irwin/mcgraw-hill © the mcgraw-hill companies, inc., 1999 3-1 representing data constants and...

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© The McGraw-Hill Companies, Inc., 1999

Irwin/McGraw-Hill

Representing DataConstants and Variables

chapter THREE

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Representing DataConstants and Variables

After studying this topic you should be able to: Differentiate between numeric and string data. Determine whether a data item should be a constant

or variable. Code constants and variables in event procedures. Describe the characteristics and uses of standard data

types. Explain scope and describe the domain of variables in

event procedures and forms. Create projects that consist of several forms.

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Data item

Decisions required to specify a data item in VB

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Data item

String

Constant Variable


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Data item

String Numeric

Constant VariableConstant Variable

First decision: numeric or string

Second decision:

constant or variable


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String Values

A collection of printable symbols. Synonym: textExample:

“CNS 1200”“Visual Programming 1 – Visual Basic ”“It was a dark and stormy night”

Must be printable ANSI charactersSee ANSI table p. 175

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Numeric values Numbers that may be used in arithmetic

calculations Examples:13 14.6 -8.77 -0.023 001.800 -.023 ‘0 to left of the decimal not required 001.800 ‘leading 0’s ignored - trailing 0’s contain

information, carried if possible 8.9E-6 = 0.0000089 1.34E+3 = 1340. Note: no commas or $ in the number

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Classification of constants in VB

Numeric or stringconstant

SymbolicLiteral

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Constants

Fixed at run timeCannot be changedLiteral constant

- the value itselfSymbolic constant

- PI = 3.1416

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Symbolic Constant

Use a symbol - SALESTAXRATE - defined once, used many times

Name - 255 characters letters & numbers1st character must be a letterSpaces not allowed, SALES_TAX_RATEReserved words not allowed - problemAll Caps is the convention

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Defining & Declaring Constants

Const is a reserved word used to declare an immutable value and its symbolic name

Examples:Const SALES_TAX_RATE = 0.081Const COMPANY_ADDRESS =

“347 Main Street, Orem UT, 84058”Const DISK_ERR= “There is no disk in A:”

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No Magic Numbers

Only 0, 1, -1, “” allowed as literal constantsNamed constant easier to understandConstant reduces a the chance for typosMakes a change throughout the program easy

and correct, especially when the same value used for two different purposes.

TAX_RATE goes from 0.081 to 0.086Const SHIPPLING_PER_TON = .081

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Visual Basic Constants

Shipped as a part of Visual BasicA large number related to various objectsBackColor, ForeColor, …

vbRed, vbYellow, vbBlue, vbGreen …Alignment

vbLeftJustify, vbRightJustify, vbCenter

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New Line Constant

Used in many languages for a carriage return and a line feed

CHR(13) = Carriage returnCHR(10) = Line feedvbNewLine = CHR(13) + CHR(10)

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Which are Valid Constant Definitions?

In-class exercise 3.2

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Variables

A named memory location used as temporary holder

Current value is replaced forever by the new Declaration DIM Name as String

Variable name Variable data type

Initialization Name = “The Wolverine” VB provides default values

Numeric variables 0 String variables “” (empty string )

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Variable Names Name must be unique & should be meaningful Keywords may not be used as variable names Case Insensitive - but matches the Dim Dim is the keyword used to tell VB to

Give it a name - to be used later in the program Set aside an amount of memory

Name 255 characters, 1st letter alphabetic no spaces or special characters %, @, ., #, !, $

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Which are Valid Variable Names?

In-class exercise 3.3

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Data Types Defined in VBString

Variable lengthFixed length

NumbersInteger, Long, Single, Double, Currency

Others including VariantAvoid Variant in this course

User defined

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Data RepresentationStrings

“Visual Basic is easy to learn and use.”“Jones” “X” “286 Main Street” “3”“” ‘ The empty string

Numbers (can be used for arithmetic)286-308.45514259578374679.00974

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Numeric data types

Whole numbers 37 -842Integer - smallLong - big

Fractional numbers 37.7223 -842.01Single - BigDouble - BiggerCurrency - Bigger without rounding errors

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String Data Type

Variable length -- 2 Billion max Example:

Dim FirstName as StringFirstName = “Christopher” FirstName= “Victoria”

Size changes automatically Memory Size: 10 bytes + 1 byte per character

Fixed length -- 65,400 max Example:

Dim ZipCode as String * 5ZipCode = “48058”

Memory Size: 1 byte per character

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Fixed String Data Type

Required for Arrays and Record (Structure)Memory - Byte per space (here 26)Dim City as String * 26Unused space is filled by NULL charactersOverflow is truncated

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String Concatenation &

Appends one string to the end of another string

Example:String1 = “The big bad”String2 = “wolf.”String3 = String1 & String2String3 now contains The big badwolf.

