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  The memory locations are called variables, because the contents of the locations can change as the program is running  You should assign a descriptive name to each variable used in an application – it should help you remember the variable’s data type and purpose

Tutorial 3: Using Variables and Constants4 Selecting a Data Type for a Variable TypeSizeTypeSize Byte1Short2 Char2Integer4 Boolean4Long8 Decimal12Single4 Double8StringVaries Date8ObjectAnything

Tutorial 3: Using Variables and Constants5 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 Constants6 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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9 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 Constants10 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 Constants11 Assigning Data to an Existing Variable  Quotation marks differentiate a string from both a number and a variable name  The Dim statement creates the intNumber variable in memory and automatically initializes it to the number 0

Tutorial 3: Using Variables and Constants12 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 Constants13 The Scope of a Variable  A variable’s scope indicates which procedures in an application can use the variable – either 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 Constants14 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 Constants15 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 Constants16 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 Constants17 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