Repetition Chapter 6 - Visual Basic Schneider 1 Loop Structure Elements of a Loop Structure ...

Repetition

Chapter 6 - Visual Basic Schneider 1

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


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


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


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


Do While condition is true statement(s) Loop



Is the condition true

Execute statementswithin

the loop

Execute statementsthat follow the loop

Yes

No

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 LoopEnd Sub


Is executed as long as the condition is True.

If condition is False then the next statement after the Loop is executed.


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.


passWord = ""Do While passWord <> "SHAZAM" passWord = UCase(InputBox("What is the

password?"))Loop


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.


num = 1Do While num <= 10 picOutput.Print num; num = num + 1Loop


Is executed until the condition becomes True

Any Do While…. Loop can be rewritten as a Do Until ….. Loop


Private Sub cmdDisplay_Click() 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 #1End Sub


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


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(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


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.


Private 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


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


num = 11Do While num <= 10 picOutput.Print num; num = num + 1Loop


num = 11Do picOutput.Print

num; num = num + 1Loop until num

<= 10

How many times will the following loops execute?

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

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


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 ^ 2Next i

End Sub


Initial Value

Control variable

Terminating value

Dim numVar As IntegerFor numVar = 1 To 5 Step 2 picOutput.Print numVar; Next numVar

Output: 1 3 5


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.


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

Each For loop must end with a Next statement.


Private Sub cmdDisplay_Click() Dim i As Integer For i = 1 To 10 picOutput.Print "*"; Next iEnd Sub

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


Private Sub cmdDisplay_Click() Dim i As Integer, stars As Integer stars = Val(InputBox("Row length (1-20) :

")) For i = 1 To stars picOutput.Print "*"; Next iEnd Sub


Dim numVar As IntegerFor numVar = 8 To 1 Step -2 picOutput.Print numVar;Next numVar

Output: 8 6 4 2


For outer = 1 To 4 For inner = 1 To 2 .. .. Next innerNext outer


Private Sub cmdDisplay_Click() Dim i As Integer, j As Integer For i = 1 To 10 For j = 1 To 10 picOutput.Print "*"; Next j picOutput.Print Next iEnd Sub


For i= -5 To -1 Step - 2 picOutput.Print i; Next ipicOutput.Print i;


For i= 1 To 5 Step 2 i=i+1

picOutput.Print i;

Next ipicOutput.Print i;

Private Sub cmdDisplay_Click()

Dim i As Integer, j As Integer

For i = 1 To 10 For j = i To 10 picOutput.Print "*"; Next j picOutput.Print Next iEnd Sub


Private Sub cmdDisplay_Click()

Dim i As Integer, j As Integer

For i = 1 To 10 For j = 1 To i picOutput.Print

"*"; Next j picOutput.Print Next iEnd Sub

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Repetition Chapter 6 - Visual Basic Schneider 1 Loop Structure Elements of a Loop Structure ...

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