chapter 4 — variables and arithmetic operations

CHAPTER FOURVariables and

Arithmetic Operations

Chapter 4: Variables and Arithmetic Operations 2

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


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



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


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 value


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 COMPLETEVariables and


chapter 4 — variables and arithmetic operations

Technology

text property

numeric variables

textbox object use code

maskedtextbox object

label object

multiline text box

program chapter

textbox net component