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# Chapter 7 Loops - PowerPoint PPT Presentation

Chapter 7 Loops. Programming In Visual Basic.NET Prepared by Johnny Tsui, [email protected] 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. Do Loops.

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### Chapter 7Loops

Programming In

Visual Basic.NET

Prepared by Johnny Tsui, [email protected]

• 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

• 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

Do {While |Until} condition

Statements to execute

Loop

OR

Do

Statements to execute

Loop {While | Until} condition

Top of Loop Condition,

Pretest

Bottom of Loop Condition,

Posttest

• Top Evaluation, not guaranteed to run once since evaluated BEFORE running

Do While … Loop

Do Until … Loop

• Example:

intA = 0

Do Until intA = 10

intA = intA + 1

Loop

• Example:

intA = 0

Do While intA < 10

intA = intA + 1

Loop

• Bottom Evaluation, will always run at least one time

Do … Loop While

Do … Loop Until

• Example:

intA = 0

Do

intA = intA + 1

Loop Until intA = 10

• Example:

intA = 0

Do

intA = intA + 1

Loop While intA < 10

number of

repetitions

known?

Must

the loop

execute

once?

NO

NO

Top Eval Do Loop

YES

YES

For Next Loop

Bottom Eval Do Loop

Is the

expression

initially

true?

Do Until Loop

Do While Loop

NO

YES

Do Loops / For Next Loops:Deciding Which To Use

• 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

For LoopIndex = InitialValue To TestValue [Step Increment]

Statements

Next [LoopIndex]

• Example:

intA = 0

intB = 0

For intA = 1 To 10

intB = intB + 1

Next

• Example:

intA = 0

intB = 0

For intA = 1 To 10 Step 2

intB = intB + 1

Next

• In some situations you may need to exit the loop prematurely

• Use theExit For statement inside the loop structure

• Example:

intA = 0

intB = 0

For intA = 1 To 10

intB = intB + 1

If intB = 3

Exit For

End If

Next

Name: Class:

• 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 0.5

Next

(b) For 4 = 1 to 10 Step 2

Next

(c) For intIndex = 100 to 0 Step -25

Next

(d) For intIndex = 0 to -10 Step -1

Next

(e) For intIndex = 10 to 1

Next

Name: Class:

• 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