1 CSE1301 Computer Programming Lecture 3: Components of an Algorithm
Jan 13, 2016
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CSE1301Computer Programming
Lecture 3:Components of an Algorithm
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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?
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Topics
• More components of an algorithm
• The software development process
• Top-down design
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Components of an AlgorithmValues and Variables
• Instruction (a.k.a. primitive)– Sequence (of instructions)– Procedure (involving instructions) – Selection (between instructions)– Repetition (of instructions)
• Documentation (beside instructions)
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Family Tree of
Programming
Languages
The components of Algorithms
That we use correspond
To “structured programming”
(Algol, Pascal, C, Modula, Ada…)
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Variables
This jarcan contain
10 cookies
50 grams of sugar
3 slices of cake
etc.
ValuesVariable
• Are containers for values – places to store values
• Example:
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Restrictions on Variables
• Variables may be restricted to contain a specific type of value
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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)
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Instructions (Primitives)
• Some action that is – simple– unambiguous– that the system knows about...– ...and should be able to actually do
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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.
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Instructions -- Application
• Some instructions can only be applied to a specific type of values or variables
• Examples:
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Instructions (Primitives) -- Recommendations
• When writing an algorithm, make each instruction simple and unambiguous
• Example:
Cut chicken into pieces and brown the pieces on all sides in a casserole dish in hot olive oil.
Cut chicken into pieces.
Heat olive oil in a casserole dish.
Brown the chicken pieces in the casserole dish.
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Instruction (Primitives)• When writing an algorithm, make the
instructions simple and unambiguous.
• Example:
Cut chicken into pieces and brown the pieces on all sides in a casserole dish in hot olive oil.
Cut chicken into pieces.
Heat olive oil in a casserole dish.
Brown the chicken pieces in the casserole dish.
A “sequence” of simple instructions
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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.
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Selection Example --Car Repair (cont)
if (motor turns) then {
CheckFuelCheckSparkPlugsCheckCarburettor
}else {
CheckDistributorCheckIgnitionCoil
}
Sequence if the condition is true.
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Selection Example --Car Repair (cont)
if (motor turns) then {
CheckFuelCheckSparkPlugsCheckCarburettor
}else {
CheckDistributorCheckIgnitionCoil
}
Sequence if the condition is false.
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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”
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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
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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
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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
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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
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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:
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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
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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...
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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
}
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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)
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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
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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 ( )}
}
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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
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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
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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
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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
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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?
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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
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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)
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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)
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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
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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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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
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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
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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
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Reading
• Deitel & Deitel, C: How to program– Chapter 3, Sections 3.8 to 3.13