1. ( Iteration / Repetition / Looping )
TOPIC 5
CONTROL STRUCTURE
PART 2
( Iteration / Repetition / Looping )

2. TOPICS TO BE COVERED
Nested iteration
Simple iteration
While statement

3. At the end, student be able to :-
OBJECTIVE
At the end, student be able to :-
define the type of repetition control structure
write pseudocode using various repetition control structures
draw flow chart for different type of repetition control structure
Trace the output based on algorithm

4. REPETITION
Repetition Control Structure also known as Looping or Iteration.The control structure that directs the computer to repeat one or more instructions while certain condition remains true.

5. THE ADVANTAGES OF LOOPING:
When you use a loop within a computer program, you can write one set of instructions that operates on multiple, separate sets of data.
A major advantage of having a computer perform complicated tasks is the ability to repeat them.
THE ADVANTAGES OF LOOPING:

6. LOOPS IN PROGRAMMING pretest loop CSC118 Syllabus Loop statements
repeat … until
while
for
do…while
pretest loop
CSC118 Syllabus
pretest loop
posttest loop
posttest loop

7. PRETEST vs POSTTEST LOOPS
POSTTEST LOOP (action before condition)
In each iteration, the loop action(s) is executed. Next, the loop control expression is tested.
If it is true, a new iteration is started; otherwise, the loop terminates.
PRETEST LOOP (condition before action)
In each iteration, the loop control expression is tested first.
If it is true, the loop action(s) is executed;
if it is false, the loop is terminated.
Action(s)
True
False
True
Action(s)
False
Posttest loop
Pretest loop

8. REQUIREMENTS OF A REPETITION STRUCTURE
An Iteration/ Repetition structure requires:
Loop Control
Variable
(LCV)
Loop
condition
Loop body
The loop body is controlled by
3 operations:
Initialization of the Loop Control Variable (LCV)
Example: set the variable to 0
Evaluation of LCV in the loop condition
Eg: Variable < 10
Update of the LCV
Eg: counter = counter + 1
A value of variable determines whether the loop body will be executed or not.
It also identify the number of loops to be performed
Example :
x < 10
If the condition is true, the loop body is executed; otherwise the loop exits.
Block of statement that had to be repeated.

9. It sets the stage for the loop actions.
AND
In addition to the loop control expression, there are two other processes associated with almost all loop:
initialization and updating.
LOOP INITIALIZATION
Before a loop can start, some preparation is usually required. Such preparation is called
loop initialization. Initialization must be done before the first execution of the body.
It sets the stage for the loop actions.
LOOP UPDATE
How can the condition that controls the loop be true for a while and then change to false…?
The answer is that something must happen inside the body of the loop to change the
condition. Otherwise, we would have an infinite loop.This changes the resulting condition from true to false (are known as loop update). Updating is done in each iteration, usually as the last action. If the body of the loop is repeated m times, then the updating is also done m times.
When you write a loop, you must control the number of repetition it performs. If you don’t, you run the risk of creating an infinite loop. Commonly, you control a loop’s repetitions by using either a counter or a sentinel value.

10. … AND Initialization Initialization Action(s) Updating Action(s)
Figure 3: Pretest loop
Figure 4: Posttest loop 10

11. LOOPING STRUCTURE

12. REPETITIONS BY USING EITHER A COUNTER OR A SENTINEL VALUE
MUST CONTROL A LOOP’S
REPETITIONS BY USING EITHER
A COUNTER OR A SENTINEL VALUE

13. COUNTER- CONTROLLED LOOP
Used when the number of time a segment code needs to be repeated is known in advance.
Example: Repeat a process for 10 times

14. SENTINEL-CONTROLLED LOOP
Used when the number of repetition depends on a certain condition.
It uses a special end-of-data value called sentinel value that serves as a signal for loop termination.
Also called event-controlled loops.The number of repetition is not known.
For example :Write a program display ‘Incorrect pin number, please try again’ as long as the code entered is not 877.

