iteration conditional loops counted loops. charting the flow of control we’ve used flow charts to...

Iteration

Conditional Loops

Counted Loops

Charting the Flow of Control

• We’ve used flow charts to visualise the flow of control.

• The simplest form is sequence.

Declare vars for input and result

get input

produce result from input

show result

Charting the Flow of Control

• We’ve also used flow charts to map several different styles of selection.

• e.g.If/Then/Else

Condition

TrueFalse

ProcessProcess

Flow of Control

• The third way control can be transferred is called iteration, repetition, or looping.

• There are several ways to describe a looping process. For example:– Repeat the following steps if conditions are right.

• conditional loop

– Repeat the following steps X times.• counted loop

Repeat the following steps if conditions are right.

• “if conditions are right” can mean different things:– … until there’s no more data.

– … while there’s still data.

• The difference is simply in how the condition is stated.

• “the following steps” implies that the condition will be tested before the steps are executed. – This is called a pre-test.

Conditional loops

In VB, loops are specified by the keywords Do and Loop.

Statements that make up “the following steps” are bracketed by this pair and will be repeatedly executed.

Do

' step 1

' step 2 ...

Loop

Conditional loops

The keywords Until or While specify the Conditions.

Note:Note:

Until counter >= Max

and

While counter < Max

are functionally equivalent.

Pre-test Do Until

Do Until counter = max

step1

step2

step3

counter += 1

Loop process data step1 step2…

stop?N

Y

increment counter

Pre-test Do While

Do While counter < max

step1

step2

step3

counter += 1

Loop process data step1 step2…

repeat?Y

N

increment counter

Post-test Loops

• There are other ways of expressing conditional loops:– Repeat steps 1, 2, 3, & 4 until there’s no more data.

– Repeat steps 1, 2, 3, & 4 while there’s still data.

• These are the same types of conditions.• The difference is that the condition will be tested

after the steps are executed each time. – This is called a post-test.

Post-test Do ... Loop Until

Do

step1

step2

step3

counter += 1

Loop Until counter = max

process datastep1step2…

stop?N

Y

increment counter

Post-test Do ... Loop While

Do

step1

step2

step3

counter += 1

Loop While counter < max

process datastep1step2…

repeat?Y

N

increment counter

C

Repeat the following steps X times.

Counted loops are so common that most programming languages have a special syntax which optimises counted loops.

In VB this structure is implemented as the

For/Next structure.

For loopIndex = start To finish

‘ body of loop

Next loopIndex

For/Next Loops

Dim counter As Integer

For counter = 1 To 5

‘ loop body

Next counter

For/Next loops use a pre-test, and have a Do Until style.

loop body

limit exceeded?

N

Y

increment loop index

initialise loop index variable

For/Next Loops

In the most general case, For/Next loops use 4 integer parameters.

Dim counter As Integer ‘loop index

Dim start As Integer ‘initial value

Dim finish As Integer ‘final value

Dim increment As Integer ‘delta value

For counter = start To finish Step increment

‘ loop body

Next counter

For/Next Loops

The scope of counter extends beyond the loop, so its value can be used or changed when the loop is finished. This may be a useful side effect.



‘loop body

Next counter

txtDisplay.Text = “Counter is ” & Str(Counter)

will display …

For/Next Loops

The scope of counter extends beyond the loop, so its value can be used or changed when the loop is finished. This may be a useful side effect.



‘loop body

Next counter

txtDisplay.Text = “Counter is ” & Str(Counter)

will display 6 .

For/Next Loops

Since they are integers they can have positive or negative values._________________________________________________________

For counter = -11 To 2 Step 4‘ loop body

Next counter_________________________________________________________

What values will counter have?_________________________________________________________

For/Next Loops

Since they are integers they can have positive or negative values._________________________________________________________

For counter = -11 To 2 Step 4‘ loop body

Next counter_________________________________________________________


counter will have the values -11, -7, -3, 1, and 5.

For/Next Loops

Another example._________________________________________________________

For counter = 10 To 0 Step -3‘ loop body

Next counter_________________________________________________________


For/Next Loops

Another example._________________________________________________________

For counter = 10 To 0 Step -3‘ loop body

Next counter_________________________________________________________


counter will have the values 10, 7, 4, 1 and -2.

For/Next Loops

The start, finish, and increment values can be specified as literals or as Integer variables declared before the loop.

The loop index must be an Integer variable. It must be declared before the loop.

For/Next Loops

VB ignores attempts to change the loop parameters within the loop,

Dim counter, start, finish, increment As Integer


start = -2

finish = 10

increment = 5

‘ will have no effect

Next counter

except…

For/Next Loops

…the loop index.


For counter = 1 To 10‘display Countercounter = counter + 3

Next counter

Simulate this program to see what values will be displayed.

skip

For/Next Simulation



‘display Counter

counter = counter + 3

Next counter

displaycounter

For/Next Simulation



‘display Counter


Next counter

1

displaycounter

For/Next Simulation



‘display Counter


Next counter

11

displaycounter

For/Next Simulation



‘display Counter


Next counter

4

11

displaycounter

For/Next Simulation



‘display Counter


Next counter

5

4

11

displaycounter

For/Next Simulation



‘display Counter


Next counter

55

4

11

displaycounter

For/Next Simulation



‘display Counter


Next counter8

55

4

11

displaycounter

For/Next Simulation



‘display Counter


Next counter 9

8

55

4

11

displaycounter

For/Next Simulation



‘display Counter


Next counter 99

8

55

4

11

displaycounter

For/Next Simulation



‘display Counter


Next counter

12

99

8

55

4

11

displaycounter

For/Next Simulation



‘display Counter


Next counter

12

99

13

8

55

4

11

displaycounter

For/Next Simulation



‘display Counter


Next counter

In general this is poor programming.

12

99

13

8

55

4

11

displaycounter

For/Next Loops

Here’s an even poorer example.


For counter = 1 To 5counter = 1

Next counter

Since counter is set to 1 each time the loop executes it can never reach 5, so the loop is infinite.

Avoid changing the loop index!

Nested Loops

The body of a loop can contain any VB elements, including a loop.

For outerIndex = 0 To 9

For innerIndex = 0 To 9

‘display (Str(outerIndex) & Str(innerIndex))

Next innerIndex

Next outerIndex

What is the ‘output’ from this loop?

Nested Loops

90

80

70

60

50

40

30

20

10

00

innerIndexouterIndex

Nested Loops

9190

8180

7170

6160

5150

4140

3130

2120

1110

0100

innerIndexouterIndexinnerIndexouterIndex

Notes

VB allows programmers to omit the loop index from the Next statement:

For counter = start To finish

‘ loop body

Next [counter]

Notes

This can make code difficult to follow.

For outerIndex = 0 To 9For middleIndex = 0 To 9For innerIndex = 0 To 9‘display (Str(outerIndex) & _(Str(middleIndex ) & Str(innerIndex))NextNextNext

Notes

• Proper use of indentation also helps.

For outerIndex = 0 To 9

For middleIndex = 0 To 9

For innerIndex = 0 To 9

‘display (Str(outerIndex) & _(Str(middleIndex ) & Str(innerIndex))

Next innerIndex

Next middleIndex

Next outerIndex

Notes

VB allows programs to exit For - Next loops before termination is reached.

For counter = start To finish

Exit For

Next

Notes

Usually this command is controlled by an If statement.

For counter = start To finishIf <some other condition> Then

Exit ForEnd If

‘ other stepsNext

iteration conditional loops counted loops. charting the flow of control we’ve used flow charts to...

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