line indicates the
bottoms of the text are aligned. Then, release the left mouse
button

Object

Aligning Text in a TextBox Object
 Scroll to the TextAlign property of the Text box
 Click the TextAlign property in the left column, and then click the
list arrow in the right
 Click Center in the TextAlign property list

Aligning Text in a TextBox Object

Creating a MultiLine Text Box
 Select the TextBox object, click the Action tag, and point to
the MultiLine check box
 Click the MultiLine check box

Creating a MaskedTextBox Object

 Drag a MaskedTextBox component from the Toolbox to the
Windows Form object. Then, click the Action tag on the TextBox
object and point to the Set Mask command
 Click Set Mask on the MaskedTextBox Tasks list and then click the
Short date mask description in the Input Mask dialog box
 Click the OK button in the Input Mask dialog

Label Objects
 Drag a Label object to the Form object to the correct location.
Name the label lblTotalCostOfDownloads. Change the label to the
appropriate font size (Century Gothic, 12 point).
 In the Text property for the Label object, enter the maximum
number of characters ($888.88) that will appear in the label during
execution of the program
 Drag the Label object up until the red snap line appears. Then
release the left mouse button

Label Objects

Accept Button in Form Properties
 The program will carry out the event handler processing
associated with the Accept button if the user clicks the button or if
the user presses ENTER
 Click a blank area in the Windows
 Scroll to and select the AcceptButton Properties.
 Click the AcceptButton property list arrow in the right column
 Click btnCalculateCost in the AcceptButton property list

Accept Button in Form Properties

Cancel Button in Form Properties
 When the user presses the ESC key, the event handler
processing for the button identified as the Cancel button will
be executed
 Click a blank area in the Windows Form object to select it
 Click the CancelButton property name in the left column in the
Properties window for the Windows Form object, and then
click the CancelButton list arrow
 Click the button name (btnClear) in the CancelButton property
list

String Data Type
 A String variable allows any character to be stored in it
 The data type for the value the user enters in a TextBox object
and that is stored in the Text property of the TextBox object is
string
 A variable is a named location in RAM where data is stored
 A String variable is a named location in RAM that can store a
string valueChapter 4: Variables and Arithmetic Operations 21

String Data Type

Assignment Statements
 One method to place data in the variable is to use an
assignment statement

 In the code editing window type Dim followed by a space. Then, type
strNumberOfSongs on your keyboard
 Press the SPACEBAR, type the word As and then press the
SPACEBAR again
 Because the entry should be String, type str on your keyboard
 Press the ENTER key

 Type strn. IntelliSense displays the variable name that starts with the
letter strn, the String variable strNumberOfDownloads
 Press the SPACEBAR, press the EQUAL SIGN key, and then press the
SPACEBAR
 Type txt to display the IntelliSense list of the Form objects, and
then type n to identify the txtNumberof Downloads Text Box object
in the IntelliSense list
 Press the PERIOD key and then, if necessary, type te to highlight the
Text entry in the IntelliSense list
 Press the ENTER key

Numeric Data Types
 A numeric data type must be used in arithmetic
operations

Numeric Data Types

Other Data Types

Miscellaneous Data Types

Variable Prefixes

Literals
 A value is called a literal when the value being used in the
assignment statement is literally the value that is required

Forced Literal Types
 Sometimes you might want a literal to be a different
data type than the Visual Basic default

Constants
 A constant variable will contain one permanent value
throughout the execution of the program

Constants
 Constant variable the definitions begins with the letters Const, not the letters Dim
 The value of the constant must be on the same line as the definition of the constant.
 At run time, you cannot attempt to change a constant variable value.
 The letter c often is placed before the prefix of the constant variable name to
identify throughout the program that it is a constant variable and cannot be
changed
 Other than the letter c, constant variable names us the same naming convention

Referencing a Variable
 When a variable is declared, it will be underlined with a green
squiggly line until it is referenced in a statement
 It is mandatory when using a variable in a program that the
variable is defined prior to using the variable in a statement

