IS 118 Introduction to
Development Tools
VB Chapter 04
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Chapter 4
Data, Operations, and Built-In
Functions
Visual Basic. NET
Copyright (c) 2003 by Prentice Hall
Objectives
Differentiate among different types of data
Declare data types properly for constants
and variables
Appreciate the importance of having the
IDE check variables for their declaration
Differentiate and appreciate the
implications of the scope and lifetime of
variables under different declarations
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Objectives
 Understand the nature of the assignment
statement
 Recognize and use various arithmetic
expressions and functions
 Understand the usefulness of strict type
checking
 Recognize and use various string functions
 Understand better how data and VB controls are
used together in a project
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Classification of Data
MsgBox(“Your age is “ & 24)
Text in quotation marks is a string
Text can be manipulated, but no computations
Numeric data can be used for computation
Data must be kept in memory before it can
be retrieved for manipulation, computation,
or display
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Using Memory Locations
Give memory locations a name and refer
to the name
If data expected to change, classify as
variable
If data not expected to change, classify as
constant
Named constant: a constant given a name and
referenced accordingly
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Declaring Constants
Some constants are predefined
Known as system constants
Examples include True and False
Others defined and declared before used
Known as symbolic constants
Must be declared with Const statement
To declare a constant: Const MyName = “Tom
McKenzie”
Defines the constant MyName and assigns a value
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Why Use Constants?
Makes code easier to read and trace
To change the value of a constant, change
it where it is declared
Change will be reflected throughout system
Name all constants for your program in
one location
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Declaring Variables
Declared with the Dim statement
Must give the variable a name
Optionally, define data type
Although it’s only optional, you should specify data
type
Can use type suffixes for data types
Code suffixes hard to read and understand
Syntax: Dim MyID as Long
Defines a variable named MyID as a long data type
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Checking for Variable Declaration
Option Explicit statement determines
whether VB requires variable declaration
When set to Off, VB will not require declaration
Default is On, so variables must be
declared before they can be used
Get in the habit of requiring declaration
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Why Require Declaration?
An undeclared or misspelled variable can
give unexpected results
You can spend hours tracing bugs
Helps you catch syntax errors
When you use variables in code, VB will
enforce same capitalization as when
declared
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Rules for Declaration
 Can contain any combination of letters and
numbers
 Can begin with letter or underscore, but not a
number
Must have at least one letter or digit if it begins with an
underscore
 Cannot contain spaces
 If it has same name as a reserved keyword, i.e.
Date, must be enclosed with brackets
 Cannot contain embedded periods or data suffix
characters
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Scope and Lifetime of Variables
Scope refers to how widely variable is
recognized throughout program
Lifetime refers to how long a variable
remains in computer memory
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Class-Level Variables
 Code window associated with form referred to as
class module
 Class level variables declared in general
declarations area (before any code is entered)
with Dim statement
 Scope: recognized by all procedures in the
class
 Lifetime: created when form is loaded and
remain in memory until form is destroyed
(closed)
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Procedure-Level Variables
 Declared inside a procedure
Also called “local” variables
 Scope: only recognized within the procedure
 Lifetime: depends on how declared
If declared with Dim keyword, reinitialized every time the
procedure is called
 String variables reinitialized to an empty string
 Numeric variables reinitialized to 0
If declared with Static keyword, values preserved rather
than reinitialized every time the procedure is called
 Destroyed when form is destroyed
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Declaration Issues
 Block-level declaration:
If declared within an If block, recognized only within the
block
 Overlapping declarations:
When variable with same name declared at class level
and procedure level, VB uses the memory location with
the narrowest type
 Take care not to overlap
 In general, declare variables at narrowest
possible scope
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Access Modifiers
With class-level variables, you need to
decide whether they will be accessible to
other modules in the project
Instead of declaring with Dim statement,
use Public or Private keyword
Public access – visible to other modules
Public MyIDNum as Integer
Private access – not visible to other modules
Private MyIDNum as Integer
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Objects
An object is actually an instance of a class
Think of the class as a template
Object is created as an instance of that class
Process called instantiation
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Declaring Objects
 To use object data types in code is a two-step
process
Declare variable of that class type
Dim M as Math declares M as an variable of Math
type
Create an instance of that class
requires the New keyword
M = New Math() associates the variable M with an
instance of the Math object
Can do both in one statement with most
objects
Dim M as New Math() combines the two previous
statements
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Numeric Data and Types
Many different ways to store and handle
numbers
Precision of data type refers to accuracy in
handling significant digits
Inadequate precision can lead to significant
rounding error in calculations
Different types require different amounts of
storage
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Data Types of Limited Values
Byte – uses one character
Often used to handle ASCII code data
Char – uses two characters
Often used to handle Unicode code data
Neither type handles negative numbers
Boolean – stores logical value of True or
False
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Data Types for Whole Numbers
 Can handle positive or negative numbers, but no
decimal places
 Range of numbers determined by storage size
Short
 Requires 2 bytes of storage
Integer
 Requires 4 bytes of storage
Long
 Requires 8 bytes of storage
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Data Types for Mixed Numbers
Uses floating point coding scheme
Mantissa: stores base value of data
Exponent: stores how the mantissa is raised (to
the power of 10)
