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CP1020 - Week 4

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

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CP1020 - Week 4

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  1. CP1020 - Week 4 Making Decisions

  2. ? Decisions Example: 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?

  3. Yes No! 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..

  4. 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

  5. Further Examples ifkettle has boiled then make tea iftemperature less than 18 C.then turn on central heating

  6. General form of IF statements IF condition THEN <action> ENDIF IF, THEN and ENDIF are RESERVED words condition is the “test”, if the answer is YES then we carry out the <action>

  7. An example program REM program : to demonstrate the IF statement REM written by : S. Garner REM date written 8/3/00 DIM iAge AS INTEGER 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" END IF END

  8. 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 else keep working

  9. Basic IF..THEN..ELSE IFconditionTHEN <action1> ELSE <action2> ENDIF

  10. Example program REM program : to demonstrate the IF statement DIM iMark AS INTEGER 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" ELSE PRINT "You have passed" PRINT "Well Done!" ENDIF END

  11. Testing We now have more than one possible “route” through our code We must TEST each of these! We should also test the “boundary”

  12. Test Data Mark Expected Actual Result Result 25 You have failed You have failed 60 You have passed You have passed 40 You have passed You have passed 39 You have failed You have failed

  13. 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

  14. Example Conditions NOTE: Brackets help to clarify! (iMark < 0) (iAge >=18) (iValueA = iValueB) (iAge >= 16 AND iAge < 65)

  15. 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 1 is the candidate at least 16 years old? • condition 2 is the candidate no older than 65? • Both condition 1ANDcondition 2must be satisfied "TRUE" in order to accept the candidate

  16. More Complex Decisions 2 • Algorithm:Step1 Get 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

  17. The code INPUT“How old is the candidate > ”; iCandidatesAge If (iCandidatesAge >= 16) And (iCandidatesAge <= 65)Then Print “You are eligible to apply” Else Print “You are outside the age range!” End If

  18. The OR condition • Alternatively test for ineligible candidates:Step1 Get 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

  19. Another Example REM program : to demonstrate the IF statement REM written by : I Coulson REM date written: 8/3/00 DIM iCandAge AS INTEGER 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" ELSE PRINT “You are eligible to apply” END IF END

  20. 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 Merit 60 - 79 • A Distinction 80 +

  21. Mark to Grade ConversionAlgorithm • Algorithm:Step1 Get a student's mark 2If (mark is greater than 0) AND (mark less than 40)2.1 Then Grade is Fail 3If (mark greater than or equal to 40) AND (mark is less than 60)3.1 Then Grade is Pass 4If (mark is greater or equal to 60) AND (mark less than 80)4.1 Then Grade is Merit 5If (mark is greater or equal to 80) AND (mark is no more than 100)5.1 Then Grade is Distinction 6 Display Grade

  22. Mark to Grade Conversion Improved Algorithm • Further improved algorithm:Step1 Get a student's mark 2If (mark is less than 40%)2.1 Then Grade is Fail2.2 ElseIf (mark is less than 60%)2.2.1 Then Grade is Pass2.2.2 Else If (mark is less than 80%)2.2.2.1 Then Grade is Merit2.2.2.2 Else Grade is Distinction 3 Display Grade • This is known as nesting decisions

  23. 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” END IF

  24. Questions 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”

  25. End of lecture

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