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Brief description on how to navigate within this presentation (ppt)
The first time a Key Term from the chapter is used in the ppt it will display in blue
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2. Variables, Constants and Calculations3
Chapter
Variables, Constants and Calculations
McGraw-Hill
© 2006 The McGraw-Hill Companies, Inc. All rights reserved.

3. 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

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)
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
When you declare a variable or a named constant, VB reserves an area of memory and assigns it a name, called an identifier
Identifier names are specified according to the rules of Basic as well as some recommended naming conventions

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 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
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
Short
Small integer in the range -32,768 to 32,767
Integer
Whole numbers in the range -2,147,483,648 to +2,147,483,647
Long
Larger whole numbers
String
Alphanumeric data: letters, digits, and other characters
Varies
Object
Any type of data
The data type of a variable or constant indicates what type of information will be stored in the allocated memory space
The data type charts displays the kind of data each type of data types hold, and the amount of memory allocated for each

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 class (data type) of variable (QUOTA_Integer)
Use mixed case for variables and uppercase for constants (quantityInteger)
Can not use reserved words or keywords that Basic has assigned a meaning such as print, name, and value
A programmer has to name (identify) the variables and named constants that will be used in a project

9. Constants Named User assigned name, data type and value
Use CONST keyword to declare
Const COMPANY_ADDRESS_String As String = "101 S. Main Street"
Const SALES_TAX_RATE_Decimal As Decimal = .08D
Constants provide a way to use words to describe a value that doesn’t change
Constants are declared using the keyword and are given a name, a data type, and a value
Once a value is declared as a constant, its value can’t be changed during the execution of the project
Data type declared and data type of the value must match
Many sets of intrinsic constants (key term) are declared in system class libraries and are available for use in VB programs

10. Assigning Values to Constants
Declare the data type of numeric constants by appending a type-declaration character
Decimal D
Decimal – D
Double R
Double – R
Integer I
Integer – I
Long L
Long – L
Short S
Single F
Single – F
If a type-declaration character is not appended any whole number is assumed to be Integer and any fractional value is assumed to be Double
Use two quotes with a string literal to avoid confusion

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
Dim customerName Strging As String
As String
Private totalSoldInteger As Integer
As Integer
Dim temperatureSingle
As Single
Dim priceDecimal
As Decimal
Private priceDecimal
VB IntelliSense feature helps you enter Private, Public, and Dim statements—after you type the space that follows VariableName As, a list pops up and displays the possible entries for data type to complete the statement
If you begin to complete the statement the list automatically scrolls to the correct section; when the correct entry is highlighted press Enter, Tab, or the spacebar to select the entry or double-click if using the mouse
The reserve word Dim is really short dimension, which means size. When declaring a variable, the amount of memory reserved depends on its data type.

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
A variable may exist and be visible for an entire project, for only one form, or for only one procedure
Visibility really means “this variable can be used or ‘seen’ in this location.”
Namespace - Available to all procedures of project
Module - Available to all procedures within that module (often a form)
Use Public or Private keywords
Local - Available only to the procedure in which it is declared
Block - Available only in block of code inside a procedure where declared
An example of a module level variable declaration is displayed on the next slide (Slide 15)

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 a Numeric 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
The class that you use depends on the data type of the variable to which you are assigning a value; for example, to convert text to an integer, use the Integer.Parse method

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
When you assign a value to a variable, you must take care to assign like types; you assign an integer value to an Integer variable and a decimal value to a Decimal variable ( chart shown on next slide, Slide 19)
There are some exception to the rule about assigning only like types See “Implicit Conversions”

18. Conversion Methods Method Convert To Integer.Parse Integer
Decimal.Parse Decimal
.ToString String

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
The arithmetic operations you can perform you can perform in VB include addition, subtraction, multiplication, division, integer division, modulus, and exponentiation
The first four operations are self explanatory, but you may not be familiar with Integer Division, Modulus, and/or exponentiation
Integer division-use to divide one integer by another giving an integer result truncating (dropping) any remainder
Modulus – returns the remainder of a division problem
Exponentiation – raises a number to the pwer specified and retruns (produces) a result of the Double data type

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
VB provides two options that can significantly change the behavior of the editor and compiler
Not using either Option Explicit and/or Option Strict can make coding somewhat easier but provide opportunities for hard-to-find errors and very sloppy programming
When Option Explicit is turned off, programmers can use any variable name without first declaring it which is a throwback to the older version of Basic and programmers spent many hours debugging programs that had small typos in a variable name
Programmers should always program with Option Explicit turned on-in VB.NET the option is turned on by default for all new projects. Programmers will need to place a an additional code line before the first line of code in a file to turn it off
When Option Strict it turned on, the editor and compiler try to help keep programmers from making hard-to-find mistakes.
By having Option Strict turned on programmers can be saved from developing poor programming habits

26. 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()
When wanting to display numeric data in the Text property of a label or text box the value must first be converted to string—the data can be formatted which controls the way the output will look

27. 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
The format specifier codes formats the display of output and are predefined
The default format of each of the formatting codes is based on the computer’s regional setting
Format specifier codes are displayed on the next slide (Slide 32) and examples are shown on the following slide (Slide 33)

28. 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

29. Format Specifier Code Examples
Variable
Value
Code
Output
totalDecimal
"C"
$1,125.67
"N0"
1,126
pinInteger
123
"D6"
000123
rateDecimal
0.075
"P"
7.50%
"P3"
7.500%
"P0" 8%
valueInteger
-10
($10.00)

30. Date Specifier Code
Format DateTime values using format codes and ToString method
Date codes are case sensitive
The strings returned are based on the computer’s regional settings and can be changed. Currently the default values are set for US-English in Windows XP
*Refer to chart on p. 118

31. 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)
Two sample message boxes

32. 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

33. 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

34. 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

35. 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
Programs often need to calculate the sum of numbers.
The technique for summing is to declare a module-level variable for the total