1. Chapter 5: Control Structure
Part 2: Repetition/Loop Control Structure

2. Lesson Outcome At the end of this chapter, students should be able to:
Use the control structure in problem solving
Understand the concept of loop control structure
Apply iteration structure on the given problems
Differentiate simple loop and nested loop
Understand the execution of pseudocode uses nested loop control structure

3. Introduction
In some cases, we may want to solve a problem that needs to do the same process for several time.
For this type of problem, it is suitable to use repetition or loop or iteration control structure.
For example, the concept of repetition is implemented when you want to check balance in your account using ATM.

4. Introduction (cont.) The steps to follow are: Insert ATM card
Choose language
Insert pin number
The machine asks you which transaction to perform. Say that you choose a check balance transaction
The machine will display the amount in your account
The machine asks you either you want to continue the next transaction or not. If you choose yes, the machine will repeat the above forth (4) step.

5. Statement for Repetition (Loop Control Structure)
There are two types of repetition that can be implemented:
Counter loop - The number of repetition is identified
Sentinel loop - The number of repetition is unknown
The statement while…endWhile can be used to implement both type of repetition control structure.

6. Statement for Repetition (Loop Control Structure) (cont.)
The pseudocode for while…endWhile statement for loop:
Initial value of a loop
while (condition)
statement(s) to be repeated
statement to update the condition
endWhile

7. Statement for Repetition (Loop Control Structure) (cont.)
Here is the flowchart for loop control structure:

8. Statement for Repetition (Loop Control Structure) (cont.)
When implementing counter loop, the following three statements must be available:
initialize counter with 1
counter ≤ N
increase counter by 1
where N represent the number of repetition.

9. Statement for Repetition (Loop Control Structure) (cont.)
Some examples of problem that we know the number of repetition needed are:
Ask the user to enter 30 numbers. Count how many odd numbers.
Determine the highest mark in a class of 35 students.
Find the average of rain received in particular city in a year.
Calculate the total salary of all staff in finance department. The number of staff will be entered by the user.

10. Example Write flowcharts and pseudocode for the following problems:
a) Display the multiplication table as follows:
1 x 2 = 2
2 x 2 = 4
3 x 2 = 6
: :
10 x 2 = 20

11. Example (cont.)
PSEUDOCODE: Start Set count  1 While (count ≤ 10) result = count x 2 Display count, “ x 2 = ”, result Display newline Add 1 to count endWhile End

12. FLOWCHART:

13. Tracing Algorithm for Counter Loop

14. Exercise 5D
What is the output is n = 4?

15. Exercise 5E (Counter Loop)
Write an algorithm using flowchart and pseudocode for the following problem statement:
A second hand store is having a seven day sale during which the price of any unsold item drops 10% each day. For example, an item that costs RM10.00 on the first day costs 10% less (RM9.00), on the second day. On the third day, the same item is 10% less than RM9.00 (RM8.10). Produce a report that shows the price of the item on each day, 1 through 7.

16. Statement for Repetition (Loop Control Structure) (cont.)
When implementing sentinel loop, the following three statements must be available:
initialize the loop
Condition of the loop
Statement to make the condition false
The three statements will make sure that the loop starts and stops correctly.

17. Statement for Repetition (Loop Control Structure) (cont.)
Some examples of sentinel loop problem:
Ask the user to enter a group of number and end the data entry with zero. Count how many odd numbers in the group.
Determine the highest mark in a class. The process stop when the user enters an invalid mark.
Calculate the total salary of all staff in finance department. The process ends when there is no more data to be process.

18. Statement for Repetition (Loop Control Structure) (cont.)
The three statements will make sure that the loop starts and stops correctly.
For example, if we miss the statement with false condition in the algorithm, we will get an infinite loop.
An infinite loop is a scenario where the loop does not stop.

19. while…endWhile Statement for Sentinel Loop
The syntax is:
All statements in between while…endWhile will be executed when the condition is TRUE. The repetition stop when the condition is FALSE.
Initial value of a loop
while(condition)
statement(s) to be repeated
statement to update the condition
endWhile

20. while…endWhile Statement for Sentinel Loop (cont.)
Example 1: Ask the user to enter a group of numbers. The last number is Display all entered numbers.
Read number
while (number is NOT 999)
Display number
endWhile

21. while…endWhile Statement for Sentinel Loop (cont.)
Example 2: Input marks of students. Display mark which is less than 50. Stop the process when the user enters invalid marks.
Read mark
while (mark ≥ 0.0 AND mark ≤ 100)
If (mark < 50)
Display mark
endIF
endWhile

22. Exercise 5F (Sentinel Loop)
Suppose that the input is What is the output of the following pseudocode?
Start
Read num
While (num is NOT -999)
Display num % 25
Display newline
endWhile
End

23. Exercise 5G (Sentinel Loop)
Suppose that the input is What is the output of the following pseudocode?
Start
Set sum  0
Read num
While (num is NOT 0)
If (num greater than 0)
sum  sum + num
Else
sum  sum – num
endIF
endWhile
Display “Sum = ” sum
End

24. Exercise 5H (Sentinel Loop)
Write an algorithm (pseudocode and flowchart) to input two numbers and display their sum. It asks the user if he/she would like to run the program. If the answer Y or y, it prompts the user to enter two numbers. After adding the numbers and displaying the results, it again asks the user if he/she would like to add more number.

