CST2601 Visual
Basic I
Notes on variables
VariablesIn Visual Basic, you use variables to temporarily store values during the execution of an application. Variables have a name (the word you use to refer to the value the variable contains) and a data type (which determines the kind of data the variable can store). You can think of a variable as a placeholder in memory for an unknown value. For example, imagine you are creating a program for a fruit stand to track the sales of apples. You don't know the price of an apple or the quantity sold until the sale actually occurs. You can use two variables to hold the unknown values let's name them ApplePrice and ApplesSold. Each time the program is run, the user supplies the values for the two variables. To calculate the total sales and display it in a Textbox named txtSales, your code would look like this:
The expression returns a different total each time, depending on what values the user provides. The variables allow you to make a calculation without having to know in advance what the actual inputs are. In this example, the data type of ApplePrice is Currency; the data type of ApplesSold is an integer. Variables can represent many other values as well: text values, dates, various numeric types, even objects. Storing and Retrieving Data in VariablesYou use assignment statements to perform calculations and assign the result to a variable:
Note that the equal sign in this example is an assignment operator, not an equality operator; the value (10) is being assigned to the variable (ApplesSold). Declaring VariablesTo declare a variable is to tell the program about it in advance. You declare a variable with the Dim statement, supplying a name for the variable: Dim variablename [As type] Variables declared with the Dim statement within a procedure exist only as long as the procedure is executing. When the procedure finishes, the value of the variable disappears. In addition, the value of a variable in a procedure is local to that procedure that is, you can't access a variable in one procedure from another procedure. These characteristics allow you to use the same variable names in different procedures without worrying about conflicts or accidental changes. A variable name:
The optional As type clause in the Dim statement allows you to define the data type or object type of the variable you are declaring. Data types define the type of information the variable stores. Some examples of data types include String, Integer, and Currency. Variables can also contain objects from Visual Basic or other applications. Examples of Visual Basic object types, or classes, include Object, Form1, and TextBox. For More Information For more information on objects, see "Programming with Objects" and "Programming with Components." Data types are discussed in detail in the section, "Data Types," later in this chapter. There are other ways to declare variables:
Implicit DeclarationYou don't have to declare a variable before using
it. For example, you could write a function where you don't need to
declare
Visual Basic automatically creates a variable with that name, which you can use as if you had explicitly declared it. While this is convenient, it can lead to subtle errors in your code if you misspell a variable name. For example, suppose that this was the function you wrote:
At first glance, this looks the same. But because
the Explicit DeclarationTo avoid the problem of misnaming variables, you can stipulate that Visual Basic always warn you whenever it encounters a name not declared explicitly as a variable. To explicitly declare variables
Had this statement been in effect for the form or
standard module containing the SafeSqr function, Visual Basic would have
recognized
Now you'd understand the problem immediately because
Visual Basic would display an error message for the incorrectly spelled Note The Option Explicit statement operates on a per-module basis; it must be placed in the Declarations section of every form, standard, and class module for which you want Visual Basic to enforce explicit variable declarations. If you select Require Variable Declaration, Visual Basic inserts Option Explicit in all subsequent form, standard, and class modules, but does not add it to existing code. You must manually add Option Explicit to any existing modules within a project. |