auto generate barcode vb net Here is a variation of Example 3.14, illustrating the use of a typical Exit For statement. in VS .NET

Generation QR in VS .NET Here is a variation of Example 3.14, illustrating the use of a typical Exit For statement.

EXAMPLE 3.16
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Here is a variation of Example 3.14, illustrating the use of a typical Exit For statement.
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sum = 0 For i = 1 To 10 sum = sum + i If sum >= 10 Then Exit For Next i
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This loop is set up to execute 10 times, but the execution will be terminated if the current value of sum equals or exceeds 10. In this particular case, the execution will terminate within the fourth pass (because 1 + 2 + 3 + 4 = 10).
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3.7 LOOPING WITH Do-Loop In addition to For-Next structures, Visual Basic also includes Do-Loop structures, which are convenient when the number of passes through a loop is not known in advance (as, for example, when a loop is required to continue until some logical condition has been satisfied). A Do-Loop structure always begins with a Do statement and ends with a Loop statement. However, there are four different ways to write a Do-Loop structure. Two of the forms require that a logical expression appear in the Do statement (i.e., at the beginning of the block); the other two forms require that the logical expression appear in the Loop statement (at the end of the block). The general forms of the Do-Loop structure are shown below. First form:
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Do While logical expression . . . . . . . .
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Second form:
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Do Until logical expression . . . . . . . .
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executable statements
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. . . . . . . . Loop
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executable statements
. . . . . . . . Loop
BRANCHING AND LOOPING
[CHAP. 3
Third form:
Do . . . . . . . .
Fourth form:
Do . . . . . . . .
executable statements
. . . . . . . . Loop While logical expression
executable statements
. . . . . . . . Loop Until logical expression
The first form continues to loop as long as the logical expression is true, whereas the second form continues to loop as long as the logical expression is not true (until the logical expression becomes true). Similarly, the third form continues to loop as long as the logical expression is true, whereas the fourth form continues to loop as long as the logical expression is not true. Note that there is a fundamental difference between the first two forms and the last two forms of the DoLoop block. In the first two forms, the logical test is made at the beginning of each pass through the loop; hence, it is possible that there will not be any passes made through the loop, if the indicated logical condition is not satisfied. In the last two forms, however, the logical test is not made until the end of each pass; therefore, at least one pass through the loop will always be carried out.
EXAMPLE 3.17
Consider the following two Do-Loop structures.
flag = "False" . . . . . . . . Do While flag = "True" . . . . . . . . Loop flag = "False" . . . . . . . . Do . . . . . . . . Loop While flag = "True"
The left loop will not execute at all, because the logical test at the beginning of the loop structure is false. The right loop will execute once, however, because the logical test is not carried out until the end of the first pass through the loop. Moreover, if the string "True" is assigned to flag during this first pass through the right loop, then the execution will continue indefinitely, until flag is reassigned.
Note that a Do-Loop structure does not involve a formal index. Thus, the programmer must provide the logic for altering the value of the logical expression within the loop. Typically, an initial assignment is made before entering the loop structure. The logical expression is then altered at some point within the loop.
EXAMPLE 3.18
Here is a Do-While loop that is comparable to the For-Next loop in Example 3.14.
sum = 0 count = 1 Do While count <= 10 sum = sum + count count = count + 1 Loop
This structure will result in 10 passes through the loop. Note that count is assigned a value of 1 before entering the loop. The value of count is then incremented by 1 during each pass through the loop. Once the value of count exceeds 10, the execution will cease.
CHAP. 3]
BRANCHING AND LOOPING
Here is another way to accomplish the same thing.
sum = 0 count = 1 Do sum = sum + count count = count + 1 Loop While count <= 10
If we choose to use an Until clause rather than a While clause, we can write the control structure in either of the following ways.
sum = 0 count = 1 Do Until count > 10 sum = sum + count count = count + 1 Loop sum = 0 count = 1 Do sum = sum + count count = count + 1 Loop Until count > 10
Note that the logical expression in these two structures (count > 10) is the opposite of the logical expression in the first two structures (count <= 10).
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