25. Exercise 5I (Sentinel Loop)
One large chemical company pays its salespeople on a commission basis. The salespeople receive RM per week plus 9 percent of their gross sales for that week. Write a pseudocode and draw a flowchart that use loop structure to input each salesperson’s gross sales for last week and calculate and display that salesperson’s earnings.
Enter sales in Ringgit Malaysia (-1 to end):
Salary is: RM650.00
Enter sales in Ringgit Malaysia (-1 to end):
Salary is: RM740.00
Enter sales in Ringgit Malaysia (-1 to end):
Salary is: RM830.00
Enter sales in Ringgit Malaysia (-1 to end): -1
Total number of salesperson: 3
Have a Good Day!!!

26. Basic Algorithm Using Loop Control Structure
The scope of problem for this chapter focuses on how to:
Calculate the total
Calculate the average
Find maximum value and minimum value
Count

27. Basic Algorithm Using Loop Control Structure Example of Problem Statement
Write an algorithm that receives the value of test scores for 30 students. Find and displays the following:
The total of the test scores
The average of the test scores
The highest test score
The lowest test score
Count the number of student that pass the test and the number of student that fail the test. Passing mark is 50.

28. Algorithm To Calculate The Total
General algorithm to calculate the total for a group of numbers:
Start
Initialize variable total with 0
repeat the following actions until end of data
Read number
Add number to variable total
endRepeat
Display total
End

29. Algorithm To Calculate The Average
General algorithm to calculate the average for a group of numbers:
Start
Set total with 0
Set count with 0
repeat the following actions until end of data
Read number
total  total + number
add 1 to count
endRepeat
average  total / count
Display average
End

30. Algorithm Using while…endWhile To Find Total and Average
Calculate the average of 5 numbers
Start
sum  0
Display “Enter 5 numbers: ”
Set counter to 1
While (counter ≤ 5)
Read number
sum  sum + number
add 1 to counter
endWhile
average  sum / 5
Display “The sum of 5 numbers: ”, sum
Display newline
Display “The average of 5 numbers: ”, average
End

31. Algorithm To Find The Maximum and The Minimum
When the limit values are known, such as student marks, we may apply the general algorithm to find the highest mark as given below:
Start
Read the first data
Assign maximum with the first data
repeat until end of data
Read next data
If (next data > maximum)
maximum  next data
endIF
endRepeat
Display maximum
End

32. Algorithm To Find The Maximum and The Minimum
General algorithm to find a minimum value:
Start
Read the first data
Set variable minimum with the first data
repeat the following actions until end of data
Read next data
If (next data < minimum)
minimum  next data
endIF
endRepeat
Display minimum
End

33. Algorithm Using while…endWhile To Find Maximum and Minimum
Find the highest and the lowest mark among 10 marks
Start
read mark
maximum  mark
minimum  mark
set count to 2
while (count ≤ 10)
If (mark > maximum)
endIF
If (mark < minimum)
Add 1 to count
endWhile
Display maximum, minimum
End

34. Algorithm To Count
When we want to count something, the initial value for variable to hold the count is zero. Then we search what we want to count. Every time we found the value that we want, then we increase the value of the count by one. Repeat the process until end of data.
Start
count_even  0
repeat until end of data
read data
If (data is even)
add 1 to count_even
endIF
endRepeat
Display count_even
End

35. Algorithm Using while…endWhile To Count
Calculate how many odd numbers in a group of 20 numbers.
Start
count_odd  0
set count to 1
while (count ≤ 20)
Read number
If (number % 2 ≠ 0)
add 1 to count_odd
endIF
Add 1 to count
endWhile
Display count_odd
End

36. Problem using Counter Loop
Task: given 5 marks, calculate the total of marks which are greater than 50.
Start
sum  0
Set counter to 1
While (counter ≤ 5)
Read mark
If (mark ≥ 50)
sum  sum + mark
endIF
counter = counter + 1
endWhile
Display sum
End

37. Exercise 5J
Write pseudocode and draw a flowchart for the following problems:
Ask the user to input the volume of monthly rain received in a particular city in a year and displays:
The total volume of rain in that city
The average rain received
The highest rain received
The lowest rain received

38. Exercise 5K
Write pseudocode and draw a flowchart for the following problems:
Ask the user to enter N numbers. Find:
How many odd numbers
How many negative numbers
Total of number which is divisible by 10

39. Exercise 5L
Write pseudocode and draw a flowchart for the following problems:
Write an algorithm that receives the name and the salary of ABC company staff. Then calculates and displays the:
Name of the staff with the highest salary
Name of the staff with the lowest salary
Average salary of all staff
Total number of staff with salary below RM2000
Use a sentinel value to exit your loop.

40. Infinite Loop
An infinite loop is a non stop loop, where the repetition keeps running and never stop.
Example:
Start
count  0
number  7
while(count ≤ 100)
Display number
Read number
endWhile
Display count
End

41. Nested Loop
Nested loop refers to a situation where there is another loop in a loop.
In this situation, we have an inner loop and outer loop.
The execution of this type of structure is, the computer will finish the inner loop for every iteration of outer loop.

42. Nested Loop (cont.)
Format for nested loop:

43. Nested Loop (cont.)
The flowchart for a nested loop is as follows:

44. Execution of Nested Loop

45. Execution of Nested Loop (cont.)
Value changes for variables out- counter and inner-counter
Expected screen:

46. Example
Given the following pseudocode, trace and provide the output:

47. Any Question