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Integer Data Type

Whole number 89, -37, -32768, ?32840?Memory set aside 2 BytesMaximum Size +-32K (-32768 to +32767) Example:

Dim Age as IntegerAge = 100

Fractional values are dropped

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Long Data Type

Whole number 89, -37, -32768, 32840Memory Set aside 4BytesMaximum Size very big

-2,147,843,648 to 2,147,843,647 Example:

Dim BankBalance as Long ‘4 bytesBankBalance = 1000895 ‘ No comma’s

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Single Data Type

Fractional number 89.6 -37.0 .00006 Maximum - Big number (+- 1.4E-45 to 3.4E38) Memory Set aside 4 Bytes 7 Significant Digits

1.7894288 .00017894288 17894288 Example:

Dim Wage as SingleWage = 5.90

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Double Data Type

Fractional number 89.6 -37.0 .000061.7894288 .00017894288 17894288

Maximum - Very big (+- 4.9324 to 1.8E308)Memory Set aside 8 Bytes15 Significant Digits

-.0009787658654388955 9875.86445679987

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Currency Data Type

Fractional number with 4 decimal places Maximum Very big (15.4 digits)Memory Set aside 8 BytesExample:

Dim HouseLoan as Currency HouseLoan = 111211121112765.9999

No rounding errors for money calculations

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Decision rules for Standard Data Types

Math to be done

Size change?

F String !

V String !

Currency?

Currency !

NoYes

NoNo Yes

Yes

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Decision rules for Math Data Types

Decimals?

> 7 Digits ? > 32 K ?

Single

Double Long

Integer

No

No No

Yes

Yes Yes

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Review of Standard Data Types

Type Bytes LimitString 10 + character 2,000,000,000Integer 2 +- 32 KLong 4 +- 2 BillionSingle 4 7 digitsDouble 8 15 digitsCurrency 8 15.4 digits

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Defining, Declaring and Initializing Variables

Dim VariableName as DataTypeExamples:

Dim Name as String ‘ Empty stringName = “Jones”Dim Age as Integer ‘ 0 value enteredAge = 84Dim WageRate as Single ‘ 0.0 value enteredWageRate = 5.76

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Variable LifetimeLocal level variables

Created when Function/Sub calledDie when Function/Sub is exited

Form level variablesCreated when Form is Loaded into memoryDie when Form is un-Loaded from memory

Global variablesCreated when program startsDie when program terminates

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Variable Lifetime

Local - while code runsExcept Static variables

Module - while the Form runsGlobal - while the Program runs

Allows re-use of memory

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Variable Scope

Depends on where and how declaredScope How DeclaredLocal Dim, Static, ReDimModule Dim, PrivateGlobal Global, Public

Right mouse click will show properties of variable

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Local variables

Variable ValueMyName “John Doe”

Variable ValueMyName “ ”

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Variable Value

Variable Value

Module-level variable

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Variable Value

Variable Value

Module-level variable

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Variable Value

Variable ValueMyName “John Doe”YourName “Mary Smith”

Variable ValueYourName “ ”

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Variable Value

Variable Value

Variable ValueOurName “John Doe”

Variable Value

Variable Value

A globalvariable

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The three levels of variable

scope

A local variable is declared in an event

procedure using a Dim statement. It

can be accessed only in the event

procedure in which it is declared.

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Use

r in

terf

ace

Cod

e

The three levels of variable

scope

FormfrmTestA

A module-level variable is declared in

the general declarations section containing the Dim

statement. It can be accessed by any event

procedure on the same form as the general

declarations statement.

