tutorial 3: using variables and constants1 tutorial 3 using variables and constants

Tutorial 3: Using Variables and Constants 1

Tutorial 3 Using Variables and Constants


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


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


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


Selecting a Data Type for a Variable

Type Size Type Size

Byte 1 Short 2

Char 2 Integer 4

Boolean 2 Long 8

Decimal 16 Single 4

Double 8 String Varies

Date 8 Object Anything


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


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



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



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


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


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”


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


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


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


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 form-level variable and is said to have module scope


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


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


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


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


Modifying the Skate-AwaySales Application

Lesson B ObjectivesAfter 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


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


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


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


The Object Browser Provides information about objects available to your application


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


Modifying the Skate-AwaySales 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


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


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


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

tutorial 3: using variables and constants1 tutorial 3 using variables and constants

Documents

store data

item of data

variables data type

variableassigning data

variableassign data

new data

timeassigning data

intnumber variable