Tutorial 3: Using Variables and Constants1 Tutorial 3 Using Variables and Constants

Tutorial 3: Using Variables and Constants2 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

Tutorial 3: Using Variables and Constants3 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

Tutorial 3: Using Variables and Constants4 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

Tutorial 3: Using Variables and Constants5 Selecting a Data Type for a Variable TypeSizeTypeSize Byte1Short2 Char2Integer4 Boolean2Long8 Decimal16Single4 Double8StringVaries Date8ObjectAnything

Tutorial 3: Using Variables and Constants6 Choose the Correct Data Type Short, Integer, LongUsed to store whole numbers Single, DoubleStore floating-point numbers DecimalStores numbers with a decimal point BooleanStores True and False CharStores one Unicode character ByteStores 8-bits of data DateStores date and time information StringStores a sequence of characters

Tutorial 3: Using Variables and Constants7 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

Tutorial 3: Using Variables and Constants8 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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10 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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Tutorial 3: Using Variables and Constants12 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

Tutorial 3: Using Variables and Constants13 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”

Tutorial 3: Using Variables and Constants14 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

Tutorial 3: Using Variables and Constants15 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

Tutorial 3: Using Variables and Constants16 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

Tutorial 3: Using Variables and Constants17 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

Tutorial 3: Using Variables and Constants18 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

Tutorial 3: Using Variables and Constants19 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

Tutorial 3: Using Variables and Constants20 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

Tutorial 3: Using Variables and Constants21 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 =  Public Const conMaxAge as Integer = 65

Tutorial 3: Using Variables and Constants22 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

Tutorial 3: Using Variables and Constants23 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 ExampleResult “Hello “ & strFirstNameHello Mary strFirstName & “ sold $“ & sngSales & “.”Mary sold $1000. intUnits & sngSales intUnits + sngSales1200

Tutorial 3: Using Variables and Constants24 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

Tutorial 3: Using Variables and Constants25 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

Tutorial 3: Using Variables and Constants26 The Object Browser  Provides information about objects available to your application

Tutorial 3: Using Variables and Constants27 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

Tutorial 3: Using Variables and Constants28 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

Tutorial 3: Using Variables and Constants29 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

Tutorial 3: Using Variables and Constants30 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

Tutorial 3: Using Variables and Constants31 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