Chapter 6

Chapter 6 - Visual Basic Schneider 1

Chapter 6Chapter 6

Repetition


Outline & ObjectivesOutline & Objectives

Loop Structure Elements of a Loop Structure Processing Lists of Data with Do Loops


Types of LOOP StructuresTypes of LOOP Structures

Do While ……. Loop Do Until …… Loop For …… Next loop


Basic DefinitionBasic Definition

Looping: the process of repeating a series of statements multiple times until a criteria is met


Basic Components of LoopsBasic Components of Loops

Loop control variable: A variable used to determine whether a loop will be executed

Loop body: The statement (s) that are executed each time a loop repeats


The Do While ……. LoopThe Do While ……. Loop

Do While condition is true

statement(s)

Loop


Flowchart for a Do While LoopFlowchart for a Do While Loop

Is the condition true

Execute statementswithin

the loop

Execute statementsthat follow the loop

Yes

No


Example Example (Displays the numbers from 1 through 10)(Displays the numbers from 1 through 10)

Private Sub cmdDisplay_Click()

Dim num As Integer

' Display the numbers from 1 to 10

num = 1

Do While num <= 10

picNumbers.Print num;

num = num + 1

Loop

End Sub


The Do While ……. LoopThe Do While ……. Loop

Is executed as long as the condition is True. If condition is False then the next statement after

the Loop is executed.


Controlling LoopsControlling Loops

Methods of controlling loops: Counter-controlled loops

repeat a specific number of times

Event-controlled loops repeat until something happens in the loop body to change

the value of loop control variable.


Example of event-controlled loopsExample of event-controlled loops

passWord = ""

Do While passWord <> "SHAZAM"

passWord = UCase(InputBox("What is the password?"))

Loop


Counter-controlled LoopsCounter-controlled Loops

Is useful when the programmer knows how many times the loop should be executed.

Initialize the counter by setting it to a beginning value before entering the loop.

The counter is incremented (or decremented) by the same value during each repetition.


ExampleExample

num = 1

Do While num <= 10

picOutput.Print num;

num = num + 1

Loop


Do Until ……. LoopDo Until ……. Loop

Is executed until the condition becomes True Any Do While…. Loop can be rewritten as a Do

Until ….. Loop


Example Example (requires the user to give a password (requires the user to give a password

before opening a file)before opening a file)


Dim passWord As String, info As String

If UCase(txtName.Text) = "SECRET.TXT" Then

Do

passWord = UCase(InputBox("What is the password?"))

Loop Until passWord = "SHAZAM"

End If

Open txtName.Text For Input As #1

Input #1, info

picItem.Cls

picItem.Print info

Close #1

End Sub


Example Example (years to deplete a saving account)(years to deplete a saving account)

Private Sub cmdEstimate_Click()

Dim amt As Single, yrs As Integer

' Years to deplete savings account

picResult.Cls

amt = 15000

yrs = 0

Do

amt = amt * 1.05 - 1000

yrs = yrs + 1

Loop Until amt <= 0

picResult.Print "It takes"; yrs; "years to deplete the account."

End Sub


Comparing While… and Until Comparing While… and Until LoopsLoops

The Do While … Loop executes while the condition is true

The Do Until….. Loop executes until the condition is true

Both can be used to create any type of loop


EOF FunctionEOF Function

EOF(n) is True if the end of the file having reference number n has been reached. Otherwise, it is False

Example: Open “PHONE.TXT” for Input As #1Do While Not EOF(1) Input #1, nom, phoneNum picOutput.Print nom, phoneNumLoopClose #1


Counters and AccumulatorsCounters and Accumulators

A counter is a numeric variable that keeps track of the number of items that have been processed in a loop.

An accumulator is a numeric variable that holds a sub-total during multiple passes through a loop.


Example:Example: CounterCounter && Accumulator AccumulatorPrivate Sub cmdAnalyze_Click() Dim numCoins As Integer, sum As Single, value As Single Open "COINS.TXT" For Input As #1 numCoins = 0 sum = 0 Do While Not EOF(1) Input #1, value numCoins = numCoins + 1 sum = sum + value Loop picValue.Print "The value of the"; numCoins; "coins is"; sum;

"cents."End Sub

CompareCompare

Do While ……. Loop Do ……. Loop While Do ……. Loop Until Do Until ……. Loop


ReviewReview

num = 11

Do While num <= 10


num = num + 1

Loop


num = 11

Do


num = num + 1

Loop until num <= 10

How many times will the following loops execute?

ReviewReview

Do While i < 10

i = i + 1

Loop


Do

i = i + 1

Loop While i < 10

Which loop is infinite?

Do Until i < 10Loop

Do i = i + 10 Loop Until i < 10


For … Next LoopFor … Next Loop

A loop where the number of iterations is determined by a range of values for a numeric variable

Syntax:

For controlVariable = initial To terminal

statement(s)

Next controlVariable


ExampleExample

Private Sub cmdDisplayTable_Click()

Dim i As Integer

‘Display a table of the first 5 numbers and their squares

For i = 1 To 5

picTable.Print i; i ^ 2

Next i

End Sub

Initial Value

Control variable

Terminating value


ExampleExample

Dim numVar As Integer

For numVar = 1 To 5 Step 2

picOutput.Print numVar;

Next numVar

Output: 1 3 5


When a For statement is When a For statement is encounteredencountered

1. The control variable is assigned the initial value.

2. After each loop iteration, the step value is added to the value of the control variable. (If there is no step value, 1 is added.)

3. Iteration continues until the terminating value is exceeded.


Rules for Using Rules for Using For ... NextFor ... Next loop loop

You should never modify the value of the loop control variable in the loop body.

Each For loop must end with a Next statement.


ExampleExample


Dim i As Integer

For i = 1 To 10

picOutput.Print "*";

Next i

End Sub

Output: **********


Example Example


Dim i As Integer, stars As Integer

stars = Val(InputBox("Row length (1-20) : "))

For i = 1 To stars


Next i

End Sub


ExampleExample

Dim numVar As Integer

For numVar = 8 To 1 Step -2

picOutput.Print numVar;

Next numVar

Output: 8 6 4 2


Nested LoopsNested Loops

For outer = 1 To 4

For inner = 1 To 2

..

..

Next inner

Next outer


Example: Display a 10x10 Example: Display a 10x10 rectangle of starsrectangle of starsPrivate Sub cmdDisplay_Click()

Dim i As Integer, j As Integer

For i = 1 To 10

For j = 1 To 10


Next j

picOutput.Print

Next i

End Sub

ReviewReview

For i= -5 To -1 Step - 2

picOutput.Print i;

Next i

picOutput.Print i;


For i= 1 To 5 Step 2

i=i+1

picOutput.Print i;

Next i

picOutput.Print i;


ReviewReview



For i = 1 To 10

For j = i To 10


Next j

picOutput.Print

Next i

End Sub



For i = 1 To 10

For j = 1 To i


Next j

picOutput.Print

Next i

End Sub

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Chapter 6