3-1

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Variables, Constants and Calculations

Chapter 33

McGraw-Hill © 2006 The McGraw-Hill Companies, Inc. All rights reserved.

3-3

Objectives (1 of 2)

Distinguish between variables, and constantsDifferentiate among the various data typesApply naming conventions incorporating standards

and indicating the data typeDeclare variables and constantsSelect the appropriate scope for a variableConvert text input to numeric values

3-4

Objectives (2 of 2)

Perform calculations using variables and constantsConvert between numeric data types using implicit

and explicit conversionsRound decimal values using the Decimal.Round

method Format values for output using the ToString method.Use Try/Catch blocks for error handlingDisplay message boxes with error messages Accumulate sums and generate counts

3-5

Data - Variables and Constants (1 of 2)

VariableMemory locations that hold data that can be changed

during project executionExample: customer’s name

Named ConstantMemory locations that hold data that cannot be changed

during project executionExample: sales tax rate

3-6

Data - Variables and Constants (2 of 2)

In Visual Basic when you declare a Variable or Named Constant An area of memory is reservedA name is assigned called an IdentifierFollow rules and naming conventions

Use Declaration Statements to establish Variables and ConstantsAssign name and data typeNot executable unless initialized on same line

3-7

Data Types Data Type Use For Storage Size in bytes

Boolean True or False value 2

Byte 0 to 255, binary data 1

Char Single Unicode character 2

Date 1/1/0001 through 12/31/9999 8

Decimal Decimal fractions, such as dollars/cents 16

Single Single precision floating-point numbers with six digits of accuracy

4

Double Double precision floating-point numbers with 14 digits of accuracy

8

Short Small integer in the range -32,768 to 32,767 2

Integer Whole numbers in the range -2,147,483,648 to +2,147,483,647

4

Long Larger whole numbers 8

String Alphanumeric data: letters, digits, and other characters

Varies

Object Any type of data 4

3-8

Naming Variables and Constants

Must follow Visual Basic Naming RulesShould follow Naming Conventions

Meaningful names consisting of letters, digits and underscores; must begin with a letter and no spaces or periods

Include the class (data type) of the variableUse mixed case for variables and uppercase for

constants (quantityInteger, QUOTA_Integer)Can not use reserved words or keywords that Basic has

assigned a meaning such as print, name, and value

3-9

Constants

NamedUser assigned name, data type and valueUse CONST keyword to declare

IntrinsicSystem defined within Visual Studio

Const COMPANY_ADDRESS_String As String = "101 S. Main Street"Const SALES_TAX_RATE_Decimal As Decimal = .08D

3-10

Assigning Values to Constants

Declare the data type of numeric constants by appending a type-declaration character

Decimal D Decimal – 850.50D

Double R Double – 52875.8R

Integer I Integer – 12345678I

Long L Long – 134257987L

Short S Short – 20064S

Single F Single – 101.25F

3-11

Declaring Variables

Declared inside a procedure using a Dim statement

Declared outside a procedure using Public, Private or Dim statements

Always declare the variable’s data typeMay declare several variables with one

statementUse IntelliSense to assist in writing statements

3-12

Declaration Statement Examples

Dim customerNameString As String

Private totalSoldInteger As Integer

Dim temperatureSingle As Single

Dim priceDecimal As Decimal

Private priceDecimal As Decimal

3-13

Scope and Lifetime of Variables (1 of 2)

Visibility of a variable is its scopeScope may be

NamespaceModule levelLocalBlock level

Lifetime of a variable is the period of time the variable exists

3-14

Module Level Variable Declaration Example

Code module-level declarations in the Declaration section at the top of the code

3-15

Calculations

Calculations can be performed with variables, constants, properties of certain objects, and numeric literals

Do not use strings in calculationsValues from Text property of Text Boxes

Are strings, even if they contain numeric dataMust be converted to a numeric data type before

performing a calculation

3-16

Converting Strings to aNumeric Data Type

Use Parse methods to convert the Text property to its numeric form before it’s used in a calculation

