1. DCP2073 Asas Pengaturcaraan C Lecture 3: Components of an Algorithm
CSE1301 Sem
DCP2073 Asas Pengaturcaraan C Lecture 3: Components of an Algorithm
Lecture 4: Components of Algorithms

2. Recall What is the problem solving process? What is an algorithm?
CSE1301 Sem
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?
Lecture 4: Components of Algorithms

3. Topics More components of an algorithm
The software development process
Top-down design

4. Components of an Algorithm
Values and Variables
Instruction (a.k.a. primitive)
Sequence (of instructions)
Procedure (involving instructions)
Selection (between instructions)
Repetition (of instructions)
Documentation (beside instructions)

5. Family Tree of Programming Languages The components of Algorithms
That we use correspond
To “structured programming”
(Algol, Pascal, C, Modula, Ada…)

6. Variables Are containers for values – places to store values Example:
CSE1301 Sem
Variables
Are containers for values – places to store values
Example:
This jar
can contain
10 cookies
50 grams of sugar
3 slices of cake
etc.
Values
Variable
One form: strain the juices, put aside and let cool …(further instruction)… now add the reserved mix
In mathematics much more precisely: Call the sum of all items X. Call the number of items N… divide X by N
Lecture 4: Components of Algorithms

7. Restrictions on Variables
CSE1301 Sem
Restrictions on Variables
Variables may be restricted to contain a specific type of value
Lecture 4: Components of Algorithms

8. Components of an Algorithm
Values and Variables
Instruction (a.k.a. primitives)
Sequence (of instructions)
Procedure (involving instructions)
Selection (between instructions)
Repetition (of instructions)
Documentation (beside instructions)

9. Instructions (Primitives)
Some action that is
simple
unambiguous
that the system knows about...
...and should be able to actually do

10. Instructions – Examples
CSE1301 Sem
Instructions – 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
Directions to perform specific actions on values and variables.
Lecture 4: Components of Algorithms

11. Instructions -- Application
Some instructions can only be applied to a specific type of values or variables
Examples:
Chop

12. Instructions (Primitives) -- Recommendations
When writing an algorithm, make each instruction simple and unambiguous
Example:
Cut chicken into pieces.
Heat olive oil in a casserole dish.
Brown the chicken pieces in the casserole dish.
Cut chicken into pieces and brown the pieces on all sides in a casserole dish in hot olive oil.

13. Instruction (Primitives)
When writing an algorithm, make the instructions simple and unambiguous.
Example:
A “sequence” of simple instructions
Cut chicken into pieces.
Heat olive oil in a casserole dish.
Brown the chicken pieces in the casserole dish.
Cut chicken into pieces and brown the pieces on all sides in a casserole dish in hot olive oil.

14. Sequence A series of instructions
...to be carried out one after the other...
...without hesitation or question
Example:
How to cook a Gourmet MealTM

15. Sequence -- Example 2. Take out Gourmet Meal™ 3. Close freezer door
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

16. Components of an Algorithm
Values and Variables
Instruction (a.k.a. primitives)
Sequence (of instructions)
Procedure (involving instructions)
Selection (between instructions)
Repetition (of instructions)
Documentation (beside instructions)

17. Procedure A named sequence of instructions So that you can Example:
Refer to it collectively (by name)
...instead of individually (by each instruction in the sequence)
Example:
Drive_To_Uni

18. Procedure -- Example { 1. find car keys 2. disable car alarm
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 }

19. Procedure – Example (cont)
procedure Do_Wednesday {
Wake_up
Have_Shower
Eat_Breakfast
Drive_To_Uni
Sit_1301_Lecture
...etc...etc...etc...
Drive_From_Uni }
procedure Do_Week {
Do_Monday
Do_Tuesday
Do_Wednesday
Do_Thursday
...etc...etc...etc... }

20. Procedure – Example (cont)
procedure Do_Wednesday {
Wake_up
Have_Shower
Eat_Breakfast
Drive_To_Uni
Sit_1301_Lecture
...etc...etc...etc...
Drive_From_Uni }
In this subject, we also use the following words to refer to a “Procedure” :
Sub-routine
Module
Function

21. Procedure – Example (cont)
procedure Do_Wednesday {
Wake_up
Have_Shower
Eat_Breakfast
Drive_To_Uni
Sit_1301_Lecture
...etc...etc...etc...
Drive_From_Uni }
We use brackets to mark the beginning and end of a sequence.

