Chapter 3

McGraw-Hill © 2009 The McGraw-Hill Companies, Inc. All rights reserved.

Chapter 3

Variables, Constants, and Calculations

McGraw-Hill © 2009 The McGraw-Hill Companies, Inc. All rights reserved. 3-2

Objectives (1 of 2)

• Distinguish between variables, constants, and controls.

• Differentiate among the various data types.

• Apply naming conventions incorporating standards and indicating the data type.

• Declare variables and constants.

• Select the appropriate scope for a variable.

• Convert text input to numeric values.


Objectives (2 of 2)

• Perform calculations using variables and constants.

• Convert between numeric data types using implicit and explicit conversions.

• Round decimal values using the Decimal.Round method.

• Format values for output using the ToString method.

• Use Try/Catch blocks for error handling.

• Display message boxes with error messages.

• Accumulate sums and generate counts.


Data — Variables and Constants (1 of 2)

Variable

•Memory locations that hold data that can be changed during project execution

•Example: customer’s name

• Named Constant

•Memory locations that hold data that cannot be changed during project execution

•Example: sales tax rate


Data — Variables and Constants (2 of 2)

• In Visual Basic, when you declare a Variable or Named Constant

• An area of memory is reserved

• A name is assigned called an Identifier

• Follow rules and naming conventions

• Use Declaration Statements to establish Variables and Constants,

• Assign name and data type,

• Not executable unless initialized on same line


Data Types

Data Type Use For Storage Size in bytes

Boolean True or False value 2

Byte 0 to 255, binary data 1

Clear 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


Naming Variables and Constants

• Must follow Visual Basic Naming Rules

• Should follow Naming Conventions• Meaningful names consisting of letters, digits, and

underscores; must begin with a letter and no spaces or periods. Include class (data type) of variable (QUOTA_Integer)

• Use mixed case for variables and uppercase for constants (quantityInteger).

• Cannot use reserved words or keywords to which Basic has assigned a meaning, such as print, name, and value


Constants

• Named• User assigned name, data type, and value

• Use CONST keyword to declare.

• Intrinsic• System defined within Visual Studio

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


Assigning Values to Constants

Decimal D Decimal – 850.50D

Double R Double – 52875.8R

Integer I Integer – 12345678I

Long L Long – 134257987L

Short S

Single F Single – 101.25F

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


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 type.

• May declare several variables with one statement.

• Use IntelliSense to assist in writing statements.


Declaration Statement Examples

Dim customerNameString As String

Private totalSoldInteger As Integer

Dim temperatureSingle As Single

Dim priceDecimal As Decimal

Private priceDecimal As Decimal


Scope and Lifetime of Variables (1 of 2)

• Visibility of a variable is its scope.• Scope may be

• Namespace

• Module level

• Local

• Block level

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


Module Level Variable Declaration Example

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


Calculations

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

• Do not use strings in calculations.

• Values from Text property of Text Boxes

•Are strings, even if they contain numeric data

•Must be converted to a numeric data type before performing a calculation


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 method.

• Parse method returns a value that can be used in calculations.

• Parse method fails if user enters nonnumeric data or leaves data blank.


Converting to String

• Values assigned to string variables or Text properties must be string.

• Convert any numeric data type to string using .ToString method.

Examples:

ResultTextBox.Text = ResultDecimal.ToString()

CountTextBox.Text = CountInteger.ToString()

IDString = IDInteger.ToString()


Conversion Methods

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


Conversion Examples

QuantityInteger =Integer.Parse(quantityTextBox.Text)PriceDecimal =Decimal.Parse(priceTextBox.Text)WholeNumberInteger =Integer.Parse(digitString)ResultTextBox.Text =ResultDecimal.ToString( )CountTextBox.Text =CountInteger.ToString( )IDString =IDInteger.ToString( )


Arithmetic Operations

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

Mod Modulus – Remainder of division

^ Exponentiation


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


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.


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


Using Calculations in Code

• Perform calculations in assignment statements.

• What appears on right side of assignment operator is assigned to item on left side.

• Assignment operators — allows shorter versions of code =, +=, -=, *=, /=, \=, &=

‘Accumulate a total.

TotalSalesDecimal += salesDecimal


Option Explicit and Option Strict

• Option Explicit forces variables to be declared before using.

• Option Strict• Makes 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


Converting Between Numeric Data Types

• Implicit (automatic) conversion• Converts 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.


Performing Calculations with Unlike Data Types

• VB performs the calculations using the wider data type.

• Use a cast if converting the result to a different data type.

Example:Convert.ToInt32(CountInteger / NumberDecimal) or

Convert.ToSingle(CountInteger / NumberDecimal).

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


Rounding Numbers

• Round decimal fractions

• Decimal.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

1,455 2 1.44

1.5 0 2

2.5 0 2


Formatting Data for Display

• To display numeric data in a label or text box, first convert value to string.

• Use ToString method

• Format the data using formatting codes.

• Specifies use of dollar sign, percent sign, and commas

• Specifies number of digits that appear to right of decimal point

DisplayTextBox.Text = NumberInteger.ToString()


Using Format Specifier Codes

• "C" code• Currency — String formatted with dollar sign,

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

• "N" code• Number — String formatted with commas

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

• Can specify number of decimal positions• Example: "C0" zero digits


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


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)


Date Specifier Code

• Format DateTime values using format codes and ToString method.

• Date codes are case sensitive.


Handling Exceptions

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

• Catching exceptions is referred to as error trapping.

• Coding to handle exception is 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.


Try/Catch Blocks

• Enclose statements that might cause an error within Try/Catch block.

• If 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.


Try Block — General Form

Try‘statements that may cause an error

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

[Finally‘statements that always execute before exit of the Try

block]End Try


Try Block — ExampleCatches Any Exception

TryQuantityInteger = Integer.Parse(QuantityTextBox.Text)QuantityTextBox.Text = QuantityInteger.ToString( )

CatchMessageLabel.Text = "Error in input data."

End Try


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


Common Exception Classes

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.


Try Block — ExampleHandling Multiple Exceptions

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.


MessageBox Object (1 of 2)

• The MessageBox is an overloaded method.• Signatures correspond to the argument list.

• There are multiple signatures to choose from.

• Do not reverse, transpose, or leave out any of the arguments.

• IntelliSense displays argument list (also called signatures).

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


MessageBox Object (2 of 2)

• TextMessage string• String literal or variable that displays message

• Title Bar text• String that appears in title bar of message box

• MessageBox Buttons• OK, OKCancel, RetryCancel, YesNo, YesNoCancel,

AbortRetryIgnore

• MessageBox Icons• Asterisk, Error, Exclamation, Hand, Information, None,

Question, Stop, Warning


Using Overloaded Methods

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

• Each argument list is 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.


Testing Multiple Fields

• Each input field presents an opportunity for an exception.

• To indicate specific fields that caused the exception, use nested Try/Catch blocks.

• Pinpoints specific errors, and after error, sets focus back to field in error

• Use SelectAll method of text box to make text appear selected to aid user.


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

Chapter 3

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