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Form frmTestA

Code module modTest

Project

Use

r in

terf

ace

Cod

e

Cod

e

Global Variablesare declared in the general declarations section of a code module using the Public statement, and can be accessed by the code in any event procedure anywhere in the project.

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Static vs Dim

Initializes variable the first time only, then the value is maintained for the remainder of the life of the program. Reuse of the variable (by entering the procedure), does not reinitialize the variable. Scope is unchanged.

Example:Static Joe As IntegerDim Joe As Integer

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Assignment operator =

Target = Source ‘ required direction 3 = A if A = 7 and B = 2 then after A = B what

is contained in both A and B ???Target must be a variableRight side of equation is completely

evaluated, then transferred over =Implicit type conversion will occur unless

explicit conversion by programmer

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Type Mismatch error

Trying to put a golf club in a gallon jug.Dim x as Integerx = “14 inches” ‘ Errorx = 14.99 ‘No Error auto truncation to

14Run time error - Depends on values when

the program is running so can’t be tested.

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Changes to the Propertyof a Control

Label1.Caption = “New Caption”Label2.Caption = Label1.CaptionMost properties can be Set or ReadMost properties can changeSome (Name, Tag,…) can not be changedSome are “Read only at run time” and can

not be changed

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Public vs Private code

Public changes all variables to GlobalAllows the code to be used by othersViolates good programming practices

Avoid Globals

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Methods vs Properties

Object.Method vs. Object.PropertyProperties

Store a Numeric or string valueCan be used in an assignment statement

MethodsCannot be used in assignment statementsMust be on a line by itself

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Additional Data Types

Byte 0 to 255Boolean True or FalseDate Jan,1, 100; Dec.31,9999Variant Any numeric or string valueObject (reference) User defined – Structures (records), Classes

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Byte Data Type

Whole positive number 0, 89, 255Memory set aside 1 ByteMaximum range 0 to 255Dim Age As ByteAge =100

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Boolean Data Type

True or FalseMemory set aside 2 Bytes

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Date Data Type

Contains both date and time information Date.TimeMemory set aside 8 BytesDate Jan.1, 100 to Dec.31, 9999Time 00:00:00 to 23:59:59

Noon = .5000, Midnight or 00:00:00 = 0

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Variant Data Type

Default data type Memory set aside 17 BytesWill hold any data type (Byte, Boolean,

Date, Double, Integer, Long, Single, String, Objects) and Empty, Error, Nothing, Null

type = VarTyp (variable name) ‘defines type of variable contained

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Object Data Type

A reference to an objectDo not use LetUse Set

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Multiple FormsAll the code is associated with the form and

its controls and is stored in the .frm fileLoad event occurs whenever a form is

displayedForm.Load -code is placed in RAMForm.Unload -code is removed from RAMForm.Show - Loads & Displays the formForm.Hide -Removes from display (but

still in RAM)

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More than one form in a project

Add form to projectEach form’s controls have access to

themselves or the controls on other formsForm1.Label1.BackColor =

Form2.BackColorCycle from one form to another with Hide

& Show properties.

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Adding & Deleting a Form

Adding a new form to a projectUse New Form Icon or Project New FormProject Explorer shows the additional form

Deleting a form from a projectHighlight in Project Explorer windowProject Remove Form

New forms generate a new file when saved

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Typical Contents of .frm fileVERSION 5.00Begin VB.Form Form1 Caption = "Assign01-Gross Pay Calculator" ClientHeight = 3465 ClientLeft = 1545 ClientTop = 1965 ClientWidth = 3600 BeginProperty Font Name = "MS Sans Serif" Size = 8.25 Charset = 0 Weight = 700 Underline = 0 'False…

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Contents of .vbp fileType=ExeForm=Assign2.FRMReference=*\G{00025E04-0000-0000-C000-

000000000046}#3.5#0#..\..\..\..\..\..\..\PROGRAM FILES\COMMON FILES\MICROSOFT SHARED\DAO\dao2535.tlb#Microsoft DAO 2.5/3.0 Compatibility Library

IconForm="Form1"Startup="Form1"ExeName32="Assign2.exe"Command32=""Name="Prob2"HelpContextID="0"

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Lab: Data Types

Complete Exercise 3.16

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