Each numeric data type class has a Parse methodParse method returns a value that can be used in

calculationsParse method fails if user enters nonnumeric data

or leaves data blank

3-17

Converting to String

Values assigned to string variables or Text properties must be string

Convert any numeric data type to string using .ToString method

3-18

Conversion Methods

Method Convert ToInteger.Parse IntegerDecimal.Parse Decimal.ToString String

3-19

Conversion Examples

quantityInteger =Integer.Parse(quantityTextBox.Text)priceDecimal =Decimal.Parse(priceTextBox.Text)wholeNumberInteger =Integer.Parse(digitString)resultLabel.Text =resultDecimal.ToString( )countTextBox.Text =countInteger.ToString( )idString =idInteger.ToString( )

3-20

Arithmetic Operations

Operator Operation+ Addition– Subtraction* Multiplication/ Division\ Integer Division

Mod Modulus – Remainder of division

^ Exponentiation

3-21

Order of Operations

Hierarchy of operations, or order of precedence, in arithmetic expressions from highest to lowest

1. Any operation inside parentheses2. Exponentiation3. Multiplication and division4. Integer division5. Modulus6. Addition and subtraction

3-22

Evaluation of Expression

1. All operations within parentheses. Multiple operations within the parentheses are performed according to the rules of precedence.

2. All exponentiation. Multiple exponentiation operations are performed from left to right.

3. All multiplication and division. Multiple operations are performed from left to right.

4. All integer division. Multiple operations are performed from left to right.

5. Mod operations. Multiple operations are performed from left to right.

6. All addition and subtraction are performed from left to right.

3-23

Mathematical Examples

Note the use of parentheses to control order of precedence

3+4*2 = 11 Multiply then add(3+4)*2 = 14 Parentheses control: add then multiply8/4*2 = 4 Same level, left to right: divide then multiply

3-24

Using Calculations in Code

Perform calculations in assignment statementsWhat appears on right side of assignment operator is

assigned to item on left sideAssignment operators – allows shorter versions of

code =, +=, -=, *=, /=, \=, &=

‘Accumulate a total.

TotalSalesDecimal += salesDecimal

3-25

Option Explicit and Option Strict

Option Explicit forces variables to be declared before using

Option StrictMakes VB a strongly typed language like C++, Java

and C#Does not allow implicit conversions from a wider

data type to a narrower one or between String and numeric data types

Best practice to always turn on either in code or in Project Properties dialog box

3-26

Converting Between Numeric Data Types

Implicit (automatic) conversionConverts value from narrower

data type to wider type where no danger of losing precision exists

Explicit conversion (casting)Uses methods of Convert

class to convert between data types

Convert Class has methods that begin with “To” for each of the data types

3-27

Performing Calculations with Unlike Data Types

VB performs the calculations using the wider data type

Perform a cast if converting the result to a different data type

VB does not convert to a different data type until it is necessary

3-28

Rounding Numbers

Round decimal fractionsDecimal.Round method returns a decimal result rounded to a

specified number of decimal positions Decimal.Round and Convert methods use technique called

“rounding toward even”

Decimal Value to Round

Number of Decimal Positions Results

1.455 2 1.46

1.445 2 1.44

1.5 0 2

2.5 0 2

3-29

Formatting Data for Display

To display numeric data in a label or text box, first convert value to stringUse ToString method

Format the data using formatting codesSpecifies use of dollar sign, percent sign and

commasSpecifies number of digits that appear to right of

decimal point

Me.displayTextBox.Text = numberInteger.ToString()

3-30

Using Format Specifier Codes

"C" codeCurrency – String formatted with dollar sign,

commas separating each group of 3 digits and 2 digits to the right of decimal point

"N" codeNumber – String formatted with commas

separating each group of 3 digits and 2 digits to the right of decimal point

Can specify number of decimal positionsExample: "C0" zero digits

3-31

Format Specifier Codes

Format Specifier Codes Name

C or c Currency

F or f Fixed-point

N or n Number

D or d Digits

P or p Percent

3-32

Format Specifier Code Examples

Variable Value Code Output

totalDecimal

1125.6744 "C" $1,125.67

totalDecimal

1125.6744 "N0" 1,126

pinInteger 123 "D6" 000123

rateDecimal 0.075 "P" 7.50%

rateDecimal 0.075 "P3" 7.500%

rateDecimal 0.075 "P0" 8%

valueInteger -10 "C" ($10.00)

