Transcript Using Variables and Constants

Tutorial 3
Using Variables and Constants
Tutorial 3: Using Variables and Constants
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Creating Variables
and Named Constants
Lesson A Objectives
After completing this lesson, you will be able to:
 Create a local and form-level variable
 Select an appropriate data type for a variable
 Select an appropriate name for a variable
 Assign data to an existing variable
 Create a named constant
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Using Variables to Store Information
 Besides storing data in the properties of
controls, a programmer also can store data,
temporarily, in memory locations inside the
computer
 The memory locations are called variables,
because the contents of the locations can
change as the program is running
 You can enter and store data in the box, but
you cannot actually see the box
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Using Variables to Store Information
 One use for a variable is to hold information
that is not stored in a control on the user
interface
 You can also store the data contained in a
control’s property in a variable
 Before learning how to create a variable in a
Visual Basic .NET application, you learn how
to select an appropriate data type and name
for the variable
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Selecting a Data Type for a Variable
Type
Size
Type
Size
Byte
1
Short
2
Char
2
Integer
4
Boolean
4
Long
8
Decimal
12
Single
4
Double
8
String
Varies
Date
8
Object
Anything
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Choose the Correct Data Type
Short, Integer, Long
Used to store whole numbers
Single, Double
Store floating-point numbers
Decimal
Stores numbers with a decimal point
Boolean
Stores True and False
Char
Stores one Unicode character
Byte
Stores 8-bits of data
Date
Stores date and time information
String
Stores a sequence of characters
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Selecting a Name for a Variable
 You should assign a descriptive name to
each variable used in an application
 The name should help you remember the
variable’s data type and purpose
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Selecting a Name for a Variable
 Figure 3-4 lists the three characters typically
associated with the Visual Basic .NET data types
 It is a common practice to type the letter m and
the three-character ID using lowercase letters,
and then use Pascal-case for the remainder of the
variable’s name
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Selecting a Name for a Variable
 Using Pascal-case, you capitalize the first
letter in each word in the name
 In addition to being descriptive, the name that
a programmer assigns to a variable must
follow several rules, which are listed in Figure
3-5
 Also included in the figure are examples of
valid and invalid variable names
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Rules for Naming Variables
 Name must begin with a letter
 Name can contain only letters, numbers,
and the underscore. No punctuation
characters or spaces are allowed
 Name cannot exceed 255 characters
 Name cannot be a reserved word
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Declaring a Variable
Accessor variablename As Datatype [ = InitialValue]
Accessor is [Public|Private|Static|Dim]
Dim intTotal As Integer
Dim sngRadius As Single = 12
Dim intYellow, intBlue As Integer
Public strName As String = “Diane Zak”
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Assigning Data to an Existing Variable
 A literal constant is simply an item of data whose
value does not change while the application is
running
 Also notice that string literal constants are
enclosed in quotation marks, but numeric literal
constants and variable names are not
 The quotation marks differentiate a string from
both a number and a variable name
 It is important to remember that a variable can
store only one item of data at any one time
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Assigning Data to an Existing Variable
 When you use an assignment statement to assign
another item to the variable, the new data
replaces the existing data
 When you run the application and click the
button, the three lines of code are processed as
follows:
 The Dim statement creates the intNumber variable
in memory and automatically initializes it to the
number 0
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Assigning Data to a Variable
 The intNumber = 500 assignment statement removes the zero
from the intNumber variable and stores the number 500 there
instead
 The intNumber = intNumber *2 assignment statement first
multiplies the contents of the intNumber variable (500) by the
number 2, giving 1000
The assignment statement is of the form
variablename = value
sngHours = 38.5
sngBonus = sngSales * 0.1
strName = “Mary”
intNumber = 500
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The Scope of a Variable
 A variable’s scope indicates which procedures in an
application can use the variable
 The scope is determined by where the Dim, Public or
Private statement is entered
 When you declare a variable in a procedure, the
variable is called a local variable and is said to have
procedure scope, because only that procedure can
use the variable
 When you declare a variable in the form’s
Declarations section, the variable is called a formlevel variable and is said to have module scope
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Creating a Local Variable
 Created with the Dim statement
 The Dim statement is entered in an object’s
event procedure
 Only the procedure in which it is declared can
use the variable
 Removed from memory when the procedure
ends
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Creating a Form-level Variable
 Created with the Public/Private statement
 Entered in a form’s General declarations
section
 Can be used by any of the procedures in the
form
 Removed from memory when the application
ends or the form is destroyed
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Constants
 Literal constant
 an item of data
whose value cannot
change while the
program is running
 Examples:
 7
 “Mary”
 Named constant
 a memory location
whose contents
cannot be changed
while the program is
running
 Examples:
 conPi
 conRate
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Creating a Named Constant
 A memory location whose value cannot change
during run time
 Syntax: [Public|Private] Const constname [As
datatype] = expression
 Examples:
 Const conPi As Single = 3.141593
 Public Const conMaxAge as Integer = 65
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Modifying the Skate-Away
Sales Application
Lesson B Objectives
After completing this lesson, you will be able to:
 Include local and form-level variables in an
application
 Concatenate strings
 Get user input using the InputBox function
 Locate the Visual Basic .NET intrinsic constants
in the Object Browser
 Include the vbNewLine constant in code
 Designate the default button for a form
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Concatenating Strings

Connecting strings together is called concatenating

You use the concatenation operator, which is the ampersand
(&), to concatenate strings in Visual Basic .NET

When concatenating strings, you must be sure to include a
space before and after the concatenation operator
Example
Result
“Hello “ & strFirstName
Hello Mary
strFirstName & “ sold $“ & sngSales & “.”
Mary sold $1000.
intUnits & sngSales
2001000
intUnits + sngSales
1200
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The InputBox Function
 The InputBox function displays one of Visual
Basic .NET’s predefined dialog boxes
 Contains a message, along with an OK button, a
Cancel button, and an input area
 Syntax: strAnswer = InputBox(prompt, title,
default)
 Use sentence capitalization for the prompt, and
book title capitalization for the title
 Has limitations: can’t control appearance and
allows user to enter only one piece of data
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The Newline Character
 The newline character, which is Chr(13) &
Chr(10), instructs the computer to issue a
carriage return followed by a line feed
 An intrinsic constant is a named constant
that is built into Visual Basic .NET itself
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The Object Browser
 Provides information about objects available to your application
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Designating a Default Button
 Can be selected by pressing the Enter key
even when the button does not have the
focus
 Set the form’s AcceptButton property to the
desired button
 If used, it is typically the first button
 If a button’s action is destructive and
irreversible, then it should not be the default
button
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Modifying the Skate-Away
Sales Application’s Code
Lesson C Objectives
After completing this lesson, you will be able to:
 Include a static variable in code
 Code the TextChanged event procedure
 Create a procedure that handles more than
one event
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Static Variables
 A static variable is a local variable that retains
its value when the procedure in which it is
declared ends
 Syntax:
Static variablename As datatype [= initialvalue]
 Removed from memory when application
ends or form is removed from memory
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Coding the TextChanged
Event Procedure
 A control’s TextChanged event occurs
when the contents of a control’s Text
property change
 This can happen as a result of either the
user entering data into the control, or the
application’s code assigning data to the
control’s Text property
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Associating a Procedure
with Different Objects or Events
 The keyword Handles appears in a procedure
header and indicates the object and event
associated with the procedure
 You can also associate a procedure with
more than one object and event; to do so, you
simply list each object and event, separated
by commas, in the Handles section of the
procedure header
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