按键盘上方向键 ← 或 → 可快速上下翻页，按键盘上的 Enter 键可回到本书目录页，按键盘上方向键 ↑ 可回到本页顶部！
————未阅读完？加入书签已便下次继续阅读！
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 107
Else
' Code3
End If
' Code4
The following code steps are executed：
o If test1 is True； then execute Code1。 After executing Code1； execute Code4。
o If test1 is False； jump to ElseIf with test2。
o If test2 is True； then execute Code2。 After executing Code2； execute Code4。
o If test2 is False； jump to Else。
o Execute Code3。 After executing Code3； execute Code4。
Here is another example：
If test1 Then
' Code1
Else
' Code2
End If
' Code3
The executed code steps are as follows：
o If test1 is True； then execute Code1。 After executing Code1； execute Code3。
o If test1 is False； jump to Else。
o Execute Code2。 After executing Code2； execute Code3。
And here is one more example：
If test1 Then
' Code1
End If
If test2 Then
' Code2
Else
' Code3
End If
// Code4
The executed code steps are as follows：
o If test1 is True； then execute Code1。 After executing Code1； jump to If with test2。
o If test1 is False； jump to If with test2。
o If test2 is True； then execute Code2。 After executing Code2； execute Code4。
…………………………………………………………Page 130……………………………………………………………
108 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
o If test2 is False； jump to Else。
o Execute Code3。 After executing Code3； execute Code4。
The following code is illegal：
Else
' Code2
End If
' Code3
And this is also illegal：
ElseIf test2 Then
' Code2
Else
' Code3
End If
It is possible to have one If statement embedded within an Else； If； or ElseIf to create a
more plex multilevel decision tree。
The condition or test＇N ＇ variables are Boolean values that can contain True or False。 You
have already seen examples of these； like this：
If CanContinueSearch（returnArray； currNode。Connections（c1）） Then
The If statement says that if the method CanContinueSearch（） returns True； then execute
the contained code。
Here is another example of a condition：
If returnArray（c1） IsNot Nothing Then
This If statement says that if the array element returnArray＇（c1） does not have a value of
Nothing； then execute the contained code。
In both examples； either the method or parison must return a Boolean value。 If a Boolean
value is not returned； the Visual Basic piler will generate an error indicating that the code
does not result in a True or False value。
It is easy to see how a method can generate a True or False value； but the array element not
equal to Nothing statement is a bit more plicated。 The statement is an example of using
operators to perform a parison。 parisons test if things are equal to each other or not
equal to each other。 Table 4…2 shows the parison operators and what they mean。
Table 4…2。 parison Operators
Expression Description
a = b Does a equal b？
a b Does a not equal b？
a 》 b Is a greater than b？
a 《 b Is a less than b？
…………………………………………………………Page 131……………………………………………………………
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 109
Table 4…2。 parison Operators
Expression Description
a 》= b Is a greater than or equal to b？
a