1 CSE1301 Computer Programming Lecture 4: Components of an Algorithm

CSE1301Computer Programming

Lecture 4:Components of an Algorithm

Recall

• What is the problem solving process?

• What is an algorithm?

• What are some examples?

• What are values and variables?

• What are instructions or primitives?

Topics

• More components of an algorithm

• The software development process

• Top-down design

Components of an AlgorithmValues and VariablesInstruction (a.k.a. primitives)

• Sequence (of instructions)

• Procedure (involving instructions)

• Selection (between instructions)

• Repetition (of instructions)

• Documentation (beside instructions)

Sequence

• A series of instructions

• ...to be carried out one after the other...

• ...without hesitation or question

• Example:

– How to cook a Gourmet MealTM

Sequence -- Example

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

Components of an Algorithm

Values and VariablesInstruction (a.k.a. primitives)Sequence (of instructions)

• Procedure (involving instructions)

Procedure

• A named sequence of instructions

• So that you can– Refer to it collectively (by name)– ...instead of individually (by each instruction in

the sequence)

• Example:

– Drive_To_Uni

Procedure -- Example procedure Drive_To_Uni { 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...

52. 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

Procedure – Example (cont)

procedure Do_Wednesday{ Wake_up Have_Shower Eat_Breakfast Drive_To_Uni Sit_1301_Lecture ...etc...etc...etc... Drive_From_Uni ...etc...etc...etc...}

procedure Do_Week

Do_Monday

Do_Tuesday

Do_Wednesday

Do_Thursday

...etc...etc...etc...

In this subject, we also use the following words to refer to a “Procedure” :

• Sub-routine• Module• Function

We use brackets to mark the beginning and end of a sequence.

An instruction invoking a procedure is known as a “procedure call”

Procedure• A procedure may have a set of parameters

procedure customerService ( myName ,timeOfDay ){ say “Good timeOfDay” say “My name is myName” say “How can I help you?”}

customerService ( “Ann”, “Morning” )

customerService (“Ann”, “Afternoon” )

customerService ( “Jeff”, “Afternoon” )

Values and VariablesInstruction (a.k.a. primitives)Sequence (of instructions)Procedure (involving instructions)

Selection• An instruction that decides which of two

possible sequences is executed

• The decision is based on a single true/false condition

• Examples:

– Car repair

– Reciprocals

Selection Example -- Car Repairif (motor turns) then {

CheckFuelCheckSparkPlugsCheckCarburettor

}else {

CheckDistributorCheckIgnitionCoil

Selection Example –Car Repair (cont)

if (motor turns) then {

}else {

Should be a true or false condition.

Selection Example --Car Repair (cont)

}else {

Sequence if the condition is true.

Selection Example --Car Repair (cont)

}else {

Sequence if the condition is false.

Selection Example -- Reciprocals

Q. Give an algorithm for computing the reciprocal of a number.

Examples:

Reciprocal of 2: 1/2

Reciprocal of -3/4: 1/(-3/4) = -4/3

Reciprocal of 0: “undefined”

Selection Example – Reciprocals (cont)

Q. Give an algorithm for computing the reciprocal of a number.

Algorithm: input Num if (Num is not equal 0) then { output 1/Num } else {

output "infinity" }

Selection Example-- Reciprocals

input Num if (Num is not equal 0) then { output 1/Num } else { output "infinity"

Algorithm:

Num is a variable whose value depends on the actual number the user provides.

Algorithm:

Condition depends on the value of Num

Algorithm:

For a given value of Num, only one of these two sequences can be executed

Algorithm:

Executed if Num is not equal to 0

Algorithm:

Executed if Num is equal to 0

Selection -- Exercise

input Num if (Num is not equal 0) then { output 1/Num } else {

output "infinity" }

Will the following algorithms produce the same output?

Algorithm 1: input Num

if (Num is not equal 0)

output 1/Num

output "infinity"

Algorithm 2:

Selection – Several Conditions

• What if several conditions need to be satisfied?if ( today is Wednesday and the time is 10.00am )

Go to CSE1301 Lecture

Go to Library

} Solution 1

Selection – Several Conditions (cont)

Solution 2

Often called a “nested selection”

if ( today is Wednesday ) then {

if ( the time is 10.00am ) then { Go to CSE1301 Lecture } } else ...etc...etc...etc...