Scope of Variables
 The scope of a variable specifies where within the program the
variable can be referenced in a Visual Basic statement
 A variable can be referenced only within the region of the
program where it is defined
 The code between the Sub statement and the End Sub
statement is a procedure

Scope of Variables (Continued)
 A variable that can only be referenced within the region of the
program where it is defined is called a local variable
 Local variables have a certain lifetime in the program
 Global variables can be used in multiple regions of a program

Converting Variable Data
 Visual Basic includes several procedures that allow you to
convert one data type to another data type
 A procedure to convert a String data type to an Integer data
type is named ToInt32
 The procedure is found in the Convert class, which is available
in a Visual Studio 2010 class library

Using a Procedure
 A procedure that performs its task but does not return a value
is called a Sub procedure
 A procedure that returns a value is called a Function procedure,
or a function
 An argument identifies a value required by a procedure
 Every procedure is part of a class

Using a Procedure

Option Strict On
 Visual Basic will, by default, automatically convert
data types if the data type on the right side of the
equal sign in an assignment statement is different
from the data type on the left side of the equal
sign
 To prevent automatic conversion of values, the
developer must insert the Option Strict On
statement in the program prior to any event
handler code in the program

Option Strict On
The Option Strict On statement explicitly disallows any
default data type conversions in which data loss would occur
and any conversion between numeric types and strings

Arithmetic Operators

Multiple Operations
(Order of Operations)
 A single assignment statement can contain multiple operations
 Hierarchy of Operations
 Exponentiation (^) is performed first
 Multiplication (*) and division (/) are performed next
 Integer division () is next
 MOD then occurs
 Addition (+) and subtraction (-) are performed last
 Within these five steps, calculations are performed left to right

Displaying Numeric Output Data

Format Specifications
for the ToString Function
 Use the format specifier to identify the format for
the numeric data to be returned by the ToString
function

Precision Specifier
 The precision specifier is a number that is included within
quotation marks in the function call to identify the number of
positions to the right of the decimal point that should be returned

Clear Procedure
 The Clear procedure clears any data currently placed
in the Text property of a TextBox object

Clearing the Text Property of a Label
 The Clear procedure cannot be used with a Label object
 You must write an assignment statement that assigns a null
length string to the Text property of a Label object

Setting the Focus
 When the focus is on a
TextBox object, the
insertion point is
located in the text box,
and will be blinking

Form Load Event
 A form load event occurs when the program starts and the
Digital Downloads form is loaded
 This event handler completes the following tasks:
 Display the cost per download heading
 Clear the placeholder from the lblTotalCostOfDownloads Text
property
 Set the focus on the txtNumberOfDownloads text box

Concatenation
 The process of joining two different values into a single string
is called concatenation
 The values being concatenated must be String data types

Concatenation

Class Scope
 When a variable is referenced in two different event handling
procedures, it must be defined at the class level instead of the
procedure (event handler) level

Debugging Your Program
 A Format Exception
occurs when the user
enters data that a
statement within the
program cannot process
properly

Debugging Your Program
 An Overflow Exception occurs when the user enters a
value greater than the maximum value that can be
processed by the statement
 It is not possible to divide by zero, so if your program
contains a division operation and the divisor is equal
to zero, the Divide By Zero Exception will occur

Program Design

Event Planning Document

Summary
 Create, modify, and program a TextBox object
 Use code to place data in the Text property of a Label object
 Use the AcceptButton and CancelButton properties
 Understand and declare String and Numeric variables
 Use assignments statements to place data in variables
 Use literals and constants in coding statements

Summary
 Understand scope rules for variables
 Convert string and numeric data
 Understand and use arithmetic operators and operations
 Format and display numeric data as a string
 Create a form load event
 Create a concatenated string
 Debug a program

CHAPTER FOUR COMPLETE
Variables and

Chapter 4 — Variables and Arithmetic Operations

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Chapter 4 — Variables and Arithmetic Operations