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Visual Basic has strict rules for handling variables.
RULES FOR NAMING VARIABLES
You can name your variables anything you like within the rules (see below), and you can store anything you like in them. However.... unless you use sensible names you will find it very difficult to understand your program as you write it, and impossible to understand it 6 months later when you may want to change something.
- It
is sensible to give your variables names that indicate what they are for.
- eg userName might store the user's name, userAge might store the user's age etc
- There
are some fixed rules for naming variables. They must:
- begin with a letter
- contain only letters, numbers and the underscore character _ (spaces and punctuation marks are not allowed)
- be no more than 255 characters
- be any word except a Visual Basic reserved word, such as End, If, Loop, For, Sub etc
VARIABLE TYPES
As well as containing numbers, variables can contain strings. Strings are any combination of letters, numbers or symbols that you want your program to treat literally.
To assign a number to a variable, you do not use quote marks. eg total = 234
To assign a string to a variable, enclose it in quote marks. eg userName = "Bill"
So if you had a phone number written as 6223-3456, the computer would treat it as 6223 minus 3456 unless you assigned it as a string type variable. To tell the program to treat strings literally, always surround your string with quote marks like this "6223-3456"
| Phone = "6223-3456" |
If you forget to put in the quote marks, Visual Basic would try to subtract 3456 form 6223 and put the value in Phone.
There are a number of other data types. Some examples are:
| Boolean | True or False |
| Date | Dates between Jan 1, 100 and Dec 31, 9999 |
| Integer | -32,768 to 32,767 |
| Long | -2,147,483,648 to 2,147,483,647 |
DECLARING VARIABLES
Before you use a variable, you should declare it. That tells the program about it.
Examples of variable declarations:
- Dim myName (would be treated as Variant data type because no type is specified)
- Dim myName as String
- Dim Correct as Boolean
- Dim Total as Integer
If you do not state the data type when you declare a variable, it is treated as 'Variant' type, which can hold numbers or strings (of almost any length).
VARIABLE SCOPE
Variable scope is where in your program your variables can be seen. Sometimes you want your variables to be seen by all the procedures within your project, and at other times you want them to be only available within a particular procedure. Where you declare a variable determines which procedures can see and use it.
A variable declared in the declarations section (this coding section can be found by pulling down the left combo box of the coding form) has a global scope and can be accessed anywhere in your project.
While a variable that is declared inside your code, between the Sub and the End Sub, can only be accessed in that sub.
- A variable that is declared within an individual procedure (using the Dim statement) can only be used within that procedure.
eg Private Sub askName
Dim userName, userAge .....
The variables userName and userAge can only be used within the procedure askName
- A
variable that is declared in the declarations section of your project (outside
of a procedure) is a public variable and can be used more widely.
Public variables can be the most convenient to use, because every procedure can access them, but be careful. Most programmers avoid using public variables because in a large program if a public variable messes up, you have to search your entire program to find the problem, whereas if a variable within a procedure messes up, you only have to search within that procedure to find the error.
