8.2 Selection

CONTROL STRUCTURE• SELECTION

8.2 Approach in Problem Solving

1

LEARNING OUTCOME

At the end of this topic, students should be able to:❑ explain the purpose of selection control

structure.❑ apply selection control structure in problem

solving.

2

CONTROL STRUCTURE : SELECTION

What is Selection Structure?• Also called decision structure.• Purpose :

• statement is used to determine which of different statements to execute depending on certain conditions

• The selection structure allows instructions to be executed non-sequentially.

• Make a decision, and then takes appropriate action based on that decision.

3


4

SELECTION Analogy

Example 1 : the bus driver can decide whether he will stop the bus to take a passenger or just proceed a driving.


5

SELECTION Analogy

Example 2 : you drive a car and approach an intersection, whether you need to turn left or turn right.


6

SELECTION Analogy

Example 3: You need to choose a drink from a vending machine.

Pepsi??

coke??100-plus??


7

SELECTION Analogy

Example 4: You need to choose either you need to move A or B which is determine a different effect.

AB

If I move B.. If I move

A..

CONTROL STRUCTURE : SELECTION• The way to form an expression or condition in

selection statement.

• Expression or condition is used to make a comparison, then generate a Boolean expression either TRUE or FALSE.

• To form an expression/condition, we are using 6 relational operator (=, ≠, ≥ , ≤, >, <)

• Example;12 > 15 is FALSE7 ≤ 8 is TRUEif mark is 100, then

mark < 80 is FALSEmark ≠ 80 is TRUE

8

CONTROL STRUCTURE : SELECTIONRelational Operator

Expression / condition Relational operator

1. Equal =2. Greater than >

3. Less than <

4. Greater or equal ≥

5. Less or equal ≤

6. Not equal ≠

9

CONTROL STRUCTURE : SELECTION• Example :

Situation/problem Expression / condition

If the mark is greater than or equal 80, the result is Pass, otherwise fail.

Mark ≥ 80

The program will receive two numbers. If the first number is greater than second number, so the message “number1 is larger than number2”.

A > BNum1>num2

The job requirement is only open for the person that age 20 and above.

Age ≠ 20

10


11

Type of Selection construct /statement

if If-else Nested if

If – else if If- if else

SELECTION → (1) if

If construct • The simplest and most common selection structure is

the if statement which is written in a statement of the form:

if (condition/expression) statement(s)End if

• The if statement tests for a particular condition (expressed as a Boolean expression) and only executes the following statement(s) if the condition is TRUE.

• But, if the condition is FALSE, then, statement for IF is ignored and the process will execute the next statement.

12

SELECTION → (1) if

Pseudocode format: flow chart format:

13

if(expression/condition) statement_1End if expression

statement_1

false

trueExample :If the student’s mark is greater than or equal 50, print “pass”

EXAMPLE : If students mark is greater than or equal to 60, then, the message will display “Passed”. Problem analysis : Flow chart ;Input : markProcess : determine mark to identify whether PassedOutput : message “passed”

Pseudocode

14

Start input mark if mark greater than or equal to 60, print “passed” End ifEnd

Mark ≥ 60

Input mark

False

True

Print “passed”

start

end

EXAMPLE 2 : If BMI value is greater than or equal to 20, then, the message will display “you are overweight”. Problem analysis : Flow chart ;Input : BMIProcess : determine BMI to identify whether you are overweightOutput : message “you are overweight”

Pseudocode

15

Start input BMI if BMI greater than or equal to 20, print “you are overweight” End ifEnd

BMI ≥ 20

Input BMI

False

True

Print “you are overweight”

end

start

if(expression) statement-1; else statement-2;End if

SELECTION → (2) if - else

16

• Often it is desirable for a program to take one branch if the condition is TRUE or another statement if the condition is FALSE. This can be done by using an if- else selection statement:

• Statement-1 will be executed if the expression value is TRUE.• Statement-2 will be executed if the expression is FALSE

• Either statement_1 or statement_2 is executed but not BOTH.

if (expression) statement_1

else statement_2

End if

SELECTION → (2) if-else

Pseudocode format: Flow chart format:

17

if (expression)

statement_1else

statement_2End if

expression

statement_1statement_2

false

true

Example :If the student’s mark is greater than or equal 50, print “pass”. Otherwise, print “fail”


Example 1: If students mark is greater than or

equal to 60, then, the message will display “Passed”. Else, the message will display “Fail”.

