Cp1020 week 4
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CP1020 - Week 4. Making Decisions. ?. Decisions. Example:

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CP1020 - Week 4

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CP1020 - Week 4

Making Decisions




Driving to a lecture you notice that you do not have much petrol left. You will need to fill up soon, and approaching you can see a petrol station. The price is reasonable, but you do not have a lot of time to spare, so don't want to have to queue to fill-up.

What would you do?



Decisions in Problem Solving

  • "If the queue at the petrol station is short then I will stop there to fill up"

  • If (the queue at the petrol station is short)Then stop there and fill up

Decisions ...decisions..

You may decide:

"if the queue at the petrol station is short then I will stop there to fill up".

We could write the algorithm for this decision as:

Ifqueue at petrol station is short then

stop there and fill up

Further Examples

ifkettle has boiled then

make tea

iftemperature less than 18 C.then

turn on central heating

General form of IF statements

IF condition THEN




condition is the “test”, if the answer is YES then we carry out the <action>

An example program

REM program : to demonstrate the IF statement

REM written by : S. Garner

REM date written 8/3/00


CLS ' clear the screen

INPUT "Please enter your age "; iAge

REM test the condition

IF iAge > 17 THEN

PRINT "You may vote at the next election"



Two way decisions

We frequently need to do either one thing or another, depending on some condition

Ifage is greater than 65 then

retire gracefully


keep working







Example program

REM program : to demonstrate the IF statement


CLS ' clear the screen

INPUT "Please enter your mark(0-100) "; iMark

REM check mark for pass or fail

IF iMark < 40 THEN

PRINT "You have failed"


PRINT "You have passed"

PRINT "Well Done!"




We now have more than one possible “route” through our code

We must TEST each of these!

We should also test the “boundary”

Test Data

MarkExpected Actual


25You have failed You have failed

60You have passed You have passed

40You have passed You have passed

39You have failed You have failed

The condition statement

Usually we check a value.

The symbols used are:

= equal<>not equal

<less than<=less than or equal

>greater than >=Greater than or equal

Example Conditions

NOTE: Brackets help to clarify!

(iMark < 0)

(iAge >=18)

(iValueA = iValueB)

(iAge >= 16 AND iAge < 65)

More Complex Decisions 1

  • Problem:You are looking for new employees for your company.One of the criteria is that the employee must be no younger than 16 and no older than 65.

  • Note: We have two conditions to satisfy:

    • condition 1is the candidate at least 16 years old?

    • condition 2is the candidate no older than 65?

  • Both condition 1ANDcondition 2must be satisfied "TRUE" in order to accept the candidate

More Complex Decisions 2

  • Algorithm:Step1Get age of candidate2If ( age at least 16) AND (age less than 65 )2.1Then candidate is eligible2.2Else reject candidate

  • AND implies thatbothconditions must be true

The code

INPUT“How old is the candidate > ”; iCandidatesAge

If (iCandidatesAge >= 16) And (iCandidatesAge <= 65)Then

Print “You are eligible to apply”


Print “You are outside the age range!”

End If

The OR condition

  • Alternatively test for ineligible candidates:Step1Get age of candidate2If ( age less than 16) OR (age greater than 65 )2.1Then reject candidate2.2Else candidate is eligible

  • OR implies that either one (or both) of the conditions needs to be satisfied

Another Example

REM program : to demonstrate the IF statement

REM written by : I Coulson

REM date written: 8/3/00


CLS ' clear the screen

INPUT "Please enter your age "; iCandAge

IF ( iCandAge < 16) OR ( iCandAge > 65) THEN

PRINT ”Sorry you are outside the age range"


PRINT “You are eligible to apply”



Problem - Student Grades

  • When a piece of work is marked, it is given a percentage mark which needs converting to a FAIL, PASS, MERIT or DISTINCTION.

  • A Fail Upto 40

  • A Pass 40 - 59

  • A Merit60 - 79

  • A Distinction80 +

Mark to Grade ConversionAlgorithm

  • Algorithm:Step1 Get a student's mark

    2If (mark is greater than 0) AND (mark less than 40)2.1Then Grade is Fail

    3If (mark greater than or equal to 40) AND (mark is less than 60)3.1Then Grade is Pass

    4If (mark is greater or equal to 60) AND (mark less than 80)4.1Then Grade is Merit

    5If (mark is greater or equal to 80) AND (mark is no more than 100)5.1Then Grade is Distinction

    6Display Grade

Mark to Grade Conversion Improved Algorithm

  • Further improved algorithm:Step1Get a student's mark

    2If (mark is less than 40%)2.1Then Grade is Fail2.2ElseIf (mark is less than 60%)2.2.1 Then Grade is Pass2.2.2ElseIf (mark is less than 80%) Grade is Merit2.2.2.2Else Grade is Distinction

    3Display Grade

  • This is known as nesting decisions

The Program

Rem Author I Coulson

DIM iPercentage AS INTEGER

INPUT ”What percentage did you get "; iPercentage

IF iPercentage < 40 THEN

Print “Fail”

ELSEIF iPercentage < 60 THEN

Print “Pass”

ELSEIF iPercentage < 80 THEN

Print “Merit”

ELSEIF iPercentage >= 80 THEN

Print “Distinction”



1 Write an algorithm to decide if a salesman should get a bonus - he needs to have sold at least £3000 worth of goods in the month.

2 Alter the algorithm such that that the salesman earns 15% commission on all sales if sells more than £3000 worth of goods in a month, but only 5% if he sells less than that.

3 Write the code to print the appropriate comment to a runner finishing a race:

1st place - “well done you are the winner”

2nd place - “congratulations you are runner up”

3rd place- “good, you have finished third”

unplaced - “You’ve finished, well done”

End of lecture

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