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pointless。 The AndAlso operator used in place of And says if a does not equal b； then stop processing
and move to the next piece of code。
The real question is why make the difference between AndAlso and And？ I can only postu
late； but in the early； early days of Visual Basic； programmers used to write code like this：
If a = b And b = code。NextStatement（） Then
The method NextStatement（） not only verifies state； but also modifies state。 If a did not
equal b； then the state was not modified。 And that was a problem； since some programmers
relied on the fact that the other piece of code would be called regardless of how it was used。
Think about this logically； and you’ll see it is not incorrect。 After all； what is the difference
between the following two pieces of code？
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110 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
If b = code。NextStatement（） and a = b Then
and
If b = code。NextStatement（） Then
The answer is nothing； but if And behaved like AndAlso by default； then you would get an
inconsistent state。
Preventing Repetition in the Route
The FindNextLeg（） method calls the CanContinue（） method； which is designed to halt the
search。 In the case of our depth…first search algorithm； the purpose is to not fly to the same city
twice。 Contained within the function is code similar to FindNextLeg（）：
Private Function CanContinueSearch（ByVal returnArray As Node（）； _
ByVal city As Node） As Boolean
For c1 As Integer = 0 To returnArray。Length 1
If returnArray（c1） IsNot Nothing Then
If returnArray（c1）。CityName。pareTo（city。CityName） = 0 Then
Return False
End If
End If
Next
Return True
End Function
The logic is that CanContinueSearch（） will iterate through the returnArray and see if the
city being considered （variable city） is already in the found path。 If the city is in the path； then
we stop searching that part of the tree； otherwise； we continue searching。
Running the Depth…First Search Algorithm
Everything has been implemented； including tests； so we are ready to run the test of finding the
flight between Montreal and Seattle。 Looking at Figure 4…2； you can see two paths： Montreal
to Los Angeles to Seattle； or Montreal to Toronto to Seattle。 However； running the algorithm
generates the following peculiar result （you have not seen how to display the results； but that is
done easily enough with a For loop that iterates over foundRoute）：
o Montreal
o New York
o Houston
o Miami
o Toronto
o Seattle
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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 111
Looking at the result； you are probably thinking that the algorithm does not work； because
the proposed flight includes every city except Los Angeles。 If a travel agent were to propose
such a flight route to you； you would probably have a panic attack。
The algorithm did not fail； rather； the CanContinueSearch（） function did not include func
tionality to optimize the flight。 Right now； the algorithm says to perform a depth…first search；
meaning to go down the tree before backtracking。 So let’s go through the structure in the Node
shared constructor again。
We started our route in Montreal； which had the following Connections definitions：
montreal。Connections = New Node（） ｛ newyork； toronto； losangeles ｝
Applying our depth…first algorithm； it means the first array element of the tree is consid
ered a connection； and thus our route takes us to New York。 New York has the following flight
connections：
newyork。Connections = New Node（） ｛ montreal； houston； miami ｝
The first connection from New York is Montreal； which is already in the flight route。 Thus；
the second array element is searched； which is Houston。 Houston has the following flight
connections：
houston。Connections = New Node（） ｛ miami； seattle； newyork ｝
Following the flight route from Houston； we travel to Miami； which has the following
connections：
miami。Connections = New Node（） ｛ toronto； houston； newyork ｝
Following the flight route from Miami； we travel to Toronto； which has the following
connections：
toronto。Connections = New Node（） ｛ miami； seattle； montreal ｝
At Toronto； the first connection is Miami； where we have already been。 The second
connection is Seattle； and that is our end destination。
So from the perspective of the algorithm； everything worked。 From the perspective of the
traveler； it’s not ideal。 This demonstrates yet again how important it is to write test routines； as
algorithms might be correct； but they will generate responses that you might not have antici
pated。 Improving the example is one of the exercises at the end of the chapter。
The Important Stuff to Remember
In this chapter； you learned about data structures and algorithms。 Here are the key points to
keep in mind：
o When developing a program； you need to think of the data structures and algorithms
that are involved。
o A single best data structure and a single best algorithm do not exist。 Every data structure
and algorithm has promises。 You need to choose the data structure and algorithm
that best s