Chapter 7Loops

Programming InVisual Basic.NET

Prepared by Johnny Tsui, CIM@VTC

2

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

3

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: