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Variables, Constants, and Calculations
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 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 reservedA name is assigned called an IdentifierFollow 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 RulesShould 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
MethodConvert 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) 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 typesConvert 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,
AbortRetryIgnoreMessageBox 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