chapter 7 loops programming in visual basic.net prepared by johnny tsui, cim@vtc
TRANSCRIPT
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Chapter 7Loops
Programming InVisual Basic.NET
Prepared by Johnny Tsui, CIM@VTC
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Loops Repeating a series of instructions Types of Loops
Do Use when the number of iterations is unknown
For Next Use when the number of iterations known
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Do Loops Ends based on a condition you specify,
either Loop While a condition is True Loop Until a condition becomes True
Condition can be located at Top of Loop, Pretest Bottom of Loop, Posttest
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Do Loop General Form
Do {While |Until} conditionStatements to execute
Loop
OR
DoStatements to execute
Loop {While | Until} condition
Top of Loop Condition,
Pretest
Bottom of Loop Condition,
Posttest
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Do's - When Evaluated Top Evaluation, not guaranteed to run
once since evaluated BEFORE runningDo While … LoopDo Until … Loop
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Example:intA = 0Do Until intA = 10
intA = intA + 1Loop
=10? Action intA
Start 0
Do Until No intA + 1 1
No intA + 1 2
No intA + 1 3
No intA + 1 4
No intA + 1 5
No intA + 1 6
No intA + 1 7
No intA + 1 8
No intA + 1 9
No intA + 1 10
Yes Exit
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Example:intA = 0Do While intA < 10
intA = intA + 1Loop
<10? Action intA
Start 0
Do Until Yes intA + 1 1
Yes intA + 1 2
Yes intA + 1 3
Yes intA + 1 4
Yes intA + 1 5
Yes intA + 1 6
Yes intA + 1 7
Yes intA + 1 8
Yes intA + 1 9
Yes intA + 1 10
No Exit
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Do's - When Evaluated Bottom Evaluation, will always run at least
one timeDo … Loop WhileDo … Loop Until
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Example:intA = 0Do
intA = intA + 1Loop Until intA = 10
Action intA =10 ?
Start 0
Loop Until intA + 1 1 No
intA + 1 2 No
intA + 1 3 No
intA + 1 4 No
intA + 1 5 No
intA + 1 6 No
intA + 1 7 No
intA + 1 8 No
intA + 1 9 No
intA + 1 10 Yes
Exit
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Example:intA = 0Do
intA = intA + 1Loop While intA < 10
Action intA <10 ?
Start 0
Loop Until intA + 1 1 No
intA + 1 2 No
intA + 1 3 No
intA + 1 4 No
intA + 1 5 No
intA + 1 6 No
intA + 1 7 No
intA + 1 8 No
intA + 1 9 No
intA + 1 10 Yes
Exit
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Do Loops / For Next Loops:Deciding Which To Use
Is thenumber ofrepetitions
known?
Is theexpression
initiallytrue?
Mustthe loopexecuteonce?
Top Eval Do Loop
Bottom Eval Do LoopFor Next Loop
Do Until Loop Do While Loop
NO
YES
NO
NO YES
YES
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For Next Loops Use when you know the number of iterations Uses a numeric counter variable, called Loop
Index, to control number of iterations Loop Index is incremented at the bottom of
the loop on each iteration Step value can be included to specify the
incrementing amount to increment Loop Index, step can be a negative number
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For Next Loop General Form
For LoopIndex = InitialValue To TestValue [Step Increment]
Statements Next [LoopIndex]
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Example:intA = 0intB = 0For intA = 1 To 10
intB = intB + 1Next
intA intA>10 ? intB
Start 0 0
For 1 No 1
2 No 2
3 No 3
4 No 4
5 No 5
6 No 6
7 No 7
8 No 8
9 No 9
10 No 10
Exit 11 Yes
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Example:intA = 0intB = 0For intA = 1 To 10 Step 2
intB = intB + 1Next
intA intA>10 ? intB
Start 0 0
For 1 No 1
3 No 2
5 No 3
7 No 4
9 No 5
Exit 11 Yes
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Manually Exiting For Next Loops In some situations you may need to exit
the loop prematurely Use the Exit For statement inside the loop
structure
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Example:intA = 0intB = 0For intA = 1 To 10
intB = intB + 1If intB = 3 Exit For
End IfNext
intA intA>10 ? intB intB=3 ?
Start 0 0
For 1 No 1 No
2 No 2 No
3 No 3 Yes
Exit
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Exercises Idenifty the statements that are correctly formed
and those that have errors. For those with errors, state what is wrong and how to correct it.
(a)For decIndex = 3.5 to 6.0, Step 0.5Next
(b)For 4 = 1 to 10 Step 2Next
(c) For intIndex = 100 to 0 Step -25Next
(d)For intIndex = 0 to -10 Step -1Next
(e)For intIndex = 10 to 1Next
Name: Class:
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Exercises How many times will the body of the
loop be executed for each of these examples?
(a) For intIndex = 1 to 3(b) For intIndex = 2 to 11 Step 3(c) For intIndex = 10 to -1 Step -1(d) For decIndex = 3.0 to 6.0 Step 0.5(e) For intIndex = 5 to 1
Name: Class: