DO
DO [
WHILE Condition ]
.
.
.
[
BREAK |
CONTINUE ]
.
.
.
LOOP [
UNTIL Condition ]
Repeats a number of statements while the initial condition remains true or until the final condition becomes true. If neither
WHILE nor
UNTIL is used, the loop becomes an endless loop that can only be exited via the
BREAK statement. If the break statement is missing in such a case, it becomes an infinite loop, a condition that is almost universally undesirable.
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Part
|
Description
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|---|
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DO
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Always the first statement of the loop.
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WHILE
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If used, states a Condition that must remain true to execute the loop.
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UNTIL
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If used, states a Condition that has to become true to stop execution of the loop.
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Condition
|
Any boolean expression.
|
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BREAK
|
Immediately jumps out of the loop and continues execution of the program with the next line after the loop.
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CONTINUE
|
Immediately leaves out all following statements in the loop and jumps to the end of the loop causing it to start all over again.
|
|
LOOP
|
Always the last statement of the loop.
|
If the initial
Condition on a
WHILE statement is false to begin with, the loop is not executed at all. Otherwise, the loop will be executed at least once, even if the final
Condition on an
UNTIL statement is true to begin with.
Examples
' A very simple loop
Dim a As Integer = 1
Do While a <= 5
Print "Hello World "; a
Inc a
Loop
Hello World 1
Hello World 2
Hello World 3
Hello World 4
Hello World 5
' The same effect with UNTIL
Dim a As Integer
Do
Print "Hello World "; a
Inc a
Loop Until a = 6
Be careful not to enter the
UNTIL loop with "a" being larger than 5. "a" will be incremented to a larger value than 6, and the only way to stop the loop is lost. You might be better off to use "
LOOP a > 5" instead to minimize the risk of infinite loops.
Examples
' This loop will never reach its end value
Dim a As Integer = 1
DO WHILE a <= 5
PRINT "Hello World "; a
INC a
IF a = 4 THEN BREAK
LOOP
Hello World 1
Hello World 2
Hello World 3
See also