Do...Loop Statement (Visual Basic)
Repeats a block of statements while a Boolean condition is True or until the condition becomes True.
Do { While | Until } condition
[ statements ]
[ Continue Do ]
[ statements ]
[ Exit Do ]
[ statements ]
Loop
-or-
Do
[ statements ]
[ Continue Do ]
[ statements ]
[ Exit Do ]
[ statements ]
Loop { While | Until } condition
Parts
Term |
Definition |
Do |
Required. Starts the definition of the Do loop. |
While |
Required unless Until is used. Repeat the loop until condition is False. |
Until |
Required unless While is used. Repeat the loop until condition is True. |
condition |
Optional. Boolean expression. If condition is Nothing, Visual Basic treats it as False. |
statements |
Optional. One or more statements that are repeated while, or until, condition is True. |
Continue Do |
Optional. Transfers control to the next iteration of the Do loop. |
Exit Do |
Optional. Transfers control out of the Do loop. |
Loop |
Required. Terminates the definition of the Do loop. |
Remarks
Use a Do...Loop structure when you want to repeat a set of statements an indefinite number of times, until a condition is satisfied. If you want to repeat the statements a set number of times, the For...Next Statement is usually a better choice.
You can use either While or Until to specify condition, but not both.
You can test condition only one time, at either the start or the end of the loop. If you test condition at the start of the loop (in the Do statement), the loop might not run even one time. If you test at the end of the loop (in the Loop statement), the loop always runs at least one time.
The condition usually results from a comparison of two values, but it can be any expression that evaluates to a Boolean Data Type (Visual Basic) value (True or False). This includes values of other data types, such as numeric types, that have been converted to Boolean.
You can nest Do loops by putting one loop within another. You can also nest different kinds of control structures within each other. For more information, see Nested Control Structures (Visual Basic).
Note
The Do...Loop structure gives you more flexibility than the While...End While Statement (Visual Basic) because it enables you to decide whether to end the loop when condition stops being True or when it first becomes True. It also enables you to test condition at either the start or the end of the loop.
Exit Do
The Exit Do statement can provide an alternative way to exit a Do…Loop. Exit Do transfers control immediately to the statement that follows the Loop statement.
Exit Do is often used after some condition is evaluated, for example in an If...Then...Else structure. You might want to exit a loop if you detect a condition that makes it unnecessary or impossible to continue iterating, such as an erroneous value or a termination request. One use of Exit Do is to test for a condition that could cause an endless loop, which is a loop that could run a large or even infinite number of times. You can use Exit Do to escape the loop.
You can include any number of Exit Do statements anywhere in a Do…Loop.
When used within nested Do loops, Exit Do transfers control out of the innermost loop and into the next higher level of nesting.
Example
In the following example, the statements in the loop continue to run until the index variable is greater than 10. The Until clause is at the end of the loop.
Dim index As Integer = 0
Do
Debug.Write(index.ToString & " ")
index += 1
Loop Until index > 10
Debug.WriteLine("")
' Output: 0 1 2 3 4 5 6 7 8 9 10
The following example uses a While clause instead of an Until clause, and condition is tested at the start of the loop instead of at the end.
Dim index As Integer = 0
Do While index <= 10
Debug.Write(index.ToString & " ")
index += 1
Loop
Debug.WriteLine("")
' Output: 0 1 2 3 4 5 6 7 8 9 10
In the following example, condition stops the loop when the index variable is greater than 100. The If statement in the loop, however, causes the Exit Do statement to stop the loop when the index variable is greater than 10.
Dim index As Integer = 0
Do While index <= 100
If index > 10 Then
Exit Do
End If
Debug.Write(index.ToString & " ")
index += 1
Loop
Debug.WriteLine("")
' Output: 0 1 2 3 4 5 6 7 8 9 10
The following example reads all lines in a text file. The OpenText method opens the file and returns a StreamReader that reads the characters. In the Do...Loop condition, the Peek method of the StreamReader determines whether there are any additional characters.
Private Sub ShowText(ByVal textFilePath As String)
If System.IO.File.Exists(textFilePath) = False Then
Debug.WriteLine("File Not Found: " & textFilePath)
Else
Dim sr As System.IO.StreamReader = System.IO.File.OpenText(textFilePath)
Do While sr.Peek() >= 0
Debug.WriteLine(sr.ReadLine())
Loop
sr.Close()
End If
End Sub
See Also
Reference
For...Next Statement (Visual Basic)
Boolean Data Type (Visual Basic)
While...End While Statement (Visual Basic)