1. CHAPTER 4
Iterative Structure

2. Content Introduction 3 types of iterative structure FOR-ENDFOR
WHILEDO-ENDWHILE
DOWHILE-ENDWHILE
Pseudocode for iterative structure
Q&A

3. Introduction aka repetition/loop structure
Same logic steps (a set of instructions) are repeated for several unique sets of data.
Process in the loop is repeated a number of times while the condition result is true
When the condition result is false, the loop exited

4. Introduction
A completion of the process must be identified which will bring the loop into a stop, otherwise the program may have an infinite loop.
For e.g.
end of file for input from file,
for interactive program, the user might indicate that the input is complete by typing an end value from the keyboard such as an ‘N’ at the “Do you wish to continue?” prompt

5. Introduction
Consider a program to read and calculate the age of all students. The age is calculated by having the current date minus the student’s birth date. These instructions are only written once and can be repeated over and over again for each student using a loop.

6. Introduction
A simple example would be :
If you want to print “Hello World” 100 times, you will not use 100 print lines, i.e.,
print “Hello World” /* line 1
print “Hello World” …
print “Hello World” /* line 100
You would only use loop and only having a single print line, i.e.,
FOR counter = 1 to 100
Print “Hello World”
END FOR

7. Repetition Structure Processes A Process
Structure chart is represented by an asterisk at the top-right corner of a rectangular box.
Processes
An asterisk to
denote
repetitive
component *
A Process

8. Types of Iterative Structure
There are 3 types of iterative structure
FOR-ENDFOR
WHILEDO-ENDWHILE
DOWHILE-ENDWHILE

9. FOR structure
Used when the exact number of loop iteration is known in advance.
Requires lowest and highest value of a range - initial_value and final_value e.g. 1 and 10 for execution of process for 10 times.
Can specify the increment value as well
Test the value of loop_index (loop-control variable) at the beginning of each loop to ensure that it is within the specified range of values

10. FOR structure General structure format
Condition is placed on top right side of the iterative component
PROCESSES
FOR loop_index = i to n [step m]
A PROCESS *
Initial value
Final value
Number of step

11. FOR structure : Example
Everyday, a weather station receives 15 temperatures expressed in degrees Fahrenheit. A program is to be written which will accept each Fahrenheit temperature, convert it to Celsius and display both temperatures on the screen. After 15 temperatures have been processed, the word “all temperatures processed” are to be displayed on the screen

12. Solution Algorithm: Example
Loop is terminated once
the counter exceeds 15
initializes the counter to 1
FOR counter = 1 to 15
Read fahrenheit
Compute
celsius = (fahrenheit – 32) *
Display fahrenheit, celsius
Display “All temperatures processed”
These
statements
will be
repeated
15 times

13. Solution Algorithm: Example
The process of FOR loop:
It initializes the loop_index (in this case, is the counter) to 1
Execute the statements in the loop
Increments the loop_index by 1 as it passes through the loop
Tests the loop_index at the beginning of each pass so that it is within the range 1 to 15
Loop is terminated once the loop_index exceeds 15.
Then the statement after the loop (i.e. statement #2) will be executed
Algorithm:
FOR counter = 1 to 15
Read fahrenheit
Compute
celsius =
(fahrenheit –
32) *
Display fahrenheit, celsius
Display “All temperatures processed”

14. FOR : Structure Chart counter is initialized to 1
and keeps on incremented
once all the steps
are executed
TEMPERATURE
CONVERSION
PROCESS
TEMPERATURES
DISPLAY
“All temperatures
processed”
FOR counter = 1 to 15
Steps are repeated
15 times
PROCESS A
TEMPERATURE *
READ
FAHRENHEIT
CELSIUS=
(FAHRENHEIT – 32) *
DISPLAY
FAHRENHEIT,
CELSIUS

15. FOR : Pseudocode
BEGIN TEMPERATURECONVERSION sequence BEGIN PROCESSTEMPERATURES iteration FOR counter = 1 to 15 BEGIN PROCESSATEMPERATURE sequence READ fahrenheit; celsius = (fahrenheit – 32) * 0.555; WRITE fahrenheit, celsius; END PROCESSATEMPERATURE sequence ENDFOR END PROCESSTEMPERATURES iteration WRITE “All temperatures processed”; END TEMPERATURECONVERSION sequence

