1. Chapter 2
Control Structures

2. Control Structures
Solving a problem using the Stepwise refinement method, the instructions that make up the algorithm and the structured chart are combinations of 3 control structures:
Sequence Structure
Selective Structure
Iterative Structure

3. Sequence Structure The most commonly used and simple structure
Consists of two or more sub components
Instructions are executed/processed one after another in a sequence, in a top to bottom manner
The structure is read from left to right
Normally used in conjunction with one or more of the other control structures; ie, selection and/or iteration

4. Sequence Structure
The instructions are represented in the subcomponents of the structured diagram , i.e., elementary components
An elementary component is one which has no lower-level components associated and is represented in a data structure diagram simply by placing its name inside a rectangle

5. Sequence Structure
The sequence control structure can be used to represent 4 basic computer operations;
to receive information e.g. Read customername, address
put out information e.g. Display pageCount
perform arithmetic e.g. add 1 to pageCount
assign values e.g. set pageCount to zero

6. Sequence Structure: GeneralB C
The component (box) A is a sequence. Boxes B and C (Elementary components) represent component parts of the sequence (sub-components)
A is a sequence of B followed by C.

7. Sequence structure: Pseudocode
use a separate line for each logical element.
align each statement vertically until a change in logic flow occurs
Sequence block is preceded by BEGIN, terminated by END

8. Sequence Structure: Pseudocode
General format:
BEGIN SEQUENCE_BLOCK_NAME sequence
sequence statement; …
END SEQUENCE_BLOCK_NAME sequence

9. Sequence Structure: Example 1
Name
Read Title
Read FirstName
Read Surname
Print full name
Sub-components of a Sequence Structure - Name

10. Sequence Structure: Pseudocode Example 1
BEGIN NAME sequence
Read
Title
FirstName
Surname
Name
Print
full name
READ title;
READ firstname;
READ surname;
WRITE title + firstname
+surname;
END NAME sequence

11. Sequence Structure: Example 2
Problem Specification:
Design a program to process an order. A process consists of accepting the quantity sold and price of an item and then display the sales value

12. Problem Analysis Initialization Input Definition Output Definition
none
Input Definition
qty, price
Output Definition
sales : $

13. Problem Analysis Processing Requirements Read qty, price
Calculate sales
sales = qty * price
Display sales
Processing Control
sales is displayed in 2 decimal places

14. Problem Analysis Test Data & Expected Result qty price sales 12 10.00
120.00 5
100.00
500.00 10
2.00
20.00

15. Sequence Structure: Example 2
Order
Display sales
sales =
qty * price
Read qty, price

16. Sequence Structure: Example 2
Pseudocode:
BEGIN ORDER sequence
END ORDER sequence
Order
Display sales
sales =
qty * price
Read qty,price
READ qty, price;
sales = qty * price;
WRITE sales;

17. Sequence Structure: Example 3
Problem Specification: Calculate the amount due on an electricity bill. The present meter reading and the previous meter readings are entered. The amount due is the difference between the two readings multiplied by the cost of a unit of electricity, currently 5 cents per unit.

18. Sequence Structure: Your task!
Do the PROBLEM ANALYSIS
Draw the STRUCTURED CHART
Write the PSEUDOCODE

19. Problem Analysis Initialization Input Definition Output Definition
Cost = 0.05
Input Definition
current, previous
Output Definition
Amount due : $

20. Problem Analysis Processing Requirements Read current, previous
Calculate bill
2.1. used = current – previous
2.2. bill = used * cost
Display bill
Processing Control
current value must be >= previous value
bill is displayed in 2 decimal places

21. Problem Analysis Test Data & Expected Result current previous used
bill
1200
680
520
26.00
5000
1328
3672
183.60
10000
13400
error

22. Sequence Structure: Example 3
Electricity Bill
Cost = 0.05
Read meters
Calculate bill
Display bill
Read current
Read previous
used = current – previous
bill = used*cost

23. Sequence Structure: Example 3
BEGIN ELECTRICITYBILL sequence
cost = 0.05;
BEGIN READMETERS sequence
READ current;
READ previous;
END READMETERS sequence
BEGIN CALCULATEBILL sequence
used = current – previous;
bill = used * cost;
END CALCULATEBILL sequence
WRITE bill;
END ELECTRICITYBILL sequence
Electricity Bill
Calculate bill
Display bill
cost =
0.05
Read
meters
Read
current
Read previous
used = current – previous
bill = used*cost

24. Sequence Structure: Example 4
A ONE DOLLAR shop wants you to write a program to read in the sales price in cents which is not to exceed one dollar. The change from one dollar is then to be calculated and printed as the most compact set of change (i.e. the fewest coins). It is assumed that the denominations are 50¢, 20¢, 10¢, 5¢ and 1¢

25. Sequence Structure: Example 4
Initialization
none
Input Definition
sales_price
Output Definition
Denomination:
50¢ - 9
20¢ - 9
10¢ - 9
5¢ - 9
1¢ - 9

26. Problem Analysis Processing Requirements Read sales_price
Calculate change from $1
change = 100 – sales_price
Calculate set of change
coin50 = change / 50
change = change – (coin50 * 50)
coin20 = change / 20
change = change – (coin20 * 20)
coin10 = change / 10
change = change – (coin10 * 10)
coin5 = change / 5
change = change – (coin5 * 5)
coin1 = change
Print coin50, coin20, coin10, coin5, coin1

27. Problem Analysis Processing Control Test Data & Expected Result
sales_price must be between 1 and 100 cents inclusive
Test Data & Expected Result
sales_price
change
coin50
coin20
coin10
coin5
coin1 40 60 1 14 86
120
error

28. Sequence Structure: Example 4
Change
Read sales_price
change = 100 – sales_price
Set of change
Display change A B

29. Sequence Structure: Example 4
coin50 = change/50
coin20 = change/20
coin10 = change/10
coin1 = change
coin5 = change/5
change=
change – (coin50*50)
change= change – (coin20* 20)
change= change – (coin10*10)
change=
change – (coin5*5)

30. Sequence Structure: Example 4B
coin50
coin20
coin10
coin5
coin1

31. Sequence Structure: Example 4
coin5 = change / 5;
change = change - (coin5*5);
coin1 = change;
END SETOFCHANGE sequence
BEGIN DISPLAYCHANGE sequence
WRITE coin50;
WRITE coin20;
WRITE coin10;
WRITE coin5;
WRITE coin1;
END DISPLAYCHANGE sequence
END CHANGE sequence
Pseudocode
BEGIN CHANGE sequence
READ sales_price;
change = 100 – sales_price;
BEGIN SETOFCHANGE sequence
coin50 = change/50;
change = change - (coin50*50);
coin20 = change / 20;
change = change - (coin20*20);
coin10 = change / 10;
change = change-(coin10*10);

32. Class Exercise
Using stepwise Refinement method (sequence control structure) analyse the following problem, draw the structured chart and write the pseudocode:
A major department store needs a program to prepare a bill for each customer. Lets assume that each customer purchases only one item type. There will be 4 input values required; customer’s name, itemID, quantity purchased and unit price. Output will be the customer’s bill after a 10% discount is taken before taxes and a 5 percent sales tax is added.