DoLoops in Visual Basic .NET

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The exit condition of a DoLoop can be at the start of the loop (after the word Do) or at the end (after the word Loop) Exit can be when a condition becomes True (Do UntilLoop, or DoLoop Until), or when a condition becomes False (Do WhileLoop, or DoLoop While) Table 51 shows these variations (using a typical condition X = Y) A DoLoop structure can be characterized as performing zero-or-more iterations if the condition is tested at the beginning (Do While or Do Until), or oneor-more iterations if the condition is tested at the end (Loop While or Loop Until) One important facet of this type of loop is that the enclosed statements must be capable of altering the condition This is important, since if the condition can not be altered within the loop, it may never exit For the example structures in Table 51, this means that either X or Y or both must be changed at some point in the enclosed statements, for an exit condition to be possible
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5311 Exiting from a DoLoop
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Table 51 lists the ways that the repeated execution of the code enclosed in a loop can be terminated DoLoop structures are organized so that it is possible to exit
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5 n Controlling Program Code
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Table 51
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DoLoop structures and their operation Explanation
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Loop Code
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When the values in X and Y become the same, the code to repeat will Do Until X = Y Code to repeat not execute any more times If X and Y are equal immediately before the loop, the enclosed statements will never be executed Loop When the values in X and Y become different, the code to repeat will Do While X = Y Code to repeat not execute any more times If X is different from Y before the loop, the enclosed statements will never be executed Loop When the values in X and Y become the same, the code to repeat will Do Code to repeat not execute any more times Even if X and Y are the same before the loop starts, the enclosed statements will be executed at least once Loop Until X = Y When the values in X and Y becomes different, the code to repeat will Do Code to repeat not execute any more times Even if X and Y are different before the Loop While X = Y loop starts, the enclosed statements will be executed at least once
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from the beginning or the end of a loop, providing for zero-or more, or one-or more iterations It is also possible to exit a loop from anywhere in the middle, using an Exit Do statement Of course, this is useless on its own, since an unconditional Exit Do would merely serve to create a loop in which only the code up to the Exit Do could ever execute (Listing 519)
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Do 'First part of the enclosed code Exit Do 'Remainder of the enclosed code (never executes) Loop Listing 519: A DoLoop with an Exit statement
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However, we can enclose the Exit Do statement in an IfThen structure to provide a condition for getting out of a loop in the middle This makes it possible to form a loop structure that will contain some initial code to set up the potential exit condition, and further code to do something with it An example is given in Listing 520
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Do ConsoleWrite("Enter a number (0 to quit):)") n = ConsoleReadLine() If n = 0 Then Exit Do End If ConsoleWriteLine("{0} squared is {1}", n, n * n) Loop Listing 520: Controlled exit from a DoLoop
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Note that this loop does not have a normal exit condition either at the start or at the end the only possible exit is from the middle of the loop This turns out to be
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