16. For Structure : example 2
Design an algorithm to accept several numbers from the keyboard. The number of input integers is indicated by the first number entered, e.g. if the first number entered is 5, then 5 input integers will follow. Your algorithm should be able to decide and display the even integers which have been entered (excluding the first integer). If there are no even number entered, an appropriate message should be displayed instead.
Task :
Do the problem analysis
Draw the structured chart
Create the Pseudocode

17. Problem Analysis evenFound = 0 numOfInteger, integerValue
Initialization
evenFound = 0
Input Definition
numOfInteger, integerValue
Output Definition
EvenValues =

18. Problem Analysis Processing Requirements Processing Control
MOD is use to find the remainder of a division
Processing Requirements
READ numOfInteger
FOR counter = 1 to numOfInteger
2.1. READ integerValue
2.2. Check Even Number
IF number MOD 2 = 0 THEN
evenValues = evenValues + integerValue
evenFound = 1
Check Even Found
IF evenFound = 1 THEN
DISPLAY evenValues
ELSE
DISPLAY “No even number found in the input data”
Processing Control
numOfInteger and integerValue must be integer

19. Problem Analysis Test Data & Expected Result numOfInteger integerValue
Output 5
10, 20, 15, 12, 3 3
13, 9, 5
No even number found in the input data
3.6
error
Error

20. Structured Chart o o o evenFound = 1 else integerValue MOD 2 = 0
PROBLEM
evenFound
= 0
READ
numOfInteger
PROCESS
INTEGERS
DISPLAY Message
evenFound = 1
else
FOR counter =
1 to numOfInteger o o
PROCESS AN
INTEGER *
DISPLAY evenValues
DISPLAY “No even
number found
in the input data”
READ
integerValue
Check Even Number
integerValue MOD 2 = 0 o
EVEN NUMBER
evenValues =
evenValues +
integerValue
evenFound=1

21. Pseudocode
BEGIN INTEGERPROBLEM sequence evenFound = 0; READ numOfInteger; BEGIN PROCESSINTEGERS iteration FOR counter = 1 to numOfInteger BEGIN PROCESSANINTEGER sequence READ integerValue; BEGIN CHECKEVENNUMBER selection IF integerValue MOD 2 = 0 THEN BEGIN EVENNUMBER sequence evenValues = evenValues + integerValue; evenFound = 1; END EVENTNUMBER sequence END IF END CHECKEVENNUMBER selection END PROCESSANINTEGER sequence ENDFOR END PROCESSINTEGERS iteration BEGIN DISPLAY MESSAGE selection IF evenFound = 1 THEN WRITE evenValues; ELSE WRITE “No even number found in the input data”; END DISPLAYMESSAGE selection END INTEGERPROBLEM sequence

22. WHILEDO structure
Used when we do not know exactly how many times to carry out the a process
Condition is tested first before the block of statements are executed
will continue to repeat a group of statements while a condition remains true.
When the condition becomes false, the loop is exited
Block of statements are executed 0 or many times

23. WHILEDO Structure
start off by first testing the condition at the beginning of the loop
a variable will need to be initialised prior to this test. In many programming languages, if you don’t provide an initial value for a variable, the variable value is unknown or garbage.
Since the loop will only terminates when the WHILE condition is false, some process must be set up within the statement block which will EVENTUALLY CHANGE the condition to becomes false.
Failure to do so results in an endless loop

24. WHILEDO structure General structure format Condition expression
PROCESSES
WHILE continue = true
A PROCESS *

25. WHILEDO structure : Example
Everyday, a weather station receives a number of temperatures expressed in degrees Fahrenheit. A program is to be written which will accept each Fahrenheit temperature, convert it to celsius and display both temperatures on the screen. After all temperatures have been processed, the word “all temperatures processed” are to be displayed on the screen

26. Solution Algorithm: Example
continue = ‘Y’
WHILE continue = ‘Y’
READ fahrenheit
CALCULATE Celcius
Celsius = (fahrenheit – 32) *
DISPLAY fahrenheit, celsius
READ continue
Display “All temperatures processed”
A variable named ‘continue’
must be initialized
These
statements
will be
repeated
until continue
is not = Y
This statement will be executed
after continue value is not Y