18


IPO Analysis:

Input: marksProcess: determine mark to identify whether Passed or FailOutput: message “passed” OR “Fail”

19


IPO Analysis:

Input: marksProcess:

if marks greater than or equal to 60print “Passed”

elseprint “Fail”

Output: message “passed” OR “Fail”

20

Mathematical operation also can be simplified by statement

if mark ≥ 60

SELECTION → (2) if-elsePseudocode

21

startread marks

if (marks ≥ 60)print “Passed”

elseprint “Fail”

End ifend


Flow chart

22

start

end

Input marks

Print “Passed”

Marks ≥ 60

true Print “Fail”

false

22


Example 2: If x is greater than y, display “x is bigger than y” else display “x is smaller than y”.

23


IPO Analysis:

Input: x,yProcess: determine x to identify whether x is bigger than y or x is smaller than yOutput: message “x is bigger than y” OR “x is smaller than y”

24

SELECTION → (2) if-elsePseudocode

25

startread x,y

if (x > y)print “x is bigger than y”

elseprint “x is smaller than y”

End ifend


Flow chart

26

start

end

Input x, y

Print “x is bigger than y”

x > y

true Print “x is smaller than y”

false

26


Example 3: the program will print the status. If CGPA less than 2.00, status is ‘F’. Otherwise, status is ‘P’.

Answer :if (CGPA<2.00)

status = ‘F’else

status = ‘P’Print statusEnd if

27

Status =‘F’

Print Status

CGPA<2.00

Status=‘P’

false

true

• IF BMI VALUE IS GREATER THAN OR EQUAL TO 20, THEN, THE MESSAGE WILL DISPLAY “YOU ARE OVERWEIGHT”. OTHERWISE, DISPLAY “YOU ARE NORMAL”

- DO PROBLEM ANALYSIS- PSEUDO CODE- FLOWCHART

TRY THIS !!!!

28

SELECTION → (3) Nested if

• The if-else construct can also be nested (placed one within another) to any depth.

• Nested if generally take they forms : if-else if and if-if else

29

SELECTION → (3) Nested if(if-else if STATEMENT)

• Occasionally a decision has to be made on the value of a variable which has more than two possibilities.

• This can be done by placing if statements within other if-else constructions.

if( expression-1 ) statement-1 else if( expression-2 ) statement-2

……… else if( expression-n ) statement-n else last statement

End if• The boolean-expression will tested from the Boolean-expression-1 until it

find the condition is True, then the statement will be executed.• Rules → Only ONE of the statements is executed which is the Boolean-

expression is meet the True condition.

30

Example 1: The program will display the following message based on following BMI.

31


BMI message>20 “you are overweight”=20 “you are normal”<20 “ you are thin”

Example 1: The program will display the following message based on following BMI.

32


BMI message>20 “you are overweight”=20 “you are normal”<20 “ you are thin”

ifelse if

else

SELECTION → (3) Nested if(if-else if STATEMENT)IPO Analysis:

Input: BMI

Process: Determine the BMI to identify whether overweight or normal or thinOutput: message /display “you are overweight” or “you are normal” or “you are thin”

33

SELECTION → (3) Nested if (if-else if STATEMENT)

Pseudocode

34

startread BMI

if (BMI > 20) print “you are overweight”else if (BMI = 20) print “you are normal”

else print “you are thin”End if

end


start

end

Input BMI

BMI > 20true Print

“you are overweight

”false

BMI = 20 Print “you are normal”false

Print “you are thin”

true

35

Example 2: The program will display the grade based on following mark.

36


Mark grade ≥ 80 A ≥ 60 B ≥ 50 C<50 F

Example 2: The program will display the grade based on following mark.

37


Mark grade ≥ 80 A ≥ 60 B ≥ 50 C<50 F

ifelse if

else if

else

SELECTION → (3) Nested if(if-else if STATEMENT)IPO Analysis:

Input: mark

Process: Determine grade A or B or C or F based on markOutput: message/display “ A” or “B” or “C” or “F”

38

SELECTION → (3) Nested if (if-else if STATEMENT)Pseudocode

39

startread mark

if (mark ≥ 80) print “A”else if (mark ≥ 60) print “B”

else if (mark ≥ 50) print “C” else print “F”End if

end


start

end

Input mark

mark ≥ 80true

Print “A”

false

mark ≥ 60

mark ≥ 50

Print“B”

Print“C”

false

falsePrint “F”

true

true

40

SELECTION → (3) Nested if(if-else if STATEMENT

Example 3:

The program read a number, if the number is greater than 0, then display a message "x is positive“, if the number is less than zero, then display the message "x is negative“, and otherwise, display the message “x is 0”.

