1. CHAPTER 3
SELECTION STRUCTURE

2. Introduction Also known as decision structure.
Selective structure allows to choose one of the many possibilities/options, depending on whether a condition is true or false.
The condition in the selection is based on the comparison of two items, and is usually expressed with one of the relational operators.

3. E.g. If JobVacant = “yes” AND QualificationMet = “yes” then
action= “accepted”,
else
action = “rejected”.
Relational operators
= Equal To
< Less Than
<= Less Than Equal To
> Greater Than
>= Greater Than Equal To
<> Not Equal To
RESULT
Condition A 1
Condition B
AND OR
NAND
NOR
XOR

4. How to construct in structured chart
Structure: a named box with small circle at the top-right corner
Selection Action o

5. Selective Structure
Selection can represents a single-alternative, dual-alternatives or more alternatives
These number of variations represented using the following structures:
IF-THEN
IF-THEN-ELSE
Combined IFs
Multiple IFs
Nested Ifs
CASE

6. IF-THEN
Used where a task is carried out only when a particular condition is true.
if the condition is false, no processing takes place – Condition will be bypassed.
For Example:
In a situation where a person who is 12 years old or older must purchase a ticket of $1 to enter the playground and children below 12 can go for free.
IF age >= 12 THEN
ticket = 1.00

7. IF-THEN : Structured Chart
Block X
Process S o
condition
IF-THEN : Pseudocode
BEGIN BLOCK-DESCRIPION Selection
IF condition THEN
Statement Process S will be executed;
ENDIF;
END BLOCK-DESCRIPTION Selection

8. IF-THEN : Example Algorithm: 1……
Problem : A discount is given if the value is
greater or equals bulk order level
Algorithm:
1……
2. If value >= bulk_order_level Then
discount = value * dis_rate/100
new_value = value - discount

9. IF-THEN : Example Process Discount value >= bulk_order_level Deduct
* dis_rate/100
new_value=
value-discount

10. IF-THEN : Example BEGIN DISCOUNT selection
IF value >= bulk_order_level THEN
BEGIN DEDUCTDISCOUNT sequence
discount = value * dis_rate/100
new_value = value – discount
END DEDUCTDISCOUNT sequence
END IF
END DISCOUNT selection

11. IF-THEN-ELSE
Used when a choice is to be made between 2 alternative paths depending on the result of the condition being true or false.
General structure format :
Block X
Process R o
Process S
condition
else

12. IF-THEN-ELSE : Pseudocode
BEGIN BLOCK-DESCRIPION Selection
IF condition THEN
Statement – Process R will be executed ;
ELSE
Statement – Process S will be executed;
ENDIF
END BLOCK-DESCRIPTION Selection
INDENTED

13. IF-THEN-ELSE : Example
Problem : counting a number of male and a number of female in a group of students
Algorithm:
1. malecount = 0, femalecount = 0
2. ………..
3. IF gender = ‘M’ Then
malecount = malecount + 1
ELSE
femalecount = femalecount + 1

14. IF-THEN-ELSE : Example
BEGIN GENDER Selection
END GENDER Selection
Check Gender
malecount =
malecount + 1 o
femalecount =
femalecount + 1
Gender = ‘M’
else
IF gender = ‘M’ THEN
malecount = malecount +1;
ELSE
femalecount = femalecount+1;
ENDIF

15. Combined IFs Used when there are two or more conditions to be tested
Conditions are connected with logical operators – AND, OR
AND / OR table
Condition1
Condition2
AND OR T
Execute true actions F
Execute false actions

16. Combined IFs General structure format : Block X Condition1 [AND][OR]
else o o
Process R
Process S

17. Combined IFs : Pseudocode
BEGIN BLOCK-DESCRIPION Selection
IF condition 1 [AND] [OR] condition 2 THEN
Statement – Process R will be executed;
[ELSE]
[Statement – Process S will be executed;]
ENDIF
END BLOCK-DESCRIPTION Selection

18. Combined IFs : Example 1
The application will be processed only if the applicant is above 21 years old and he/she is in category ‘A’.
Algorithm:
Structured chart: Pseudocode:
1…….
2. IF age > 21 AND category = ‘A’ Then
Do processApplication()
Check Applicant
Process Application o
age > 21 and
category = ‘A’
BEGIN CHECKAPPLICANT selection
IF age > 21 AND category = ‘A’ THEN
DO processapplication()
ENDIF
END CHECKAPPLICANT selection

19. Combined IFs : Example 2
Discount will be given for sales price between $1 and $100
Discount
discAmount =
sales_price * disc o
sales_price >= 1 and
sales_price <=100
Pseudocode is similar to the IF-THEN and IF-THEN-ELSE

20. Multiple IFs Using two or more IF-THEN statement
All conditions will be tested even though the first IF-THEN condition is true.
General structure format :
Block X
condition1
Block Q
Process Q o
condition n
Block R
Block S
condition2 ….
Process R o
Process S

21. Multiple Ifs : Pseudocode
BEGIN BLOCK-DESCRIPION sequence
BEGIN BLOCK_Q selection
IF condition1 THEN
Statement - Process Q will be executed ;
ENDIF
END BLOCK_Q selection
BEGIN BLOCK-R selection
IF condition2 THEN
Statement - Process R will be executed;
ENDIF;
END BLOCK_R selection …
BEGIN BLOCK-S selection
IF condition n THEN
Statement - Process S will be executed;
END BLOCK-S selection
END BLOCK-DESCRIPTION sequence

22. Multiple IF : Example
Displaying Subjects taken by a student. Check each of the subject, whether the mark is -1, if yes, meaning they are not taking that subject. Else, it would show the subject name in the message.

