Variable Declaration 

Variable Declaration 2.2
Variable Names
Variable names are given depending on the information the variable will store. For example the variable name FirstName will be given to a variable that will hold a persons first name. Again The variable name is declared or named in the general declarations area of a program as show in the Variable Description section.

Rules for Declaring Variables
There are certain standards that we must follow when declaring variables. They include:

1. The variable name must begin with a character. For variables that are declared in an event the first character must be a letter. The first character for variables declared in the form or module the first letter must indicate whether the variable is declared in the form's (f_) or module's (m_) general declarations area followed by an underscore and the variable name.
   

eg. f_Age as Integer
The concepts of declaring variables in events, forms or modules will be discussed in the Scope of Variables section.
NOTE: for smaller programs we will be declaring variables in forms or in objects programs. For larger programs with multiple forms we will declare variables in modules.

1. All memory variables will be declared in the most appropriate location and appear in lower case.
2. The name cannot be more than 40 characters long.
3. The name cannot contain punctuations or blank spaces
4. All variables must have a data type that is suitable to the variable use in the program.
5. The name cannot be a reserved word (such as Print, Name, Hour), because reserved words have special meaning in Visual Basic.

See Appendix A for a list of Reserved Words.

Examples of Proper Variable Declarations

Valid Variable Name	Invalid Variable Name	Reason for Invalidity
f_Hourswkd	Hours	hours is a reserved word
f_Salary97	97Salary	name must not begin with a number
c_Discount_Price	Discount Price	name cannot contain a space
c_FirstName	Name	reserved word
m_FireDepartment	Fire.Department	name cannot contain punctuation

The Dim Statement for Data Types

Before a variables is declared programmers must include the Dim syntax. This indicates to the program that the variables value could change as the program runs.
Syntax
Variable Declaration in an Event:
Dim <NameYouCreate> as Data Type

Variable Declaration in a form's General Declarations:
Dim <f_NameYouCreate> as Data Type

Variable Declaration in a module's General Declarations:
Dim <m_NameYouCreate> as Data Type

Examples

Dim CanadianDollar as Currency
Dim f_ Year as Integer
Dim m_FirstName as String
Dim CorrectAnswer as Boolean
Dim f_Temperature as Long
Dim m_FractionCalc as Single

Dim f_TheCalculation ' this makes f_TheCalculation have the variant data type.

The variant data type takes up a lot of space in memory. It is not good programming practice to declare variables with the variant data type.
You can also declare variables on the same line. It is good programming practice to keep all the same variables of the same data type on the same line.
Dim NumberofDays as Integer, HoursWorked as Integer
Dim m_FirstName as String, m_MiddleName as String, m_LastName as String
Dim f_TaxRate as Integer, f_Subtotal as Integer, f_Total As Integer

The Scope of Variables

Variables may be declared in the following levels in Visual Basic:

Variable Type	variable name = expression	Description
Event	Dim Fname as Type ReDim Fname as Type Static Fname as Type	declared in any procedure. available only in the procedure or object in which it is called. ReDim is used to change the size of an array, only in a procedure. Static is when the programmer wishes to have a procedure retain a variable's values for the duration of the application. also known as local to the event, local to the object or control or procedure
Form	Dim f_Fname as Type	in any procedure. in the declaration section of a form. available to all procedures in that module, but not to any other form or code module.
Module	Public m_Fname as Type (for VB 4.0) Global m_Fname as Type (for VB 3.0)	in declaration section of a module, cannot be declared anywhere. available in every procedure in all forms and code modules in the application.

Variable Assignment Statements
The easiest way to create a variable is to assign a value to the variable's name in a code statement.
An assignment statement assigns a value to the property of an control/object. After the period is the property of the object followed by the equal sign and the value. The value could be a variable or a piece of information.

Syntax
<Object.Property> = <value property >

EXAMPLE	CHANGES THE...
txtLname.Text = "Jones"	the text displayed in the text box to Jones
HourlyIncome = 10.55
f_FirstName = "John"
g_PresentAge = 16

Variables can either hold numbers called numeric data or characters called string data. String data must be placed in quotation marks. Numeric data need not be in quotation marks.
Variables can also be assigned to the value of another variable, the value of an object/control or the value of a mathematical expression.
txtHourlyIncome.Text = f_HourlyIncome

If you prefer output to be tested you can print any output on the form. All you need to do is separate the variables or anything as text with a semi-colon. Remember that variables need not have quotes around them, but the programmers setup must.

For example:
print "The "; f_Chlorine ; "is too high and the"; f_AlgaeCount ; "is normal."
Visual Basic Variable Declaration Exercises
Activity 2.2.1

1. Complete the following table in your computer science binders.

Variable Declaration Or Assignments	Explanation Why Correct or Incorrect?	Correction or Better Choice
Dim Name as Integer
Dim m_FirstName as Integer
Dim m_Money Earned as Integer
Dim f_Currency as String
Dim f_ Person as String
Dim m_Name as String
Dim GoldbachNo as Integer
Dim m_PizzaOrder
Dim !Factorial as Double
Dim m_Chemicalppms as Double
Dim x, y, z as Integer
Dim m_Num1 as Integer, m_Num2 as Integer
txtCarPayment.txt = m_CarPayment
TextOrderNum.Text = m_OrderNum
txtCarPayment.txt = m_CarPayment
Distance = txtDistance
print "Show me the $ "; m_Money "!!!"
m_FibonacciNum = "45"

Activity 2.2.2

TOPICS: Variable Text Boxes, Label Boxes, Command Boxes and the Message Box.
PART A
You need to create the following:

1. 5 Label Objects
2. 4 Text Boxes
3. 3 Command Boxes
4. Change the form CAPTION to THE PERSONAL INFORMATION PROGRAM.

