Visual Basic 6 Programming Blue Book: The Most Complete, Hands-On Resource for Writing Programs with Microsoft Visual Basic 6!:The Basics Of Basic
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Visual Basic 6 Programming Blue Book: The Most Complete, Hands-On Resource for Writing Programs with Microsoft Visual Basic 6!
(Publisher: The Coriolis Group)
Author(s): Peter G. Aitken
ISBN: 1576102815
Publication Date: 08/01/98
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Table 4.4 Operator precedence.
Operator
Precedence
Exponentiation ^
1
Multiplication (*), division (/)
2
Integer division (\)
3
Modulus (MOD)
4
Addition (+), subtraction (-)
5
String concatenation (&)
6
expression, you force operators inside parentheses to be evaluated first regardless of precedence. If you write
A + B * C
the precedence rules will cause the multiplication to be performed first, and the result will not be what you want. If, however, you write the expression like this
(A + B) * C
the parentheses force the addition to be performed first, and the expression evaluates properly. You can use as many parentheses in an expression as you need, as long as they always come in pairs; each left parenthesis must have a matching right parenthesis. If you create an expression with an unmatched parenthesis, Visual Basic displays an error message when you try to move the editing cursor off the line. When parentheses are nested (one set inside another set), execution starts with the innermost set and proceeds outward.
You can use parentheses in an expression even when they are not needed to modify the order of operator precedence. Particularly with long, complex expressions, parentheses can help make the expression easier to read and understand.
A Need To Control
You’ve seen that Basic code consists of a series of statements, one to a line. When a chunk of Basic code executes, the normal execution order begins with the first statement and then executes all the statements in order, top-to-bottom. Sometimes, however, this just won’t do. One of the most powerful Basic programming tools available to you is the ability to control program execution—to determine which Basic statements execute, when they execute, and how many times they execute. Before I show you how to do this, however, you need to know about logical expressions and Visual Basic’s comparison and logical operators.
Logical Expressions
Computer programs often need to deal with yes/no questions. When a question or an ex-pression has only these two possible outcomes, it is called a logical expression. In computer programming, the two possible outcomes are referred to as True and False. As an example, consider the question, “Is the value stored in the variable X larger than the value stored in the variable Y?” Clearly, either it is (answer = True) or it is not (answer = False). Because computers use numbers for everything, it has become standard practice to use 0 for False and -1 for True. Now let’s take a look at how to construct and manipulate logical expressions.
Visual Basic has a special data type, Boolean, designed specifically to hold logical values. Such variables are sometimes referred to as flags. For example, if your program needed a way to track whether the current data has been saved to disk, you could declare a flag variable as follows:
Dim DataSaved As Boolean
When the user saves the data to disk, assign a value like this:
DataSaved = True
Likewise, when the data has been modified since the last save:
DataSaved = False
Even though Visual Basic uses numbers internally to represent the Boolean values True and False, you can (and should) use the predefined identifiers True and False in your code.
Comparison Operators
Visual Basic provides a number of operators you can use to construct logical expressions by asking questions about the data in your programs. Specifically, these comparison operators perform comparisons between expressions, returning a value of True or False, depending on the result of the comparison. Table 4.5 lists the comparison operators.
We can see that if X is equal to 10 and Y is equal to 5, then the expression X < Y will evaluate as False and X <> Y will evaluate as True. Assuming both Q and Z have been declared as type Integer, we can use the numerical value of a logical expression like this
Q = X < Y
Z = X <> Y
which would result in the variable Q having the value 0 and Z having the value -1. More often, however, logical expressions are used in program control, as you’ll see soon. But first, we need to take a look at how to combine two or more logical expressions to arrive at a single True/False answer.
Table 4.5 Comparison operators.
Operator
Comparison
Example
Meaning
=
Equal to
X = Y
Is X equal to Y?
>
Greater than
X > Y
Is X greater than Y?
<
Less than
X < Y
Is X less than Y?
>=
Greater than or equal to
X >= Y
Is X greater than or equal to Y?
<=
Less than or equal to
X <= Y
Is X less than or equal to Y?
<>
Not equal to
X <> Y
Is X not equal to Y?
Logical Operators
The logical operators work to combine two or more logical expressions into a single True or False answer. Why would we want to do this? Here’s an example from everyday life. You receive a call inviting you to join a group of friends for dinner. If Mary is coming along, you’d like to go, because you’re a bit sweet on her; otherwise, you’ll pass. This is a single logical condition, and easy to understand. Now, assume for a moment that you also like Helen; if she’s coming with the group, you’d also like to go. But if both Mary and Helen are coming, the situation will get a bit too complicated, so you again will pass. In this situation, we have two True/False questions (“Is Mary coming?” “Is Helen coming?”) that you need to combine in some way before answering the True/False question: “Are you going?”
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