41


IPO Analysis:

Input: x

Process: Determine x is positive or negative or 0 number

Output: display/ message “x is positive“ or "x is negative“ or “x is 0”

42


Pseudo code

43

startread x

if (x > 0) display "x is positive" else if (x < 0) display "x is negative" else display "x is 0“ End if

end


Flow chart

44

start

end

Input x

(x > 0)true Print "x

is positive”

false(x < 0)

Print “x is

negative”falsePrint “x is

0”

true

• The next nested if ; if-if else • This can be done by placing if statements within other if

statement. if(expression-1 ) if(expression-2 )

if(expression-3) statement-1

elsestatement-2

end ifelse

statement-3 end if

elsestatement-4

End if

SELECTION → (3) Nested if(if-if else STATEMENT)

45

• Expression-1 is evaluated. If it is FALSE, statement-4 is executed and the entire if statement is terminated. If TRUE, control goes to the second if and expression-2 is evaluated.

• If it is FALSE, statement-3 is executed; if TRUE, control goes to third if and expression-3 is evaluated.

• If it is FALSE, statement-2 is executed; if TRUE, statement-1 is executed.


• Example 1; The job vacancy is open for the person that is Male and have more than 5 years experience. Display a message “eligible” if the person meet both requirement. Otherwise, display “not eligible”.

46


• IPO ANALYSISInput : sex, year experienceProcess : Determine sex and year experience to identify whether eligible or not eligible

Output :display/message “eligible” or “not eligible”

47


• pseudocodeStart

Input sex and year experienceif (sex=‘m’)

if(year experience>5)display “eligible”

elsedisplay “not eligible”

end ifEnd if

End

48


• flowchart

49

Input sex, year experience

Sex=‘m’

Year experience =

5Print “not eligible”

Print “eligible”

True

True

false

false

start

end


• Example 2; The program will display the message based on CGPA and Program enrolled by student.

50

CGPA PROGRAM MESSAGE<2.00 PST GO TO PDT

PDT FAIL TO CONTINUE

≥ 2.00 PST PASS

PDT PASS


• IPO ANALYSISInput : CGPA and ProgramProcess : Determine CGPA and program to identify whether go to PDT or fail to continue.Output :message “pass” or “go to PDT” or “fail to continue”

51


• pseudocodeStart

Input CGPA and Programif (CGPA<2.00)

if(program=PST)display “go to PDT”

elsedisplay “fail to continue”

End ifelse

display “pass”End if

End

52


• flowchart

53

Input CGPA, Program

CGPA<2.00

Program= PST

Print “Pass”

Print “fail to

continue”

Print “Go to PDT ”

True

True

false

false

start

end

54

1. Prepare a program that enter marks for student sit for a test. Passing marks is 50. Pass students were been identify by two groups which above and equal to 75 is excellent, else is good.

2. Student enter their CGPA and the system print or identify CGPA for student. If CGPA below than 2.0 the status is failed else the status is pass.

3. Prepare a program that user can enter two number and the program will identify which number is bigger.

4. User inserts two numbers in the program. The program will determine if the two numbers is not less than 0.0 the program will calculate and display three mathematical calculations – addition, multiplication and modulus based on the numbers given.

5. Prepare a program that reads marks and attendance for student. Student sit for a test and the status is pass if the student get marks not less than 40 and attendance to class is not less than 30 times. If not pass give reasons. Display the mark and attendance of student.

6. Requirement for membership of an exclusive club is not earning less than RM5000 per month and the age of members not less than 20 years. Prepare a program that can determine a person is eligible or not to join the club if given monthly income and age. If not eligible, give reasons.

CONTROL STRUCTURE• SELECTION

55

CGPA Range Degree Class

3.75<=CGPA<=4.00 First Class

3.0<=CGPA<3.75 Second Class Upper

2.75<=CGPA<3.0 Second Class Lower

2.0<=CGPA<2.75 General Degree

CGPA<2.0 Failed

•Prepare a program that can determine undergraduate class for universities student base on the CGPA table below:

8.2 Selection

Education