Selection – At Least One of Several Conditions

• What if at least one of several conditions needs to be satisfied?if ( I feel hungry or the time is 1.00pm or my

mate has his eye on my lunch )

Eat my lunch now

Selection – Several Courses of Action• What if there are several courses of action? if ( button pressed is 1 )then{ CheckAccountBalance}else{ if ( button pressed is 2 ) then { TransferFunds } else { if ( button pressed is 3 ) then

{ PayBills } else { if ( button pressed is 4 ) then { ExitPhoneBanking } else { say “Invalid option” } } }} Form 1

Selection – Several Courses of Action (cont)

if ( button pressed is 1 )then{ CheckAccountBalance}else{ if ( button pressed is 2 ) then { TransferFunds } else { if ( button pressed is 3 ) then

{ PayBills } else { if ( button pressed is 4 ) then { ExitPhoneBanking } else { say “Invalid option” } } }}

Form 1

then { PayBills } else { if ( button pressed is 4 ) then { ExitPhoneBanking } else { say “Invalid option” } } }}

Form 1

One condition per course of action

Form 1

“Default” sequence (may be optional)

if ( button pressed is 1 )then{ CheckAccountBalance}else if ( button pressed is 2 )then{ TransferFunds}else if ( button pressed is 3 )then{ PayBills}

else if ( button pressed is 4 )then{ ExitPhoneBanking}else{ say “Invalid option”}

Form 2“Cascaded” selection.

if ( button pressed is 1 )then{ CheckAccountBalance}else if ( button pressed is 2 )then{ TransferFunds}else if ( button pressed is 3 )then{ PayBills}

else if ( button pressed is 4 )then{ ExitPhoneBanking}else{ say “Invalid option”}

Form 2

Optional default sequence.

Components of an AlgorithmValues and VariablesInstruction (a.k.a. primitives)Sequence (of instructions)Procedure (involving instructions) Selection (between instructions)

Repetition

• Repeat an instruction...– ...while (or maybe until) some true or

false condition occurs– Test the condition each time before

repeating the instruction• Also known as iteration or loop• Example:

– Algorithm for getting a date

Repetition -- Exampleprocedure 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 zerowhile (reply is "No" and begging count < 100){ Say("Oh please!")

add 1 to begging count

ListenToReply ( )}

Repetition – Example (cont)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 zerowhile ( reply is "No" and begging count < 100 ){ Say("Oh please!") add 1 to begging count ListenToReply ( )}

Condition is tested before sequence

Repetition – Example (cont)

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 zerowhile (reply is "No" and begging count < 100){ Say("Oh please!") add 1 to begging count ListenToReply ( )}

Sequence may not get executed at all

Repetition – Example (cont)procedure AskOnDate ( name, time, location ){

Ensure initial values of variables used in the conditions are set correctly

procedure AskOnDate ( name, time, location ){

Ensure the variables used in the conditions are updated in each iteration

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 zerowhile (reply is "No" and begging count < 100){ Say("Oh please!")

}Infinite loop

• What if we don’t increment the begging count?

Repetition – Variation

decide on Time and Location

initialise booking to “unsuccessful”

while ( not successfully booked )

get next Name in little black book

AskOnDate(Name, Time, Location)

DetermineBookingSuccess

SighWithRelief

Repetition– Pre-tested Loop

while ( not successfully booked )

SighWithReliefpre-tested loop

Repetition – Pre-tested Loop

until ( successfully booked )

SighWithRelief

pre-tested loop

Repetition – Post-tested Loop

} while ( not successfully booked )

SighWithReliefpost-tested loop

Sequence is executed at least once

Repetition – Post-tested Loop

repeat

} until ( successfully booked )

SighWithRelief

Repetition -- Variations

decide on Time and Locationinitialise booking to “unsuccessful”loop { get next Name in little black book AskOnDate(Name, Time, Location)

DetermineBookingSuccessif ( successfully booked )then{ break loop}

}SighWithRelief

Components of an AlgorithmValues and VariablesInstruction (a.k.a. primitives)Sequence (of instructions)Procedure (involving instructions) Selection (between instructions)Repetition (of instructions)

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 -- ExampleThink of something romantic to dodecide on time and location

Work through address book to look for a personinitialise booking to “unsuccessful”until (successfully booked){ get next Name in little black book AskOnDate(Name, Time, Location) DetermineBookingSuccess}

Assumes that I will find someone in the book before it runs outSighWithRelief

Values and VariablesInstruction (a.k.a. primitives)Sequence (of instructions)Procedure (involving instructions) Selection (between instructions)Repetition (of instructions)Documentation (beside instructions)

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

Top-down Algorithm 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 – i.e., until it is a single instruction

• Example:– Learning

Top-down Design -- Example

Prepare

Reinforce

ReadMake notesPrepare questions

Attend lectureListen and thinkComplete pracAttend tute

Record answers to questions

Revise notes

Read lecture notesRead textbook

Read exerciseDesign algorithmCode solutionTest and document

Record insights

Reading

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

1 CSE1301 Computer Programming Lecture 4: Components of an Algorithm

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