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2. 3 Chapter Variables, Constants and Calculations McGraw-Hill © 2006 The McGraw-Hill Companies, Inc. All rights reserved.

3. Objectives (1 of 2) • Distinguish between variables, and constants • 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

4. 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

5. 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

6. 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

7. Data Types

8. 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 the class (data type) of the variable • Use 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

9. 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

10. Declare the data type of numeric constants by appending a type-declaration character Assigning Values to Constants

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 type • May declare several variables with one statement • Use IntelliSense to assist in writing statements

12. Declaration Statement Examples

13. 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

14. Module Level Variable Declaration Example Code module-level declarations in the Declaration section at the top of the code

15. 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

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 method • Parse method returns a value that can be used in calculations • Parse method fails if user enters nonnumeric data or leaves data blank

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

18. Conversion Methods • Method Convert To • Integer.ParseInteger • Decimal.ParseDecimal • .ToStringString

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( )

20. Arithmetic Operations • Operator Operation • + Addition • – Subtraction • * Multiplication • / Division • \ Integer Division • Mod Modulus – Remainder of division • ^ Exponentiation

21. Order of Operations • Hierarchy of operations, or order of precedence, in arithmetic expressions from highest to lowest 1. Any operation inside parentheses 2. Exponentiation 3. Multiplication and division 4. Integer division 5. Modulus 6. Addition and subtraction

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.

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 multiply 8/4*2 = 4 Same level, left to right: divide then multiply

24. 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

25. 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

26. 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

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

28. 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”

29. 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 Me.displayTextBox.Text = numberInteger.ToString()

30. 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

31. Format Specifier Codes

32. Format Specifier Code Examples

33. Date Specifier Code • Format DateTime values using format codes and ToString method • Date codes are case sensitive

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

35. 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

36. Try Block - General Form Try statements that may cause error Catch [VariableName As ExceptionType] statements for action when an exception occurs [Finally statements that always execute before exit of Try block] End Try

37. Try Block - ExampleCatches Any Exception Try quantityInteger = Integer.Parse(quantityTextBox.Text) Me.quantityLabel.Text = quantityInteger.ToString( ) Catch Me.messageLabel.Text = "Error in input data." End Try

38. Try Block - ExampleCatches Specific Exception • This Catch statement catches bad input data that cannot be converted to numeric CatchtheException As FormatException messageLabel.Text="Error in input data." End Try

39. Common Exception Classes

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.

41. Try Block - ExampleHandling Multiple Exceptions Try 'Statements that may cause errors. CatchtheException As FormatException ' Statements for nonnumeric data. CatchtheException As ArithmeticException ' Statements for calculation problem. CatchtheException As Exception ' Statements for any other exception. End Try

42. MessageBox Object (1 of 2) • MessageBox flavors • 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)

43. 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

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

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

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 Integer • saleCountInteger += 1 • averageDiscountedSaleDecimal = discountedPriceSumDecimal / saleCountInteger