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S
The notion of a left…hand side and a right…hand side as two separate parts is important
when you consider this code：
a = a ＋ 1
In the example； the variable a is added with the value 1 and assigned to the variable a； but
these operations do not happen at the same time； they happen sequentially。 First； the right
hand side is executed； and then the left…hand side is executed。 By executing the left…hand side；
the existing value of the variable a is overwritten。
Let’s consider another example：
b = a = a ＋ 1
If you were to run this code； b would not be assigned； because the example has mixed
operators； which translates to this：
Dim b As Integer = CInt（…（（a = （a ＋ 1）） 》 False））
The result is not what you expected； and not even close to what you were trying to achieve
（and isn’t even allowed with Option Strict set to On）。 What this illustrates is that you need to be
careful with operators。
Bitwise Operators
Bitwise operators are used to access and manipulate individual bits in a whole number。
As you learned in Chapter 2； the puter sees numbers as binary； with only two unique
identifiers。 Thus； whole numbers could be viewed as arrays of Boolean values； as there are only
two valid values： 1 and 0 or True and False。
Let’s look at example of using the bitwise operators。 Say that you want to know whether a
person is tall； wears hats； and runs slowly。 Using Boolean data members； you would write the
following code。
Class PersonWithAttributes
Public IsPersonTall As Boolean
Public WearsHats As Boolean
Public RunsSlowly As Boolean
End Class
In this definition； each attribute of PersonWithAttributes is its own data member。 Another
way to achieve the same effect is to consider each attribute as its own binary flag that is encoded
into PersonWithAttributes。 The class would be rewritten as follows：
Class PersonWithAttributes
Public Attributes As Integer
End Class
In the rewritten example； the Boolean attributes are encoded into the data member
Attributes using bitwise operators。
To encode a flag in a number means to tweak the individual binary bits of a number。 To
access the individual bits； you need to create constant numbers that are a power of 2。 We use a
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power of 2 convention because binary is a counting system based on the power of 2。 Thus； the
three Boolean data members are converted to constants：
Const isTall As Integer = 1
Const wearsHats As Integer = 2
Const runsSlow As Integer = 4
Now you could create a person that is tall and runs slowly； like this：
Dim person As PersonWithAttributes = New PersonWithAttributes（）
person。Attributes = isTall Or runsSlow
The variable person。Attributes is assigned the operations of isTall and runsSlow。 So if
isTall is 0001； and runsSlow is 0100； taking the bitwise OR （Or） of the two values results in 0101。
A bitwise OR is when you pare two values； and if either value is nonzero； then the
answer is nonzero。 If you were to convert the binary number into a decimal number； you would
get the value of 5。 You could verify that the person isTall like this：
If （person。Attributes And isTall） 0 Then
Console。WriteLine（〃Person is tall〃）
Else
Console。WriteLine（〃Person is not tall〃）
End If
A bitwise shift is the shifting of the bits to the left or to the right by the number of places indi
cated by the shift。 The following code demonstrates shifting the bits two places to the left （》 2
The result is 2； which has the binary value 10。
For the most part； you probably will not use the binary shift operators。 Unless you are dealing
with hardware…related programming （such as GUI interactions）； whatever you can do with
flags can be replicated using a bination of Boolean types and structures。 However； when
you do encounter bitwise operators； you will recognize what they are doing。
Modulus
The modulus operator （Mod） is very useful when you want to perform division operations and
need to know how much is left over。 For example； the following calculation yields a value of 3。
Dim a As Integer = 17 / 5
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420 CH AP T E R 1 6 ■ L E A R N I N G A B OU T O TH E R V IS U AL B A SI C TE C H N IQ U E S
Yet because it is an integer division； you don’t know if the value is evenly divided by 5。 The
following calculation lets you figure out the remainder for the division。
Dim remainder As Integer = 17 Mod 5
Increment and Decrement
The increment （＋=） and decrement （ …=） operators are useful in place of the addition and subtrac
tion operators。 Visual Basic also has the *= and /= operators； which you can use in place of the
multiplication and division operators。 The idea behind these operators is to perform an operation
on the variable itself。
Let’s say you execute this code：
Dim a As Integer = 2
a ＋= 1
Then after the increment operation； the variable a will have a value of 3。
Overloading Operators
Suppose you want to implement a special type； such as a plex number type； and want to
support the ＋ sign。 To do so； you would need to implement the ＋ operator； which； in turn； over
loads the ＋ operator。
As an example； let’s walk through implementing the ＋ operator for the plex number
type。 plex numbers are numbers that have two parts： real and imaginary。 For example； a ＋ b
is a plex number； where a is the real part and b is the imaginary part。 To add a plex
number； you add the real parts； and then add the imaginary parts； and that gives you the new
number。 The following is the plex number type； plexType； declaration。
Public NotInheritable Class plexType
Private ReadOnly _imaginary As Double
Private ReadOnly _real As Double
Public Sub New（ByVal real As Double； ByVal imaginary As Double）
_real = real
_imaginary = imaginary
End Sub
Public Overrides Function ToString（） As String
Return String。Concat（New Object（） ｛〃（〃； _real； 〃） （〃； _imaginary； 〃）i〃｝）
End Function
Public ReadOnly Property Imaginary（） As Double
Get
Return _imaginary
End Get
End Property
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