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level after another。 You stop traversing down the tree once you reach a room that you have
already traversed。 Then you go back one level and traverse the room beside the room that
you have already traversed。 This could mean the search path found by the puter would
be similar to Figure 4…1。 That’s because the puter is as dumb as you are； albeit the puter is
not saying to itself； “If only I had started in the hall。”
Realize that there is no magic wand to find the keys for you。 The technique that you and
the puter used is called brute force； and it is putationally expensive and typically avoided。
In this case though； brute force is the only real solution； because you don’t know where the
keys are—they could be anywhere in the house。 It just happened to be your bad luck that the
keys were found in the last place you looked。
Let’s try to improve the situation。 Imagine for a moment that your keys are tagged with a
clapper。 The idea is that with your keys going “beep； beep； beep；” you will be able to instantly
find them and avoid the brute…force searching。 But suppose that the keys are in the upper…right
corner of the child bedroom。 Listening for the beeping； you think that you hear the keys either
in the kitchen or the child bedroom。 So； where do you go first？
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CH AP T E R 4 ■ L E A R N I N G A B OU T D AT A S TR U CT U R E S； DE CI SI ON S； A N D L O OP S 81
Which path es first or is the most efficient is a mon problem； and you may witness
it every day if you have a global positioning system （GPS） for you car。 A search algorithm is
monly used in GPS devices。 You punch in a set of coordinates； and the GPS device will
attempt to find the quickest or shortest way。 In the abstract sense； the search algorithm the GPS
makers apply is identical to the search algorithm that we are going to develop in this chapter。
Implementing User…Defined Types
The data structure on which the algorithm will operate is a user…defined type。 In the examples
up to this point； we have been using types like Double and String； which are CLR…provided
types。 For the example in this chapter； we will define our own type； which we will use to repre
sent a node in the tree。
Declaring Structures and Classes
A user…defined type can be implemented in one of two ways： as a structure or as a class。 An
example of each is shown in Figure 4…3。
A type declaration requireA type declaration requiress a a CCLRLR In Visual Basic； an appropriate
conconsstruct （e。g。； truct （e。g。 SStructure or tructure or CClalassss） and） and keyword （e。g。； Class） opens a block。
an identifier （e。g。； Node）an identifier （e。g。 Node） To close the block； the keyword is
bined with the End identifier。
Everything in between belongs to
Structure Node Class Node the keyword。 So； if the keyword is
End Structure End Class Class； everything within the block
belongs to the creation of a class
Structure is used to define
a custom value type （e。g。；
Double）； which has some Class is used to define a custom reference type
restrictions and is used only （e。g。； String）； which is the most mon type
in certain situations used for development
Figure 4…3。 Choices for implementing the Node custom type
As shown in Figure 4…3； you can choose to create a user…defined type as a value type
（Structure） or reference type （Class） type。 For the most part； developers use a reference type；
because it has fewer constraints and is easiest to use in a general context。 A value type has some
constraints due its behavior of storing everything on the stack； as discussed in the following
sections。
Value Type Constraints
The constraints of using a value type relate to the fact that data is copied。 This has an impact on
what happens when you embed reference types in value types and use value types as parame
ters to methods。
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82 CH AP T E R 4 ■ L E A R N IN G AB OU T D AT A S TR U CT U R E S； DE CI SI ON S； A N D L O OP S
The Effects of Copying Data
When one value type is assigned to another value type； the contents of the value types are copied。
To see this in action； consider the declarations in Figure 4…4。
Declaration of the identifier
Custom value type declaration value is inside the block； so it
belongs to the custom type
Structure MyValueType
Public value As Integer Declaration of value is associated
End Structure with a type identifier （Integer） and a
Class MyReferenceType scope identifier （Public）
Public value As Integer
End Class
Custom reference type declaration
Both custom types declare a variable
that is a value type； which is called a
data member because it belongs to the
custom type declaration
Figure 4…4。 Declaring custom types