16. EXAMPLE 1 : A WHILE LOOP THAT PRINTS “HELLO” FOUR TIMES COUNTER
Start
Start
count = 0
WHILE count < 4
print “Hello”
count = count + 1
ENDWHILE
num count = 0
End
Yes
count < 4?
Print “Hello” No
End
count = count + 1

17. EXAMPLE 2 : COMPUTE THE SUM OF 3 NUMBERS

18. TRY THIS OUT
Write a pseudocode (from the given flowchart) to find the sum of all the even numbers from 2 to 20.

19. EXERCISE 2 TRY THIS OUT Given the program segment below:
set n = 1; while (n <= 5)
n = n + 1;
cout << n << “ ”;
Write the statement/expression that corresponds to the loop operation.
Answer:
Initialization: Evaluation : Update :

20. EXERCISE 3 TRY THIS OUT - TRACING TRACING 1: i = 0 WHILE (i <= 20)
Print i
i = i + 5 ENDWHILE
Output:
TRACING 2:
i = 0
WHILE (i <= 20)
i = i + 5 Print i
ENDWHILE
Output: 20

21. Print “Do you want to see next year’s balance?
EXAMPLE 3 : LOOPING BANK BALANCE PROGRAM
SENTINEL
Get response
WHILE (response == ‘Y’)
Print bankBal
bankBal = bankBal + bankBal *intRate Print “Do you want to see next year’s
balance? Y or No…”
ENDWHILE
Print “Have a nice day”
Get response
response
== ‘Y’?
Yes
Print bankBal No
bankBal = bankBal + bankBal * intRate
Print “Have a nice day”
Print “Do you want to see next year’s balance?
Y or N…”
End
Get response

22. LOOP

23. INTRODUCTORY TO NESTED LOOP
Program logic gets more complicated when you must use loop within loops, or nested loop.
When one loop appears inside another, the loop that contains the other loop is called the outer loop, and the loop that is contained is called the inner loop.
You need to create nested loops when the values of two (or more) variables repeat to produce every combination of values.
For example:
Suppose you want to write a program that produces a quiz answer sheet like the one shown in Figure 11. The quiz has five parts with three questions in each part, and you want a fill-in-the-blank line for each question. You could write a program that uses 21 separate print statements to produce the sheet, but it is more efficient to use nested loop.

24. NESTED LOOP EXAMPLE START SET PARTS = 5 SET QUESTIONS = 3
SET PART_LABEL = “Part “
SET LINE = “. ”
SET partCounter = 1
WHILE partCounter <= PARTS PRINT PART_LABEL, partCounter questionCounter = 1
WHILE questionCounter < = QUESTIONS PRINT questionCounter, LINE
questionCounter = questionCounter + 1 ENDWHILE
partCounter = partCounter + 1 ENDWHILE
END 1. 2. 3.
Part 2 1. 2. 3.
Part 3 1. 2. 3.
Part 4 1. 2. 3.
Part 5 1. 2. 3.
A quiz sheet answer
Pseudocode for AnswerSheet program

25. EXERCISE 4
NESTED LOOP
The number of times a loop executes can depend on a constant or a value that varies.
Suppose you own a factory and have decided to place a label on every product you manufacture. The label contains the words “Made for you personally by” followed by the first name of one of your employees. Assume that for one week’s production, you need 100 personalized labels for each employee.
(Note: the user is prompted for an employee’s name, while the user does not type the QUIT value (“ZZZ”), the program continues).
Try to solve it by your own self…

26. EXERCISE 5
NESTED LOOP
Sometimes, you don’t want to be forced to repeat every pass through a loop the same number of times…
For example, instead of printing 100 labels for each employee, you might want to vary the number of labels based on how many items a worker actually produces. That way, high-achieving workers won’t run out of labels, and less productive workers won’t have too many. Instead of printing the same, constant number of labels for every employee, a more sophisticated program prints a different number of labels for each employee, depending on that employee’s usual production level.
Try to solve it by your own self… 26