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1
CSE1301Computer Programming
Lecture 4:Components of an Algorithm
2
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?
3
Topics
• More components of an algorithm
• The software development process
• Top-down design
4
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)
5
Sequence
• A series of instructions
• ...to be carried out one after the other...
• ...without hesitation or question
• Example:
– How to cook a Gourmet MealTM
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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
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Components of an Algorithm
Values and VariablesInstruction (a.k.a. primitives)Sequence (of instructions)
• Procedure (involving instructions)
• Selection (between instructions)
• Repetition (of instructions)
• Documentation (beside instructions)
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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
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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
}
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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...
}
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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...}
In this subject, we also use the following words to refer to a “Procedure” :
• Sub-routine• Module• Function
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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...}
We use brackets to mark the beginning and end of a sequence.
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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...}
An instruction invoking a procedure is known as a “procedure call”
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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” )
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Components of an Algorithm
Values and VariablesInstruction (a.k.a. primitives)Sequence (of instructions)Procedure (involving instructions)
• Selection (between instructions)
• Repetition (of instructions)
• Documentation (beside instructions)
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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
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Selection Example -- Car Repairif (motor turns) then {
CheckFuelCheckSparkPlugsCheckCarburettor
}else {
CheckDistributorCheckIgnitionCoil
}
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Selection Example –Car Repair (cont)
if (motor turns) then {
CheckFuelCheckSparkPlugsCheckCarburettor
}else {
CheckDistributorCheckIgnitionCoil
}
Should be a true or false condition.
19
Selection Example --Car Repair (cont)
if (motor turns) then {
CheckFuelCheckSparkPlugsCheckCarburettor
}else {
CheckDistributorCheckIgnitionCoil
}
Sequence if the condition is true.
20
Selection Example --Car Repair (cont)
if (motor turns) then {
CheckFuelCheckSparkPlugsCheckCarburettor
}else {
CheckDistributorCheckIgnitionCoil
}
Sequence if the condition is false.
21
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”
22
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" }
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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.
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Selection Example – Reciprocals (cont)
input Num if (Num is not equal 0) then { output 1/Num } else { output "infinity"
}
Algorithm:
Condition depends on the value of Num
25
Selection Example – Reciprocals (cont)
input Num if (Num is not equal 0) then { output 1/Num } else { output "infinity"
}
Algorithm:
For a given value of Num, only one of these two sequences can be executed
26
Selection Example – Reciprocals (cont)
input Num if (Num is not equal 0) then { output 1/Num } else { output "infinity"
}
Algorithm:
Executed if Num is not equal to 0
27
Selection Example – Reciprocals (cont)
input Num if (Num is not equal 0) then { output 1/Num } else { output "infinity"
}
Algorithm:
Executed if Num is equal to 0
28
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)
then
{
output 1/Num
}
output "infinity"
Algorithm 2:
29
Selection – Several Conditions
• What if several conditions need to be satisfied?if ( today is Wednesday and the time is 10.00am )
then
{
Go to CSE1301 Lecture
}
else
{
Go to Library
} Solution 1
30
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...
31
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 )
then
{
Eat my lunch now
}
32
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
33
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
34
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
then { PayBills } else { if ( button pressed is 4 ) then { ExitPhoneBanking } else { say “Invalid option” } } }}
Form 1
One condition per course of action
35
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
36
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
37
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
38
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
“Default” sequence (may be optional)
39
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 2“Cascaded” selection.
40
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 2
Optional default sequence.
41
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)
42
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
43
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 ( )}
}
44
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
45
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
46
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 ( )}
}
Ensure initial values of variables used in the conditions are set correctly
47
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 ( )}
}
Ensure the variables used in the conditions are updated in each iteration
48
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!")
}
}Infinite loop
• What if we don’t increment the begging count?
49
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
50
Repetition– Pre-tested Loop
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
}
SighWithReliefpre-tested loop
51
Repetition – Pre-tested Loop
decide on Time and Location
initialise booking to “unsuccessful”
until ( successfully booked )
{
get next Name in little black book
AskOnDate(Name, Time, Location)
DetermineBookingSuccess
}
SighWithRelief
pre-tested loop
52
Repetition – Post-tested Loop
decide on Time and Location
initialise booking to “unsuccessful”
do
{
get next Name in little black book
AskOnDate(Name, Time, Location)
DetermineBookingSuccess
} while ( not successfully booked )
SighWithReliefpost-tested loop
Sequence is executed at least once
53
Repetition – Post-tested Loop
decide on Time and Location
initialise booking to “unsuccessful”
repeat
{
get next Name in little black book
AskOnDate(Name, Time, Location)
DetermineBookingSuccess
} until ( successfully booked )
SighWithRelief
54
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
55
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)
56
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)
57
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
58
Components of an Algorithm
Values and VariablesInstruction (a.k.a. primitives)Sequence (of instructions)Procedure (involving instructions) Selection (between instructions)Repetition (of instructions)Documentation (beside instructions)
59
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
60
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
61
Top-down Design -- Example
Learn
Prepare
Study
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
62
Reading
• Deitel & Deitel, C: How to program– Chapter 3, Sections 3.8 to 3.13