27. WHILEDO : Structure Chart
TEMPERATURE
CONVERSION
continue =‘Y’
PROCESS
TEMPERATURES
DISPLAY
“All temperatures
processed”
WHILE continue = ‘Y’
Repeated
steps
PROCESS A
TEMPERATURE *
READ
fahrenheit
celsius=
(fahrenheit – 32) *
DISPLAY
fahrenheit,
celsius
READ
continue

28. WHILEDO : pseudocode
BEGIN TEMPERATURECONVERSION sequence continue = ‘Y’; BEGIN PROCESSTEMPERATURES iteration WHILE continue = ‘Y’ BEGIN PROCESSATEMPERATURE sequence READ fahrenheit; celsius = (fahrenheit – 32) * 0.555; WRITE fahrenheit, celsius; READ continue ; END PROCESSATEMPERATURE sequence ENDWHILE END PROCESSTEMPERATURES iteration WRITE “All temperatures processed”; END TEMPERATURECONVERSION sequence

29. DOWHILE structure Same as WHILEDO structure BUT
Condition is tested after the block of statements are executed once.
Block of statements are executed 1 or many times.

30. DOWHILE structure General structure format Condition expression
PROCESSES
DOWHILE continue = true
A PROCESS *

31. DOWHILE structure : Example
Everyday, a weather station receives a number of temperatures expressed in degrees Fahrenheit. A program is to be written which will accept each Fahrenheit temperature, convert it to celsius and display both temperatures on the screen. After all temperatures have been processed, the word “all temperatures processed” are to be displayed on the screen

32. Solution Algorithm: Example
These
statements
will be
executed
and then
repeated
until continue
is not = Y DO
READ fahrenheit
CALCULATE
Celsius = (fahrenheit – 32) *
DISPLAY fahrenheit, celsius
READ continue
WHILE continue= ‘Y’
DISPLAY “All temperatures processed”

33. DOWHILE : structured chart
TEMPERATURE
CONVERSION
PROCESS
TEMPERATURES
DISPLAY
“All temperatures
processed”
DOWHILE continue = ‘Y’
PROCESS A
TEMPERATURE *
READ
fahrenheit
celsius=
(fahrenheit – 32) *
DISPLAY
fahrenheit,
celsius
READ reply

34. DOWHILE : Pseudocode
BEGIN TEMPERATURE_CONVERSION sequence BEGIN PROCESS_TEMPERATURES iteration DO BEGIN PROCESS_A_TEMPERATURE sequence READ fahrenheit; celsius = (fahrenheit – 32) * 0.555; WRITE fahrenheit, celsius; READ continue; END PROCESS_A_TEMPERATURE sequence WHILE continue = ‘Y’; END PROCESSTEMPERATURES iteration WRITE “All temperatures processed”; END TEMPERATURE_CONVERSION sequence

35. Class Exercise 1 Class Exercise 2
Write the algorithm of a program which will compute and display sum of all odd numbers from 1 to ,3,5,7,9…….99.
Class Exercise 2
A manager of a cattle ranch is looking for the great sheep. The first input indicates how many ‘weight of sheep’ records to follow. Records , each containing the sheep number, age in years and weight in kilogram, follow the first input record. All weight must be validated so that the value is more than zero. The program should be able to find the average weight, the detail of the lightest and the heaviest sheep. Write down the algorithm.

36. Solution for Exercise 2 totalweight = 0, lightest=1000, heaviest=0
READ noofsheep
FOR count = 1 to noofsheep
3.1 Read sheepno, age
3.2 DO
3.2.1 Read weight
3.2.2 IF weight <= 0 THEN
Display “Invalid weight. Reenter weight”
WHILE weight <= 0
3.3 totalweight = totalweight + weight
3.4 If weight < lightest
l_sheepno = sheepno, l_age=age, lightest = weight
3.5 If weight > heaviest
h_sheepno = sheepno, h_age = age, heaviest =weight
avgweight = totalweight/noofsheep
Display avgweight
Display l_sheepno, l_age, lightest
Display h_sheepno, h_age, heaviest