Different types handle different numbers of
significant digits and need different
storage
Single: 7 significant digits (4 bytes storage)
Double: 15 significant digits (8 bytes storage)
Decimal: 29 significant digits (16 bytes storage)
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Characteristics of Numeric Types
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Implicit Type Conversion
 Often bugs are created when the compiler
converts different data types
Can use data conversion functions to explicitly
convert data type by code
 Option Strict On forces you to observe strict
data conversion rules
You can implicitly convert from narrower type to
wider (e.g., from Integer to Long)
 To convert to narrower type, use data conversion
functions
 You should use this option to avoid
unexpected data conversion errors
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Assignment Statement
 OvertimePay = StandardPay * 1.5 moves data
resulting from the operation on the right side to
the memory location on the left
Variable on right must be properly declared before
operation can be moved to left
Variable on left must have the capacity to handle the
result of the operation on the right
 if not, an overflow error occurs
Left side must be a variable or a control property whose
value can be set at run time
 txtOT.Text = StandardPay * 1.5 is a valid assignment
statement
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Numeric Operations
 Code on right side referred to as expression
Can be a constant, a variable, or any numeric
operation
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Operational Precedence
 Order of Preference
Power
Unary negation
Multiplication and division
Integer division
Mod
Addition and subtraction
 To enforce a different order or eliminate
confusion, enclose expression in parentheses
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An Application Example:
Computing Net Sales
 Analyze and define requirements
An application to compute net receipts
 Determine controls needed
Mainly labels and text boxes
Right align text boxes so decimal places line up
 Declaring variables and constants
Use variables to hold data entered into text boxes
Use a constant to hold credit card processing rate
Use variables to hold net processing and net receipts
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An Application Example:
Computing Net Sales
Computations
Decide when to store information entered in text
boxes
Create formulas for computation
Decide how to display result of computations
Format function displays results with defined format
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Computing Total Receipts
CreditCardRate defined as class-level
constant
FmtStandard defined as class-level
constant
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The Visual Interface
All text boxes have the
same format, right-aligned
with two decimal places
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Built-In Numeric Functions
 Mathematical Functions
Rnd function generates a random number >= 0.0 and <
1.0
 Often used in simulations
 Math Object contains many built-in methods for
computations
Examples include Abs (absolute value) and Sqrt (square
root)
 Date/Time functions return a number of date or
time related values
Today returns the system date as defined on the
computer
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Conversion Functions
 Used to explicitly convert a variable from
one data type to another
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String Data
Declared with the String data type
All strings are variable-length strings
String Operations:
Concatenation combines 2 strings
Valid concatenation operators: & and +
If you use + and either string contains only numbers,
you will get unexpected results
You should always use & to concatenate
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Common String Functions
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An Application Example:
The Calculating Vending Machines
 Analyze and define the requirements
A form a user can use to purchase items
User will select an item from a list box
 Setting Up the Vending Machine
Use the Items.Add method of the list
Create 2 columns in the list by including a tab
character (vbTab) between item and price
Use the With block to add multiple items save
keystrokes adding the items
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Application Example:
The Calculating Vending Machine
Determining the Item Chosen
Must determine name chosen
Must also determine price for item
Displaying Selected Items
If user selects more than one item, must
concatenate into a single string
Obtaining the Price
Price will be a string in the list box
Must convert into a number
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Application Example:
The Calculating Vending Machine
 Accumulating the total
When user selects an item, the cost must be added to
the accumulated total
 Declaring variables
Give special consideration to scope and type
 Precision will be very important
 Checking out
Need a mechanism to allow user to indicate he/she has
finished ordering
Need a routine in case the user checks out without
buying anything
Reinitialize class-level variables after checking out
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Loading the Form
Class-level variables used by several
event procedures
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Calculating Items Bought
Tab divides line into
fields, use InStr function
to find Tab
Add carriage return and item to
class-level ItemsBot variable
Accumulate total; must
convert text to single
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Summary
 Data can be classified into two broad categories:
numeric and string
 Data can be classified into variables and constants
Variables require a name
Constants do not require a name, though it is
advisable
 Constants declared with keyword Const
 Variables declared with either Dim or Static keyword
 Variables and constants classified by scope
If declared at class level, recognized by all procedures
in the class
If declared within procedure, recognized only by that
procedure
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Summary
 Variables and constants classified by lifetime
 If declared at class level, they exist as long as class (form)
exists
 If declared inside procedure with Dim keyword, reinitialized
every time the procedure is called
 If declared inside procedure with Static keyword, they exist
as long as the class exists
 If declared inside block, they are out of scope as soon as the
block is terminated
 Declare variables with as narrow a scope as possible
 Declare constants with as broad a scope as possible
 Numeric data can be classified into different types, for
different computational needs
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Summary
 VB can perform a wide range of mathematical
operations
 VB provides many built-in mathematical,
date/time and data conversion functions
 VB provides many functions for manipulating
string data
 The Format function can be used to format
numeric data
 Use the Option Strict On setting to enforce
explicit data conversion
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