3-33

Date Specifier Code

Format DateTime values using format codes and ToString method

Date codes are case sensitive

3-34

Handling Exceptions

Use structured exception handling to easily catch errors before run-time error occurs

Catching exceptions referred to as error trapping

Coding to handle exception called error handling

Error handling in Visual Studio.NET is standardized for all languages using the Common Language Runtime, CLR, which improves on previous versions of VB

3-35

Try/Catch Blocks

Enclose statements that might cause an error within Try/Catch blockIf an exception occurs while statements in the Try

block are executing, program control is transferred to the Catch Block

If a Finally statement is included, the code in that section executes last, whether or not an exception occurred

3-36

Try Block - General Form

Trystatements that may cause error

Catch [VariableName As ExceptionType]statements for action when an exception occurs

[Finallystatements that always execute before exit of Try block]

End Try

3-37

Try Block - ExampleCatches Any Exception

TryquantityInteger = Integer.Parse(quantityTextBox.Text)Me.quantityLabel.Text = quantityInteger.ToString( )

CatchMe.messageLabel.Text = "Error in input data."

End Try

3-38

Try Block - ExampleCatches Specific Exception

This Catch statement catches bad input data that cannot be converted to numeric

Catch theException As FormatExceptionmessageLabel.Text="Error in input data."

End Try

3-39

Common Exception Classes

Exception Caused By

FormatException Failure of a numeric conversion

InvalidCastException Failure of conversion operation

ArithmeticException A calculation error

System.IO.EndOfStreamException

Failure of an input or output operation

OutOfMemoryException

Not enough memory to create an object

Exception Generic

3-40

The Exception Class

Each exception is an instance of the Exception class. The properties of this class allow you to determine the code location of the error, the type of error and cause.

3-41

Try Block - ExampleHandling Multiple Exceptions

Try' Statements that may cause errors.

Catch theException As FormatException' Statements for nonnumeric data.

Catch theException As ArithmeticException' Statements for calculation problem.

Catch theException As Exception' Statements for any other exception.

End Try

3-42

MessageBox Object (1 of 2)

MessageBox.Show (TextMessage, TitlebarText, _ MessageBoxButtons, MesssageBoxIcon)

MessageBox flavorsSignatures correspond to the argument listThere are multiple signatures to choose fromDo not reverse, transpose or leave out any of the argumentsIntelliSense displays argument list (also called signatures)

3-43

MessageBox Object (2 of 2)

TextMessage stringString literal or variable that displays message

Title Bar textString that appears in title bar of message box

MessageBox ButtonsOK, OKCancel, RetryCancel, YesNo, YesNoCancel,

AbortRetryIgnoreMessageBox Icons

Asterisk, Error, Exclamation, Hand, Information, None, Question, Stop, Warning

3-44

Using Overloaded Methods

This OOP feature allows the Show method to act differently for different arguments

Each argument list if called a signature so the Show method has several signatures

Supplied arguments must exactly match one of the signatures provided by the method

IntelliSense in Visual Studio editor helps when entering arguments so that they don’t need to be memorized

3-45

Testing Multiple Fields

Each input field presents an opportunity for an exception

To indicate specific field that caused the exception use nested Try/Catch blocks

Pinpoints specific errors and after error set focus back to field in errorUse SelectAll method of text box to make text appear

selected to aid user

3-46

Counting and Accumulating Sums

Declare module-level variables, since local level variables reset to 0 each time the procedure is called

Summing Numbers

Counting

Calculating an Average

discountedPriceSumDecimal += discountedPriceDecimal

Private saleCountInteger As IntegersaleCountInteger += 1

averageDiscountedSaleDecimal = discountedPriceSumDecimal / saleCountInteger