Chapter 3
Variables, Constants
and Calculations
Programming In
Visual Basic .NET
Data - Variables and Constants
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
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Data - Variables and Constants
(continued)
• 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
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Data Types (p 87 Table 3.1)
•
•
•
•
•
•
•
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Boolean
Byte (0 to 255)
Char
Date
String
Decimal
Object
• Short (-32,768 to 32,767)
• Integer (-2,147,483,648 to
2,147,483,647)
• Long (larger whole numbers)
• Single (floating point accuracy to 6
digits)
• Double (floating point accuracy to 14
digits)
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Data Types – Memory Usage
•
•
•
•
•
•
•
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Boolean – 2 bytes
Byte – 1 byte
Char – 2 bytes
Date – 8 bytes
String – varies
Decimal – 16 bytes
Object – 4 bytes
•
•
•
•
•
Short – 2 bytes
Integer – 4 bytes
Long – 8 bytes
Single – 4 bytes
Double – 8 bytes
© 2005 by The McGraw-Hill Companies, Inc. All rights reserved.
Naming Variables and Constants
• Must follow Visual Basic Naming Rules
• Should follow Naming Conventions
– Meaningful names
– Include class (data type) of variable
– Use mixed case for variables and uppercase for
constants
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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
• Intrinsic
– System defined within Visual Studio
– In Chapter 2 we used the Intrinsic Color Constants
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Assigning Values to Constants
• Declare the data type of numeric constants by appending a
type-declaration character
• Decimal - D
• Double - R
• Integer - I
Integer – 12345678I
Decimal – 850.50D
Long – 134257987L
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• Long - L
• Short - S
• Single - F
Single – 101.25F
Double – 52875.8R
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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
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Declaration Examples
Dim customerNameString
Private totalSoldInteger
Dim temperatureSingle
Dim priceDecimal
Private priceDecimal
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As String
As Integer
As Single
As Decimal
As Decimal
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Scope and Lifetime of Variables
• 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
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Scope and Lifetime of Variables
(continued)
• Namespace
– Available to all procedures of project
– Good programming practice excludes use of Namespace
variables
• Module
– Available to all procedures within that module (often a
form)
– Use Public or Private keywords
– Place in declaration section of module (form)
• Local
– Available only to the procedure it is declared in
– Declare with Dim keyword and place at top of procedure
• Block (not used until later in this course)
– Available only in block of code where declared
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Module Level Variable Declaration
Example
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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
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Converting Strings to a
Numeric Data Type
• Use Parse methods to convert Text to numeric data
• 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
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Converting to String
• Values assigned to string variables or Text properties must
be string
• Convert any numeric data type to string using .ToString
method
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Conversion Methods
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Method
Convert To
Integer.Parse
Decimal.Parse
.ToString
Integer
Decimal
String
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Conversion Examples
quantityInteger
priceDecimal
=
=
wholeNumberInteger =
resultLabel.Text
=
countTextBox.Text =
idString
=
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Integer.Parse(quantityTextBox.Text)
Decimal.Parse(priceTextBox.Text)
Integer.Parse(digitString)
resultDecimal.ToString( )
countInteger.ToString( )
idInteger.ToString( )
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Arithmetic Operations
Operator
+
–
*
/
\
Mod
^
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Operation
Addition
Subtraction
Multiplication
Division
Integer Division
Modulus – Remainder of division
Exponentiation
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Order of Operations
• 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
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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
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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
=, +=, -=, *=, /=, \=, &=
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Option Explicit and Option Strict
• Option Explicit forces variables to be declared before using
• Option Strict
– Makes VB a strongly typed language
– 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
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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
• ToDecimal, ToSingle, ToDouble, ToInt16 (Short),
ToInt32 (Integer), ToInt64 (Long)
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Rounding Numbers
• Round decimal fractions
– Use Decimal.Round method
– Decimal.Round and Convert methods use technique
called “rounding toward even”
• See Appendix B for additional mathematical, financial and
string functions
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Formatting Data for Display
• To display numeric data in a label or text box, first convert
value to string
– Use ToString method
resultLabel.Text = resultDecimal.ToString( )
countTextBox.Text = countInteger.ToString( )
• 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
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Using Format Specifier Codes
• "C" code
– Currency – String formatted with dollar sign, commas
separating each group of three digits and two digits to
the right of decimal point
• "N" code
– Number – String formatted with commas separating
each group of three digits and two digits to the right of
decimal point
• Can specify number of decimal positions
– Example: "C0" zero digits
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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
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Format Specifier Codes 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)
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Date Specifier Code
• Format DateTime values using format codes and ToString
method
• Date codes are case sensitive
d
Short date
D
Long date
t
Short time
T
Long time
f
Full date/time
F
Full date/time
g
General (short time) G
General (long time)
M or m Month
R or r GMT pattern
• ToLongDateString, ToShortDateString, ToLongTimeString,
ToShortTimeString methods also available
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Handling Exceptions
• Use structured exception handling to catch errors before
run-time error occurs
• Catching exceptions referred to as error trapping
• Code to handle exception called error handling
• Error handling in Visual Studio .NET is standardized for
all languages using the CLR
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Try/Catch Blocks
• Enclose statements that might cause an error within
Try/Catch block
– If an error occurs 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
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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
See page 114 for list of common Exception Classes
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Try Block - Example
Catches Any Exception
Try
quantityInteger = Integer.Parse(quantityTextBox.Text)
quantityLabel.Text = quantityInteger.ToString( )
Catch
messageLabel.Text = "Error in input data."
End Try
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Try Block - Example
Catches Specific Exception
Try
quantityInteger = Integer.Parse(quantityTextBox.Text)
quantityLabel.Text = quantityInteger.ToString( )
Catch theException As FormatException
messageLabel.Text="Error in input data."
End Try
Failure of numeric conversion, usually
blank or nonnumeric data
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Try Block - Example
Handling 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
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Displaying Messages in Message
Boxes
• Use Show method of MessageBox to display message box,
a special type of window
• Arguments of Show method
– Message to display
– Optional Title Bar Caption
– Optional Button(s)
– Optional Icon
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MessageBox Object
• 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)
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MessageBox Object (continued)
• 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
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Testing Multiple Fields
• Each input field presents a chance for an error
• To indicate specific field that caused error use nested
Try/Catch blocks
• After error set focus back to field in error
– Use SelectAll method of text box to make text appear
selected to aid user
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Counting and Accumulating Sums
• Declare module-level variables, since local level variables
reset to 0 each time the procedure is called
• Summing Numbers
discountedPriceSumDecimal += discountedPriceDecimal
• Counting
Private saleCountInteger As Integer
saleCountInteger += 1
• Calculating an Average
averageDiscountedSaleDecimal = discountedPriceSumDecimal / saleCountInteger
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