23. Multiple IF : Example Algorithm
…..
If MathMark <> -1 Then
Message = Message + “ Math”
If GeometryMark <> -1 Then
Message = Message + “ Geometry”
If EnglishMark <> -1 Then
Message = Message + “ English”
If BiologyMark <> -1Then
Message = Message + “ Biology”

24. Multiple IFs : Example Subject Math Geometry English Biology Message =
MathMark
<>-1
Message =
Message +
“ Math” o
GeometryMark
<>-1
Message =
Message +
“ Geometry” o
EnglishMark
<>-1
Message =
Message +
“ English” o
BiologyMark
<>-1
Message =
Message +
“ Biology” o

25. Multiple IF : Example BEGIN ENGLISH selection
IF EnglishMark <> -1 THEN
Message = Message +
“ English”;
ENDIF
END ENGLISH selection
BEGIN BIOLOGY selection
IF BiologyMark <> -1 THEN
“ Biology”;
END BIOLOGY selection
END SUBJECT sequence
BEGIN SUBJECT sequence BEGIN MATH selection IF MathMark <> -1 THEN Message = Message + “ Math”; ENDIF END MATH selection BEGIN GEOMETRY selection IF GeometryMark <> -1 THEN “Geometry”; END GEOMETRY selection

26. Nested IF IF-THEN-ELSE within another IF-THEN_ELSE structure
General structure format :
Block X
Process Q o
Block_R
Condition1
else
Process R1 o
Process R2
Condition2
else

27. Pseudocode – nested IF BEGIN BLOCK-DESCRIPION selection
IF condition1 THEN
Statements - Process Q will be executed;
ELSE
BEGIN BLOCK_R selection
IF condition2 THEN
Statements - Process R1 will be executed;
Statements - Process R2 will be executed;
ENDIF;
END BLOCK_R selection
END BLOCK-DESCRIPTION Selection

28. Nested IF : Example
Consider a grading problem below. If the student mark is
80 and above, grade is distinction.
less than 80 but more than or equal to 65, grade is Merit
Less than 65 but more than or equal to 50, grade is Pass
Below 50, grade is Fail

29. BEGIN GRADE selection IF mark>=80 THEN grade = “Distinction”; ELSE BEGIN M_TEST selection IF mark>=65 THEN grade = “Merit”; BEGIN P_TEST selection IF mark>=50 THEN grade = “Pass”;
Nested IF : Example
Grade
mark>=80
else o
grade =
‘Distinction’ o
M_Test
mark>=65
ELSE grade = “Fail”; ENDIF END P_TEST selection END M_TEST selection END GRADE Selection
else o o
grade =
‘Merit’
P_Test
mark>=50
else o o
grade =
‘Pass’
grade =
‘Fail’

30. CASE STRUCTURE Another way to express nested IF
If one condition is true, the following conditions will not be tested
General structure format :
Block X
Process R o
Process S
condition1
Process Q ….
else
Condition n

31. Pseudocode – CASE BEGIN BLOCK_DESCRIPTION selection SELECT expression
CASE condition1
Statements - Process Q will be executed;
CASE conditionn
Statements - Process R will be executed; ELSE
Statements - Process S will be executed; ENDCASE
END BLOCK_DESCRIPTION selection

32. CASE : Example commission is determined by the product code
Product code ‘A’, 10% on sales
Product code ‘B’, 15% on sales
Product code ‘C’, 20% on sales
others, 5% on sales

33. CASE : Example Algorithm..
CASE product_code = ‘A’:
comm = 10/100 * sales
CASE product_code = ‘B’:
comm = 15/100 * sales
CASE product_code = ‘C’:
comm = 20/100 * sales
ELSE
comm = 5/100 * sales

34. CASE : Example Commission …… Check code productCode = ‘A’ else =‘B’
10/100 *sales o
comm =
15/100 *sales o
comm =
20/100*sales o
comm =
5/100 * sales o

35. CASE : Example
……. BEGIN CHECKCODE selection SELECT productCode CASE ‘A’: comm = 10/100 * sales; CASE ‘B’: comm = 15/100 * sales; CASE ‘C’: comm = 20/100 * sales; ELSE comm = 5/100 * sales; ENDCASE END COMMISSION selection

36. Class Exercise 1
A program is required to accept an object’s length and width and then decide whether it is a square or a rectangle. Do the algorithm and structured chart.

37. Class Exercise 2
The Fantastic Floral Company sells to wholesale and retail buyers. The wholesale buyer needs a resale number in order to buy at no tax and receive discounts. The retail buyer pays 6% tax. The following are the discounts to the wholesale buyer:
Amount Discount
less $ %
$100 – less $500 5%
$500 or more 10%
Given an amount of purchase and buyer type, how much will the customer owe the company? Using stepwise refinement method, do problem analysis, structured chart and pseudocode

38. Class Exercise 3
A transaction record on a sales commission file contains the retail price of an item sold, a transaction code that indicates the sales commission category to which an item can belong, and the employee number of the person who sold the item. The transaction code can contain the values S, M, L, which indicate that the percentage commission will be 5%, 7% or 10%, respectively. Using stepwise refinement method, do problem analysis, structured chart and pseudocode that will read a record on the file, calculate the commission owing for that record, and print the retail price, commission and employee number.