Al-Khwarizmi 2 (Lecture 3 of CSE1301) Dr Damian Conway Room 132, Building 26 (Consult hours on the door)

Algorithms 2 (Lecture 3 of CSE1301) Dr Damian Conway Room 132, Building 26 (Consult hours on the door)

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Recall What is the problem solving process? What is an algorithm? What are some examples? What are the "variables" in the Australian Cricket team?

Topics More components of an algorithm The software development process Top-down design Conway's Game of Life

Components of an algorithm What kind of instructions do we give? What are the building blocks of those instructions?

Components of an algorithm Instruction Sequence (of instructions) Procedure (involving instructions) Selection (between instructions) Repetition (of instructions) Documentation (beside instructions)

Instruction Some action that is simple......and unambigous......that the system knows about......and should be able to actually do.

Instruction Examples –Take off your shoes. –Count to 10. –Cut along dotted line. –Knit 1. –Purl 2. –Pull rip-cord firmly. –Sift 10 grams of arsenic into the batter.

Sequence A series of instructions...to be carried out one after the other......without hestitation or question. Example: –Cooking a Gourmet Meal

Sequence 1. Open freezer door 2. Take out Gourmet Meal™ 3. Close freezer door 4. Open microwave door 5. Put Gourmet Meal™ on carousel 6. Shut microwave door 7. Set microwave on high for 5 minutes 8. Start microwave 9. Wait 5 minutes 10. Open microwave door 11. Remove Gourmet Meal™ 12. Close microwave door

Procedure A named sequence Refer to it collectively (by name)...instead of individually (by each instruction in sequence) Example: –Driving to Uni

Procedure procedure DriveToUni { 1. find car keys 2. disable car alarm 3. open car door 4. get in car 5. shut car door 6. put keys in ignition 7. start car 8. back car out of driveway 9. drive to end of street 10. turn right 11. drive to end of street 12. turn left...etc...etc...etc...etc...etc...etc find parking space 53. pull into parking space 54. turn off engine 55. remove keys from ignition 56. open car door 57. get out 58. shut car door 59. lock car door 60. enable alarm }

Selection Choose between two courses of action That is, between two instructions (or sequences or procedures) Decision based on a single true/false question Examples: –Car repairs –Reciprocals –Before driving to uni

Selection if (motor turns) then { CheckFuel CheckSparkPlugs CheckCarburettor } else { CheckDistributer CheckIgnitionCoil }

Selection input Number if (Number ≠ 0) then { output 1/Number } else { output "infinity" }

Selection Before driving to uni

Repetition Repeat an instruction......while (or maybe until) some condition occurs Test the condition each time before repeating the instruction Also known as iteration Example: –Getting a date

Repetition procedure AskOnDate(name, time, location) { Phone(name) Say("Hey", name, "it's your lucky day!") Say("Wanna come to", location, "at", time, "?") ListenToReply start begging count at zero while (reply is "No" and begging count < 100) { Say("Oh please!") add 1 to begging count } }

Repetition decide on time and location until (successfully booked) { get next name in little black book AskOnDate(name, time, location) } SighWithRelief

Documentation Records what the algorithm does Describes how it does it Explains the purpose of each component of the algorithm Notes restrictions or expectations Example: –Getting a date (again)

Documentation Thing of something romantic to do (e.g. footy, video games, chess) decide on time and location Work through address book looking for someone who's forgotten what a lousy date I am until (successfully booked) { get next name in little black book AskOnDate(name, time, location) } Will only reach here if someone's memory (or taste) is bad SighWithRelief

The Software Development Process Define the problem clearly Analyse the problem thoroughly Design an algorithm carefully Code the algorithm efficiently Test the code thoroughly Document the system lucidly

Designing an algorithm Top-down Bottom-up Object-oriented Prototyping Evolutionary

Top-down design Write down what you have to do Break that into 3-7 smaller steps Break each step into 3-7 smaller steps Keeping subdividing until each individual step is easy enough to do That is, until it's a single instruction Example: –Learning

Top-down design Learn Prepare Study Reinforce Read Make notes Prepare questions Attend lecture Listen and think Complete prac Attend tute Record answers to questions Revise notes Read lecture notes Read textbook Read exercise Design algorithm Code solution Test and document Record insights

The Game of Life Invented by mathematician John Conway (no relation) As seen on millions of screen savers around the world Now in the comfort of your own lecture theatre...

The Game of Life stand up if your age is even while (sign says GO) { count how many people next to you are standing if (exactly 3 of your neighbours are standing) { raise your hand } otherwise if (exactly 2 of your neighbours are standing AND you are standing too) { raise your hand } CHANGE wait till the sign says CHANGE CHANGE wait till the sign doesn't say CHANGE if (your hand is up) { stand up } otherwise { sit down } put your hand down }

stand up if your age is even while (sign says GO) { count how many people next to you are standing if (exactly 3 of your neighbours are standing) { raise your hand } otherwise if (exactly 2 of your neighbours are standing AND you are standing too) { raise your hand } CHANGE wait till the sign says CHANGE CHANGE wait till the sign doesn't say CHANGE if (your hand is up) { stand up } otherwise { sit down } put your hand down } The Game of Life GO

stand up if your age is even while (sign says GO) { count how many people next to you are standing if (exactly 3 of your neighbours are standing) { raise your hand } otherwise if (exactly 2 of your neighbours are standing AND you are standing too) { raise your hand } CHANGE wait till the sign says CHANGE CHANGE wait till the sign doesn't say CHANGE if (your hand is up) { stand up } otherwise { sit down } put your hand down } The Game of Life CHANGE

stand up if your age is even while (sign says GO) { count how many people next to you are standing if (exactly 3 of your neighbours are standing) { raise your hand } otherwise if (exactly 2 of your neighbours are standing AND you are standing too) { raise your hand } CHANGE wait till the sign says CHANGE CHANGE wait till the sign doesn't say CHANGE if (your hand is up) { stand up } otherwise { sit down } put your hand down } The Game of Life STOP

Reading Deitel & Deitel, C: How to program –Chapter 3, Sections 3.8 to 3.13