Types of LOOP Structures

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

Basic Definition

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

Basic 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 ……. Loop

Do While condition is true

statement(s)

Loop

Flowchart for a Do While Loop

Is the condition true

Execute statementswithin

the loop

Execute statementsthat follow the loop

Yes

No

Example

What is the output?

Output (Displays the numbers from 1 through 10)

The 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 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 loops

strPassWord = ""

Do While strPassWord <> “BIS"

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

Loop

Counter-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.

Example

intNum = 1

Do While intNum <= 10

picOutput.Print num;

intNum = intNum + 1

Loop

Do Until ……. Loop

Is executed until the condition becomes True

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

$15000 is deposited at 5% interest. If $1000 years to deplete a saving account) is withdrawn at the end of each year, how many years will it take to empty the account?

Comparing While… and Until Loops 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 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 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: Counter & Accumulator

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

For … 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

Example

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

Example

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 encountered

The control variable is assigned the initial value.

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.)

Iteration continues until the terminating value is exceeded.

Rules for Using For ... Next 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.

Example

Private Sub cmdDisplay_Click()

Dim i As Integer

For i = 1 To 10

picOutput.Print "*";

Next i

End Sub

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

Example

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

Example

Dim numVar As Integer

For numVar = 8 To 1 Step -2

picOutput.Print numVar;

Next numVar

Output: 8 6 4 2

Nested Loops

For outer = 1 To 4

For inner = 1 To 2

..

..

Next inner

Next outer

Example: Display a 10x10 rectangle of stars

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 i

End Sub