CSCI 220
Programming With Visual Basic
Lecture 4- Variables and Scope
Variables
Description
A container for information
A name that refers to a chunk of computer memory
Variables have Values
Name can be used in statements rather than value
Value can change
Principal uses
Store information gotten from user, environment, or another source
Work with information stored in object properties
Manipulate information (info garage)
Types
Computer stores different types of information in different ways
(Chart on page 105 of book)
Strings : text
Fixed or dynamic length
Integers :Whole and Negative Numbers
Integers and Long Integers
Real Numbers: Numbers with decimal values
Single and Double Precision
Currency
8 digits + four decimal places
Variant
Computer takes its best guess. Risky
Type conversions
Allow you to convert from one data type to another
Val - Graps a numeric value from a string
Str$ - Converts a numeric value to a string
Frequently used to convert from variant to more specific type
Variable Declaration Modes
Implicit
Simply by using variable, you declare its existence
Used by older forms of BASIC
Only way of specifying type was type declaration characters ($%&#, etc)
No way to check spelling of variable names -- common errors
Explicit
Each variable must be declared in a DIM statement
Syntax: Dim VariableName As Type
Example: Dim UserName As String
Varaibles should be declared before code in appropriate code area
Set up in Environment dialog of Options Menu: Require Variable Declaration
Can also be set by typing OPTION EXPLICIT in General area of form
Advantages:
Code is easier to read
Mispelling variable names is caught by compiler
Easier to set scope
More professional
Scope
Description:
Variables can be accessed only in specified parts of the program
Prevents errors -esp. in large programs
Allows for compartmentalization of program.
Variables are available only to those parts of the program that need it
Form - Level Variables
Declared in general area of form
Available to all objects, code windows attached to form
Value of variable accessible to any code in form
Object - Level Variables
Declared in code window of a specific object
Exists only when that code is running.
If same variable name is used in a different code window, they are DIFFERENT variables.
Also called local variable
Preferred when possible.
Which to use?
If only one object will use the info stored in the variable, make it Object Level
If more than one object needs it, make it Form Level
More to come.....
Simple dialog Boxes:
MsgBox
Easy way to send short message to user
Syntax : MsgBox (String Value to send)
Examples: MsgBox ("Hi there")
Dim Message as String
Message = "Hello, world!"
MsgBox (Message)
Inputbox and Inputbox$
Easy way to get quick input from user
Inputbox gets numeric value, Inputbox$ gets string
Syntax: VariableName = Inputbox$(String value representing question)
Example:
Dim UserName as String
UserName = inputbox$("What is your name?")
or
Dim Answer as Integer
Answer = Inputbox("What is 5+3?")
IF - THEN - ENDIF
Allows simple evaluations
Simple form
Syntax: IF Expression THEN Command
Example:
If Name = "Wally" then MsgBox("You're a NERD")
If Answer <> 12 then MsgBox ("Sorry. 5+7 = 12")
Block Form
Syntax:
IF Expression THEN
Code
ELSE
More Code
END IF
Example:
If Ans4 = Total Then
MsgBox ("Very Nice!")
Else
MsgBox ("Too Bad!)
End if
Your project, The Math Machine (VB4)
Purpose:
The purpose of this program is to illustrate your understanding of variable
declaration principles, the use of input and message boxes, and basic
understanding of variable scope
Description:
The program will begin with an input box asking the user for her name.
After entering a value, the user will be shown a screen with two text
boxes, one for a value of X and one for a value of Y. The screen will
also show some simple formulas manipulating X and Y: X+Y, X-Y, X*Y, X/Y.
Initially there will be no values shown by these labels. There should
be some sort of calculate button on the form. When the user chooses to
calculate, the formulas will show the calculated values. When the user
quits, she should receive a goodbye message that includes her name.
Procedure:
Developing interface
Write your own sketch for this one. Decide what it will look like, then
build it.
My version had ten labels, 2 text boxes, and 2 command buttons.
Name any controls that will contain code or be referred to by code
Setting up
Go to the general area of the form
Set explicit variable declaration (by typing Option Explicit)
Type in your general remark statements
Getting the user's name
When will this happen in the program's execution?
Where should you put the code so this happens?
What kind of function should you invoke to get the user's name?
Which type of variable will the user's name go into?
What should the SCOPE of that variable be?
Will another procedure use the value of this variable?
Don't write any code for this section until you have answered ALL
the questions above.
Dimension the variable in the proper place
Write the code in the proper place
Writing the calculation code
Where will the code go that does all the calculations
(Be careful here! Think!!!)
Which variables will this code need?
What scope should those variables have?
Write the code that handles the addition operation
Save, test, and debug
Continue with the other operations until they all work
The Quit Button
This is a little trickier than usual. (Great, mess with the one
thing we know... (Grin!))
When the user wants to quit, send a dialog box that says goodbye
and uses their name.
If the name doesn't work, check the variable name and scope.
Have FUN!!!!!!!