22. Procedure – Example (cont)
procedure Do_Wednesday {
Wake_up
Have_Shower
Eat_Breakfast
Drive_To_Uni
Sit_1301_Lecture
...etc...etc...etc...
Drive_From_Uni }
An instruction invoking a procedure is known as a “procedure call”

23. 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” )

24. Components of an Algorithm
Values and Variables
Instruction (a.k.a. primitives)
Sequence (of instructions)
Procedure (involving instructions)
Selection (between instructions)
Repetition (of instructions)
Documentation (beside instructions)

25. 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

26. Selection Example -- Car Repair
if (motor turns)
then {
CheckFuel
CheckSparkPlugs
CheckCarburettor }
else
CheckDistributor
CheckIgnitionCoil

27. Selection Example – Car Repair (cont)
if (motor turns)
then {
CheckFuel
CheckSparkPlugs
CheckCarburettor }
else
CheckDistributor
CheckIgnitionCoil
Should be a true or false condition.

28. Selection Example -- Car Repair (cont)
if (motor turns)
then {
CheckFuel
CheckSparkPlugs
CheckCarburettor }
else
CheckDistributor
CheckIgnitionCoil
Sequence if the condition is true.

29. Selection Example -- Car Repair (cont)
CSE1301 Sem
Selection Example -- Car Repair (cont)
if (motor turns)
then {
CheckFuel
CheckSparkPlugs
CheckCarburettor }
else
CheckDistributor
CheckIgnitionCoil
Sequence if the condition is false.
Lecture 4: Components of Algorithms

30. Selection Example -- Reciprocals
Examples:
Reciprocal of 2: /2
Reciprocal of -3/4: /(-3/4) = -4/3
Reciprocal of 0: “undefined”
Q. Give an algorithm for computing the reciprocal of a number.

31. Selection Example – Reciprocals (cont)
Algorithm:
input Num
if (Num is not equal 0)
then {
output 1/Num }
else
output "infinity"
Q. Give an algorithm for computing the reciprocal of a number.

32. Selection Example-- Reciprocals
Algorithm:
input Num
if (Num is not equal 0)
then {
output 1/Num }
else
output "infinity"
Num is a variable whose value depends on the actual number the user provides.

33. Selection Example – Reciprocals (cont)
Algorithm:
input Num
if (Num is not equal 0)
then {
output 1/Num }
else
output "infinity"
Condition depends on the value of Num

34. Selection Example – Reciprocals (cont)
Algorithm:
input Num
if (Num is not equal 0)
then {
output 1/Num }
else
output "infinity"
For a given value of Num, only one of these two sequences can be executed

35. Selection Example – Reciprocals (cont)
Algorithm:
input Num
if (Num is not equal 0)
then {
output 1/Num }
else
output "infinity"
Executed if Num is not equal to 0

36. Selection Example – Reciprocals (cont)
Algorithm:
input Num
if (Num is not equal 0)
then {
output 1/Num }
else
output "infinity"
Executed if Num is equal to 0

37. Selection -- Exercise Algorithm 1: Algorithm 2:
Will the following algorithms produce the same output?
Algorithm 1:
Algorithm 2:
input Num
if (Num is not equal 0)
then {
output 1/Num }
else
output "infinity"
input Num
if (Num is not equal 0)
then {
output 1/Num }
output "infinity"

38. 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

39. Selection – Several Conditions (cont)
if ( today is Wednesday )
then {
if ( the time is 10.00am )
Go to CSE1301 Lecture }
else
...etc...etc...etc...
Often called a “nested selection”
Solution 2

40. 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 }

41. Components of an Algorithm
Values and Variables
Instruction (a.k.a. primitives)
Sequence (of instructions)
Procedure (involving instructions)
Selection (between instructions)
Repetition (of instructions)
Documentation (beside instructions)

42. Repetition Repeat an instruction...
CSE1301 Sem
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
Lecture 4: Components of Algorithms

43. Repetition -- Example 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
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 zero while ( 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 zero while (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)
Ensure initial values of variables used in the conditions are set correctly
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 ListenToReply ( ) }

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 zero while (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)
CSE1301 Sem
Repetition – Example (cont)
What if we don’t increment the begging count?
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!") }
Infinite loop
Lecture 4: Components of Algorithms

49. Repetition – Variation
CSE1301 Sem
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
Lecture 4: Components of Algorithms

50. Components of an Algorithm
Values and Variables
Instruction (a.k.a. primitives)
Sequence (of instructions)
Procedure (involving instructions)
Selection (between instructions)
Repetition (of instructions)
Documentation (beside instructions)

51. 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)

52. Documentation -- Example
Think of something romantic to do
decide on time and location
Work through address book to look for a person
initialise 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 out
SighWithRelief

53. Components of an Algorithm
Values and Variables
Instruction (a.k.a. primitives)
Sequence (of instructions)
Procedure (involving instructions)
Selection (between instructions)
Repetition (of instructions)
Documentation (beside instructions)

54. 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

55. Top-down Algorithm Design
CSE1301 Sem
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
Lecture 4: Components of Algorithms

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

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