1. Conditional Loops Counted Loops
Iteration
Conditional Loops
Counted Loops

2. 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

3. Charting the Flow of Control
We’ve also used flow charts to map several different styles of selection.
e.g. If/Then/Else
Condition
True
False
Process

4. 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

5. 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.

6. 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

7. Conditional loops The keywords Until or While specify the Conditions.
Note:
Until counter >= Max
and
While counter < Max
are functionally equivalent.

8. Pre-test Do Until Do Until counter = max step1 step2 step3
process data
step1
step2…
stop? N Y
increment counter
Do Until counter = max
step1
step2
step3
counter += 1
Loop

9. Pre-test Do While Do While counter < max step1 step2 step3
Loop Y
repeat? N
process data
step1
step2…
increment counter

10. 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.

11. Post-test Do ... Loop Until
step1
step2
step3
counter += 1
Loop Until counter = max
process data
step1
step2…
stop? N Y
increment counter

12. Post-test Do ... Loop While
step1
step2
step3
counter += 1
Loop While counter < max
process data
step1
step2…
repeat? Y N
increment counter C

13. 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

14. For/Next Loops Dim counter As Integer For counter = 1 To 5 ‘ loop body
limit exceeded? N Y
increment loop index
initialise loop index variable
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.

15. 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

16. 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.
Dim counter As Integer
For counter = 1 To 5
‘loop body
Next counter
txtDisplay.Text = “Counter is ” & Str(Counter)
will display …

17. 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.
Dim counter As Integer
For counter = 1 To 5
‘loop body
Next counter
txtDisplay.Text = “Counter is ” & Str(Counter)
will display 6 .

18. For/Next Loops For counter = -11 To 2 Step 4 ‘ loop body Next counter
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?

19. For/Next Loops For counter = -11 To 2 Step 4 ‘ loop body Next counter
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?
counter will have the values -11, -7, -3, 1, and 5.

20. For/Next Loops For counter = 10 To 0 Step -3 ‘ loop body Next counter
Another example.
_________________________________________________________
For counter = 10 To 0 Step -3
‘ loop body
Next counter
What values will counter have?

21. For/Next Loops For counter = 10 To 0 Step -3 ‘ loop body Next counter
Another example.
_________________________________________________________
For counter = 10 To 0 Step -3
‘ loop body
Next counter
What values will counter have?
counter will have the values 10, 7, 4, 1 and -2.

22. 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.

23. For/Next Loops
VB ignores attempts to change the loop parameters within the loop,
Dim counter, start, finish, increment As Integer
For counter = 1 To 5
start = -2
finish = 10
increment = 5
‘ will have no effect
Next counter
except…

24. For/Next Loops …the loop index.
Dim counter As Integer
For counter = 1 To 10
‘display Counter
counter = counter + 3
Next counter
Simulate this program to see what values will be displayed.
skip

25. For/Next Simulation display counter Dim counter As Integer
For counter = 1 To 10
‘display Counter
counter = counter + 3
Next counter
display
counter

26. For/Next Simulation 1 display counter Dim counter As Integer
For counter = 1 To 10
‘display Counter
counter = counter + 3
Next counter 1
display
counter

27. For/Next Simulation 1 display counter Dim counter As Integer
For counter = 1 To 10
‘display Counter
counter = counter + 3
Next counter 1
display
counter

28. For/Next Simulation 4 1 display counter Dim counter As Integer
For counter = 1 To 10
‘display Counter
counter = counter + 3
Next counter 4 1
display
counter

29. For/Next Simulation 5 4 1 display counter Dim counter As Integer
For counter = 1 To 10
‘display Counter
counter = counter + 3
Next counter 5 4 1
display
counter

30. For/Next Simulation 5 4 1 display counter Dim counter As Integer
For counter = 1 To 10
‘display Counter
counter = counter + 3
Next counter 5 4 1
display
counter

31. For/Next Simulation 8 5 4 1 display counter Dim counter As Integer
For counter = 1 To 10
‘display Counter
counter = counter + 3
Next counter 8 5 4 1
display
counter

32. For/Next Simulation 9 8 5 4 1 display counter Dim counter As Integer
For counter = 1 To 10
‘display Counter
counter = counter + 3
Next counter 9 8 5 4 1
display
counter

33. For/Next Simulation 9 8 5 4 1 display counter Dim counter As Integer
For counter = 1 To 10
‘display Counter
counter = counter + 3
Next counter 9 8 5 4 1
display
counter

34. For/Next Simulation 12 9 8 5 4 1 display counter
Dim counter As Integer
For counter = 1 To 10
‘display Counter
counter = counter + 3
Next counter 12 9 8 5 4 1
display
counter

35. For/Next Simulation 12 9 13 8 5 4 1 display counter
Dim counter As Integer
For counter = 1 To 10
‘display Counter
counter = counter + 3
Next counter 12 9 13 8 5 4 1
display
counter

36. For/Next Simulation 12 9 13 8 5 4 1 display counter
Dim counter As Integer
For counter = 1 To 10
‘display Counter
counter = counter + 3
Next counter
In general this is poor programming. 12 9 13 8 5 4 1
display
counter

37. Avoid changing the loop index!
For/Next Loops
Here’s an even poorer example.
Dim counter As Integer
For counter = 1 To 5
counter = 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!

38. 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?

39. Nested Loops 9 8 7 6 5 4 3 2 1
innerIndex
outerIndex

40. Nested Loops 9 1 8 7 6 5 4 3 2
innerIndex
outerIndex

41. Notes
VB allows programmers to omit the loop index from the Next statement:
For counter = start To finish
‘ loop body
Next [counter]

42. Notes This can make code difficult to follow. For outerIndex = 0 To 9
For middleIndex = 0 To 9
For innerIndex = 0 To 9
‘display (Str(outerIndex) & _
(Str(middleIndex ) & Str(innerIndex))
Next

43. 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

44. Notes
VB allows programs to exit For - Next loops before termination is reached.
For counter = start To finish
Exit For
Next

45. Notes Usually this command is controlled by an If statement.
For counter = start To finish
If <some other condition> Then
Exit For
End If
‘ other steps
Next