Visual Basic 1002005 [A Developers Notebook] [Electronic resources] نسخه متنی

2.10. Skip to the Next Iteration of a Loop

The Visual Basic language provides a
handful of common flow control statements, which
let you direct the execution of your code. For example, you can use
Return to step out of a function, or
Exit to back out of a loop. However, before VB
2005, there wasn't any way to skip to the next
iteration of a loop.

Note: VB's new Continue keyword gives you a
quick way to step out of a tangled block of code in a loop and head
straight into the next iteration.

2.10.1. How do I do that?

The Continue statement is one of those language
details that seems like a minor frill at first, but quickly proves
itself to be a major convenience. The Continue
statement exists in three versions: Continue For,
Continue Do, and Continue
While, each of which is used with a different type of loop
(For ... Next,
Do ... Loop, or
While ... End While).

To see how the Continue statement works consider
the following code:

For i = 1 to 1000
If i Mod 5 = 0 Then
' (Task A code.)
Continue For
End If
' (Task B code.)
Next

This code loops 1,000 times, incrementing a counter
i. Whenever i is divisible by
five, the task A code executes. Then, the Continue
For statement is executed, the counter is incremented, and
execution resumes at the beginning of the loop, skipping the code in
task B.

In this example, the continue statement isn't really
required, because you could rewrite the code easily enough as
follows:

For i = 1 to 1000
If i Mod 5 = 0 Then
' (Task A code.)
Else
' (Task B code.)
End If
Next

However, this isn't nearly as possible if you need
to perform several different tests. To see the real benefit of the
Continue statement, you need to consider a more
complex (and realistic) example.

Example 2-5 demonstrates a loop that scans through
an array of words. Each word is analyzed, and the program decides
whether the word is made up of letters, numeric characters, or the
space character. If the program matches one test (for example, the
letter test), it needs to continue to the next word without
performing the next test. To accomplish this without using the
Continue statement, you need to use nested loops,
an approach that creates awkward code.

Example 2-5. Analyzing a string without using the Continue statement

' Define a sentence.
Dim Sentence As String = "The final number is 433."
' Split the sentence into an array of words.
Dim Delimiters( ) As Char = {" ", ".", ","}
Dim Words( ) As String = Sentence.Split(Delimiters)
' Examine each word.
For Each Word As String In Words
' Check if the word is blank.
If Word <> " Then
Console.Write("'" + Word + "'" & vbTab & "= ")
' Check if the word is made up of letters.
Dim AllLetters As Boolean = True
For Each Character As Char In Word
If Not Char.IsLetter(Character) Then
AllLetters = False
End If
Next
If AllLetters Then
Console.WriteLine("word")
Else
' If the word isn't made up of letters,
' check if the word is made up of numbers.
Dim AllNumbers As Boolean = True
For Each Character As Char In Word
If Not Char.IsDigit(Character) Then
AllNumbers = False
End If
Next
If AllNumbers Then
Console.WriteLine("number")
Else
' If the word isn't made up of letters or numbers,
' assume it's something else.
Console.WriteLine("mixed")
End If
End If
End If
Next

Now, consider the rewritten version shown in Example 2-6 that uses the Continue
statement to clarify what's going on.

Example 2-6. Analyzing a string using the Continue statement

' Examine each word.
For Each Word As String In Words
' Check if the word is blank.
If Word = " Then Continue For
Console.Write("'" + Word + "'" & vbTab & "= ")
' Check if the word is made up of letters.
Dim AllLetters As Boolean = True
For Each Character As Char In Word
If Not Char.IsLetter(Character) Then
AllLetters = False
End If
Next
If AllLetters Then
Console.WriteLine("word")
Continue For
End If
' If the word isn't made up of letters,
' check if the word is made up of numbers.
Dim AllNumbers As Boolean = True
For Each Character As Char In Word
If Not Char.IsDigit(Character) Then
AllNumbers = False
End If
Next
If AllNumbers Then
Console.WriteLine("number")
Continue For
End If
' If the word isn't made up of letters or numbers,
' assume it's something else.
Console.WriteLine("mixed")
Next

2.10.2. What about...

...using Continue in a
nested loop? It's possible. If you nest a
For loop inside a Do loop, you
can use Continue For to skip to
the next iteration of the inner loop, or Continue
Do to skip to the next iteration of the outer
loop. This technique also works in reverse (with a
Do loop inside a For loop), but
it doesn't work if you nest a loop inside another
loop of the same type. In this case, there's no
unambiguous way to refer to the outer loop, and so your
Continue statement always refers to the inner
loop.

2.10.3. Where can I learn more?

For the language lowdown on Continue, refer to the
index entry "continue statement" in
the MSDN Help.