CSC106 Intermed PC Appls and Programming - VB Variables, Constants, and Assignment

VB Variables, Constants, and the Assignment Statement

Constants

In programming, a constant is a specific value. Some values are numbers, and some are characters (called strings in VB).

Numeric constants can be divided into two groups: integers (whole numbers) and reals (numbers with decimal points). Some examples of integer constants are:

     0   256   -99   6   16384   -7531

Some examples of real constants are:

     0.05   3.14159   -15.25   99.99   8020.1

Notice that we don't put commas into a numeric constant.

A specific group of characters (a name, a word, a phrase, or any text) is called a string constant. A string constant can contain letters, digits, and/or special characters. Any character on the keyboard can be in a string constant. In VB, a string constant is always written inside of double quotes ("). Some examples of string constants are:

     "http://YouTube.com"   "Shadow of the Colossus"   "hi"   "R2D2"

Variables

In programming, a variable is a location in memory where a value can be stored. Variables are used to hold values that are read from forms, and values that are calculated during execution of a program. Every variable has a name and a type.

The name of a variable is used whenever we need to refer to the variable. There are rules for creating variable names:

must begin with a letter
can contain letters, digits, and _
must have between 1 and 255 characters
cannot be a reserved word, which is a word that has special meaning in the Basic language; some sample reserved words are sub, end, true, false, and dim (the VB editor will automatically capitalize a reserved word and display it in blue)

The type of a variable determines what kind of data we can store in the variable. There are other types, but the basic types we will use are:

Type	Values	Examples
Integer	whole numbers from -32,768 to 32,767	389, -7, 0, 1024
Double	real numbers from -1.8 * 10³⁰⁸ to 1.8 * 10³⁰⁸, with about 14 digits of precision	2.65, -.0005, 1139.25
String	characters	"last name", "hello!!" "P"

Declaring Variables

Before you use a variable in an event procedure, you should declare it using the Dim statement. The form of the Dim statement is

   Dim variable As type

For example,

   Dim total As Double
   Dim temperature As Integer
   Dim name As String

When you declare a variable with a Dim statement, the variable is given an initial value. For numeric variables, the initial value is 0. For string variables, the initial value is the empty string (no characters) "". You can change the value of a variable with an assignment statement.

Assignment Statements

The assignment statement is used to store a value in a variable. The general form of the assignment statement is:

   variable = expression

The left hand side of the assignment statement is a variable. The right hand side is an expression. An expression can be either a constant, a variable, or a calculation involving variables and/or constants. The effect of the assignment statement is to calculate the value of the right hand side, and then store that value in the variable on the left hand side.

For example, the following are all valid assignment statements that change the value of the variable on the left hand side.

   count = 0
   temperature = 47
   num1 = num2
   interest = balance * intrate
   name = "Jennifer Hudson"
   num2 = (num1 + 12) * 4
   count = count + 1

In a program, the statements are executed in order from first to last. An assignment statement replaces the old value of a variable with a new value. The old value is lost. Here is an example of how the variables change as a program executes:

Statement	Variable Values After Execution
	num1	num2	i	count	phrase
Dim num1 As Double	0
Dim num2 As Double	0	0
Dim i As Integer	0	0	0
Dim count As Integer	0	0	0	0
Dim phrase As String	0	0	0	0
i = 9	0	0	9	0
count = i	0	0	9	9
num1 = i / 2	4.5	0	9	9
count = count + 1	4.5	0	9	10
i = count * 2	4.5	0	20	10
phrase = "Dragon Days"	4.5	0	9	10	Dragon Days
num2 = (num1 + count) * 2	4.5	29.0	9	10	Dragon Days

Order of Evaluation

The order of evaluation determines how to evaluate expressions with more than one operation. For example, if there is a multiplication and an addition, these rules tell us which operation is performed first. These rules should be familiar to you, because they are the same rules that we use in math:

operations within parentheses
exponentiation (^); if there is more than one exponentiation, they are performed from left to right
multiplication (*) and division (/); if there is more than one multiplication and/or division, they are performed from left to right
addition and subtraction; if there is more than one addition and/or subtraction, they are performed from left to right

For example, the expression 10 + 5 * 12 - 8 / 2 * 3 would be evaluated as follows:

     10 + 5 * 12 - 8 / 2 * 3
     10 + 60 - 8 / 2 * 3
     10 + 60 - 4 * 3
     10 + 60 - 12
     70 - 12
     58