Tutorial 3 Using Variables and Constants Tutorial 3
Tutorial 3 Using Variables and Constants Tutorial 3: Using Variables and Constants 1
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 Constants 2
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 Constants 3
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 Constants 4
Selecting a Data Type for a Variable Type Size Byte 1 Short 2 Char 2 Integer 4 Boolean 4 Long 8 Decimal 12 Single 4 Double 8 String Varies Date 8 Object Anything Tutorial 3: Using Variables and Constants 5
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 Tutorial 3: Using Variables and Constants 6
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 Constants 7
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 Tutorial 3: Using Variables and Constants 8
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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 Tutorial 3: Using Variables and Constants 10
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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 Constants 12
Declaring a Variable Accessor variablename As Datatype [ = Initial. Value] Accessor is [Public|Private|Static|Dim] Dim int. Total As Integer Dim sng. Radius As Single = 12 Dim int. Yellow, int. Blue As Integer Public str. Name As String = “Diane Zak” Tutorial 3: Using Variables and Constants 13
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 Constants 14
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 int. Number variable in memory and automatically initializes it to the number 0 Tutorial 3: Using Variables and Constants 15
Assigning Data to a Variable § The int. Number = 500 assignment statement removes the zero from the int. Number variable and stores the number 500 there instead § The int. Number = int. Number *2 assignment statement first multiplies the contents of the int. Number variable (500) by the number 2, giving 1000 The assignment statement is of the form variablename = value sng. Hours = 38. 5 sng. Bonus = sng. Sales * 0. 1 str. Name = “Mary” int. Number = 500 Tutorial 3: Using Variables and Constants 16
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 Constants 17
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 Constants 18
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 Constants 19
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: § con. Pi § con. Rate Tutorial 3: Using Variables and Constants 20
Creating a Named Constant § A memory location whose value cannot change during run time § Syntax: [Public|Private] Const constname [As datatype] = expression § Examples: § Const con. Pi As Single = 3. 141593 § Public Const con. Max. Age as Integer = 65 Tutorial 3: Using Variables and Constants 21
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 Input. Box function § Locate the Visual Basic. NET intrinsic constants in the Object Browser § Include the vb. New. Line constant in code § Designate the default button for a form Tutorial 3: Using Variables and Constants 22
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 “ & str. First. Name Hello Mary str. First. Name & “ sold $“ & sng. Sales & “. ” Mary sold $1000. int. Units & sng. Sales 2001000 int. Units + sng. Sales 1200 Tutorial 3: Using Variables and Constants 23
The Input. Box Function § The Input. Box 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: str. Answer = Input. Box(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 Constants 24
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 Constants 25
The Object Browser § Provides information about objects available to your application Tutorial 3: Using Variables and Constants 26
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 Accept. Button 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 Constants 27
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 Text. Changed event procedure § Create a procedure that handles more than one event Tutorial 3: Using Variables and Constants 28
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 Constants 29
Coding the Text. Changed Event Procedure § A control’s Text. Changed 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 Constants 30
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 Constants 31
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