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1
CHAPTER 3
SELECTION STRUCTURE
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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.
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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 0 0 1 1
Condition B 0 1 0 1
AND 0 0 0 1
OR 0 1 1 1
NAND 1 1 1 0
NOR 1 0 0 0
XOR 0 1 1 0
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How to construct in structured chart
Structure: a named box with small circle at the top-right corner
Selection Actiono
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Selective Structure
Selection can represents a single-alternative, dual-alternatives or more alternatives
These number of variations represented using the following structures:1. IF-THEN2. IF-THEN-ELSE3. Combined IFs4. Multiple IFs5. Nested Ifs6. CASE
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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
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IF-THEN : Structured Chart
Block X
Process So
condition
BEGIN BLOCK-DESCRIPION SelectionIF condition THEN
Statement Process S will be executed; ENDIF;
END BLOCK-DESCRIPTION Selection
IF-THEN : Pseudocode
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IF-THEN : Example
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/100new_value = value - discount
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IF-THEN : Example
ProcessDiscount
DeductDiscount
new_value=value-discount
discount= value * dis_rate/100
value >= bulk_order_level
o
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IF-THEN : ExampleBEGIN DISCOUNT selection
IF value >= bulk_order_level THENBEGIN DEDUCTDISCOUNT sequence
discount = value * dis_rate/100
new_value = value – discountEND DEDUCTDISCOUNT sequence
END IFEND DISCOUNT selection
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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 Ro
Process So
condition else
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IF-THEN-ELSE : Pseudocode
Pseudocode :BEGIN BLOCK-DESCRIPION Selection
IF condition THENStatement – Process R will be executed ;
ELSEStatement – Process S will be executed;
ENDIFEND BLOCK-DESCRIPTION Selection
IND
EN
TE
D
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IF-THEN-ELSE : Example
Problem : counting a number of male and a number of female in a group of studentsAlgorithm:1. malecount = 0, femalecount = 02. ………..3. IF gender = ‘M’ Then
malecount = malecount + 1 ELSE
femalecount = femalecount + 1
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Check Gender
malecount = malecount + 1
ofemalecount = femalecount + 1
o
Gender = ‘M’else
IF-THEN-ELSE : Example
BEGIN GENDER Selection
END GENDER Selection
IF gender = ‘M’ THENmalecount = malecount +1;
ELSEfemalecount = femalecount+1;
ENDIF
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Combined IFs
Condition1 Condition2 AND OR
T T Execute true actions Execute true actions
T F Execute false actions Execute true actions
F T Execute false actions Execute true actions
F F Execute false actions Execute false actions
Used when there are two or more conditions to be tested
Conditions are connected with logical operators – AND, OR
AND / OR table
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Combined IFs General structure format :
Block X
Process Ro
Process So
Condition1[AND][OR]condition2 else
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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
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Combined IFs : Example 1
Check Applicant
Process Applicationo
age > 21 and category = ‘A’
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:
BEGIN CHECKAPPLICANT selectionIF age > 21 AND category = ‘A’ THEN
DO processapplication()ENDIF
END CHECKAPPLICANT selection
1…….2. IF age > 21 AND category = ‘A’ Then
Do processApplication()
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Combined IFs : Example 2
1. 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
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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 Qo
condition n
Block R Block S
condition2
….
Process Ro
Process So
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Multiple Ifs : Pseudocode
BEGIN BLOCK-DESCRIPION sequenceBEGIN BLOCK_Q selection
IF condition1 THENStatement - Process Q will be executed ;
ENDIFEND BLOCK_Q selectionBEGIN 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;
ENDIF;END BLOCK-S selection
END BLOCK-DESCRIPTION sequence
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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.
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Multiple IF : Example Algorithm1. …..2. If MathMark <> -1 Then
Message = Message + “ Math”
3. If GeometryMark <> -1 Then Message = Message + “ Geometry”
4. If EnglishMark <> -1 Then Message = Message + “ English”
5. If BiologyMark <> -1Then Message = Message + “ Biology”
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Multiple IFs : Example
Subject
GeometryMath
MathMark<>-1
Message = Message +
“ Math”
o
GeometryMark<>-1
Message = Message +
“ Geometry”
o
English
EnglishMark<>-1
Message = Message + “ English”
o
Biology
BiologyMark<>-1
Message = Message + “ Biology”
o
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Multiple IF : Example
BEGIN SUBJECT sequence BEGIN MATH selection
IF MathMark <> -1 THENMessage = Message + “ Math”;
ENDIF END MATH selection BEGIN GEOMETRY selection
IF GeometryMark <> -1 THEN
Message = Message + “Geometry”;
ENDIF END GEOMETRY selection
BEGIN ENGLISH selection IF EnglishMark <> -1 THEN
Message = Message + “ English”;
ENDIF END ENGLISH selection BEGIN BIOLOGY selection
IF BiologyMark <> -1 THEN
Message = Message + “ Biology”;
ENDIF END BIOLOGY selection END SUBJECT sequence
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Nested IF IF-THEN-ELSE within another IF-THEN_ELSE
structure General structure format :
Block X
Process Qo
Block_Ro
Condition1 else
Process R1o
Process R2o
Condition2 else
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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; ELSE
Statements - Process R2 will be executed; ENDIF;END BLOCK_R selection
ENDIF;END BLOCK-DESCRIPTION Selection
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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
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Nested IF : Example
BEGIN GRADE selection IF mark>=80 THEN
grade = “Distinction”; ELSE
BEGIN M_TEST selection IF mark>=65 THEN
grade = “Merit”; ELSE
BEGIN P_TEST selection
IF mark>=50 THEN
grade = “Pass”;
ELSE grade =
“Fail”;ENDIF
END P_TEST selection
ENDIF END M_TEST selectionENDIF
END GRADE Selection
Grade
grade = ‘Distinction’
o
mark>=80 else
mark>=65 else
grade = ‘Pass’
ograde =
‘Fail’
omark>=50 else
M_Test
grade = ‘Merit’
P_Test oo
o
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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 Ro
Process S
condition1
Process Qo
….
o
elseCondition n
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Pseudocode – CASE
BEGIN BLOCK_DESCRIPTION selectionSELECT expressionCASE condition1
Statements - Process Q will be executed;CASE conditionn
Statements - Process R will be executed; ELSE
Statements - Process S will be executed; ENDCASEEND BLOCK_DESCRIPTION selection
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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
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CASE : Example Algorithm
1. ..2. 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
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CASE : Example
comm = 15/100 *sales
o
productCode = ‘A’
comm = 10/100 *sales
oelse=‘B’
comm =20/100*sales
o comm =5/100 * sales
o=‘C’
…… Check code
Commission
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CASE : Example…….BEGIN CHECKCODE selection
SELECT productCodeCASE ‘A’:
comm = 10/100 * sales;CASE ‘B’:
comm = 15/100 * sales; CASE ‘C’:
comm = 20/100 * sales; ELSE
comm = 5/100 * sales; ENDCASEEND COMMISSION selection
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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.
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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 2%
$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
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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.
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