While...End While Loop Statement in Visual Basic 2008
The While. . .End While loop executes a block of statements as long as a condition is True. The loop has the following syntax:
If condition is True, all statements in the bock are executed. When the End While statement is reached, control is returned to the While statement, which evaluates condition again. If condition is still True, the process is repeated. If condition is False, the program resumes with the statement following End While.
The loop in Listing 3.5 prompts the user for numeric data. The user can type a negative value to indicate he's done entering values and terminate the loop. As long as the user enters positive numeric values, the program keeps adding them to the total variable.
Listing 3.5: Reading an Unknown Number of Values
I've assigned the value 0 to the number variable before the loop starts because this value isn't negative and doesn't affect the total.
Sometimes, the condition that determines when the loop will terminate can't be evaluated at the top of the loop. In these cases, we declare a Boolean value and set it to True or False from within the loop's body. Here's the outline of such a loop:
You may also see an odd loop statement like the following one:
It's also common to express the True condition as follows:
This seemingly endless loop must be terminated from within its own body with an Exit While statement, which is called when a condition becomes True or False. The following loop terminates when a condition is met in the loop's body:
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