Tutorial 3 Using Variables and Constants

1. Tutorial 3 Using Variables and Constants

2. 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

3. 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

4. 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

5. Selecting a Data Type for a Variable

6. Choose the Correct Data Type

7. 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

8. 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

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

12. 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

13. Declaring a Variable

14. 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

15. 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

16. 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

17. 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

18. 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

19. 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

20. 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 Constants

21. 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

22. Modifying the Skate-AwaySales ApplicationLesson 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

23. 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

24. 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

25. 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

26. The Object Browser • Provides information about objects available to your application

27. 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

28. Modifying the Skate-AwaySales Application’s CodeLesson 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

29. 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

30. 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

31. 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