Your form should appear similar to the following form:

The following will help you create your program:

Label1
CHANGE THE NAME TO: lblTPersonal
CHANGE THE CAPTION TO: MY PERSONAL INFORMATION
Change the FontSize to 18 point.

Label2
CHANGE THE NAME TO:lblFirst
CHANGE THE CAPTION TO: First Name

Label3
CHANGE THE NAME TO: lblLast
CHANGE THE CAPTION TO: Last Name

Label4
CHANGE THE NAME TO: lblHeight
CHANGE THE CAPTION TO: Height

Label5
CHANGE THE NAME TO: lblHair
Please use American spelling of color, since this is American software.
CHANGE THE CAPTION TO: Hair Colour

Text1 Diagram #1

CHANGE THE NAME TO: txtFirstName
CHANGE THE TEXT TO: null, delete text1
As see in Diagram #1



Text2
CHANGE THE NAME TO: txtLastName
CHANGE THE TEXT TO: delete Text2

Text3
CHANGE THE NAME TO: txtHeight
CHANGE THE TEXT TO: delete Text3

Text4
CHANGE THE NAME TO: txtHairColor
CHANGE THE TEXT TO: delete Text4

Command1
CHANGE THE NAME TO: cmdDisplay
CHANGE THE TEXT TO: DISPLAY

Command2
CHANGE THE NAME TO: cmdClear
CHANGE THE TEXT TO: CLEAR

Command3
CHANGE THE NAME TO: cmdExit
CHANGE THE TEXT TO: E&XIT

Double click on the command button, type in End between the sub and end sub.

1. Assignment Statements
2. In the general declarations area declare the following variables.
   

REMEMBER!!!
Accessing the general declarations area: double click on the form, beside Object click on the arrow down choose (General), beside Proc click on the arrow down choose (Declarations). You are using f_ because your variables are declared at the form level. This will allow you to use the variables anywhere else in the program.

f_FirstName - string
f_LastName - string
f_Height - integer

1. Double Click on cmdDisplay.
2. Assign the 3 text boxes to the 3 appropriate variables

f_FirstName = txtFirstName.Text
f_LastName = txtLastName.Text
f_Height = txtHeight.Text

1. Using the MSGBOX Statement
   

The MessageBox displays one of Visual Basic's predefined dialog boxes.

Example #1
MsgBox txtFirstName.Text
or
MsgBox f_FirstName

This will display your first name in the message box.
For string expressions, you must enclose them in quotations

Example #2

MsgBox "Hello"
Syntax

<ReturnValue> = MsgBox (prompt [,buttons][,title][helpfile][,context]

You can also omit the return value and the equal sign.
If you want to display more than one item in the message box it is possible using the ampersand sign (&) . You can use quotes to type in text and for blank spaces between you name.



Example #3

MsgBox f_FirstName & " " & f_LastName & ". " & "Your height is " & f_Height

1. Add in a text box for hair colour and declare a variable for hair colour.
2. Double Click on cmdDisplay. Type in the following code below your assignment statements:

NOTE: Make sure that you type the line below in one line. The ampersand (&) sign does not need to be bold and large.

MsgBox f_FirstName & " "& f_LastName & " your height is " & f_Height & " and your hair color is " & f_HairColor & ". "

THE KEY FOR UNDERSTANDING:
" " = a space
". " = a period and a space
" and your hair color is" = a space and your hair color is

Double Click on the cmdClear.
Assign all the variables to a null string to clear the text boxes.

txtFirstName.Text = " "
txtLastName.Text = " "
txtHeight.Text = " "

Remember to add in your hair colour text box.

Run the program use F5.
Save the Form File as: frmVariablePractice
Save the project as: VariablePractice.vbp

1. Enter in your first name, last name, height in centimeters and your hair color.
2. Click on Display.

What is the output? Note the output word for word and space for space.


If you have syntax errors check cmdDisplay. Make sure you have all the quotes and ampersand signs. Ask a friend in the class for help.

Here is what your output could look like:

 

Activity 2.2.3
Part B

ENHANCEMENTS:

Add some color to your form by changing the properties of the objects. Change the color of the text boxes and label boxes as well as the text.
See is you can add a label box and a text box for your favorite food. Don't forget to declare the variable for your favorite food and assign it to the text box.
Label6
NAME: lblFood
CAPTION: Favorite Food

Text5
NAME: txtFood
TEXT: delete Text5

Assign the food text box to the variable f_Food. Using the ampersand sign add in f_Food to the code in cmdDisplay. For example:
MsgBox f_FirstName & " "& f_LastName & " your height is " & f_Height & " and your hair color is " & f_HairColor & ". " & "Your favorite food is " & f_Food " ."

Note the output in your computer science binders.