CE131: Introduction to Algorithms - egec.yolasite.com · Introduction to Computing Lecture 00: Part 2 Introduction to Algorithms and Flowcharts ... •Algorithms •Flow Charts

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Introduction to Computing

Lecture 00: Part 2 Introduction to Algorithms and

Flowcharts

Assist.Prof.Dr. Nükhet ÖZBEK

Ege University

Department of Electrical & Electronics Engineering

[email protected]

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Topics

• Solving problems

• Software Development Method

• Algorithms

• Flow Charts

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Software Development

• Programming is a problem-solving activity

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How do we solve problems? • We "just do"

• Guesswork-and-luck

• Trial-and-error

• Experience (possibly someone else's)

• "Scientifically"

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The Problem-solving Process

Requirement Analysis

Analysis

Design

Implementation

Test and Verification

Maintenance

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Requirement analysis

• Formally specify the problem and the needed solution

• Unimportant aspects are removed from problem definition

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Analysis

• Detailed identification of the problem and the desired solution

• Detailed specification of input data and the required output is worked out

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Design

• Specification of the process(es) needed to transform the input to the desired output

• Sequence of instructions specifying the steps required to accomplish a task is called an algorithm

• Designed algorithm must be checked for correctness

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Implementation

• Designed algorithm is translated into a chosen computer language

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Test and verification

• To see that developed program works as intended

• Program must be run with several different data sets to verify the program

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Maintenance

• Developed program is modified to correct unexpected errors encountered in the program and add new features to the program if needed

• Maintenance can be a tedious process

– Programs must be well documented and structured

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• A sequence of instructions specifying the steps required to accomplish some task

• Named after:

Muhammad ibn Musa al-Khwarizmi

of Khowarezm (now Khiva in Uzbekistan)

Algorithm

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• A sequence of instructions describing how to do a task

Algorithm – Working Definition

[As opposed to actually executing

the instructions]

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Algorithm -- Examples

• A cooking recipe

• Assembly instructions for a model

• The rules of how to play a game

• Directions for driving from A to B

• A knitting pattern

• A car repair manual

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Components of an Algorithm

• Variables and values

• Instructions

• Sequences

• Procedures

• Selections

• Repetitions

• Documentation

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Values

• Represent quantities, amounts or measurements

• May be numerical or alphabetical (or other things)

• Often have a unit related to their purpose

• Example:

– Recipe ingredients

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Variables

This jar

can contain

10 cookies

50 grams of sugar

3 slices of cake

etc.

Values Variable

• 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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Instructions – Examples

• Take off your shoes

• Count to 10

• Cut along dotted line

Directions to perform

specific actions on values

and variables.

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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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Values and Variables

Instruction (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_Thursday

{

Wake_up

Have_Shower

Eat_Breakfast

Drive_To_Uni

Sit_EE134_Lecture


Drive_From_Uni


}

procedure Do_Week

{

Do_Monday

Do_Tuesday

Do_Wednesday

Do_Thursday


}

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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 ( “Ali”, “Morning” )

customerService (“Ayşe”, “Afternoon” )

customerService ( “Veli”, “Night” )

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{

Wake_up

Have_Shower

Eat_Breakfast

Drive_To_Uni

Sit_EE134_Lecture


Drive_From_Uni


}

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

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{

Wake_up

Have_Shower

Eat_Breakfast

Drive_To_Uni

Sit_EE134_Lecture


Drive_From_Uni


}

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

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{

Wake_up

Have_Shower

Eat_Breakfast

Drive_To_Uni

Sit_EE134_Lecture


Drive_From_Uni


}

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

• Sub-routine

• Module

• Function

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Procedure (involving instructions)

• Selection (between 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

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Selection Example - Car Repair

if (motor turns)

then

{

CheckFuel

CheckSparkPlugs

CheckCarburettor

}

else

{

CheckDistributor

CheckIgnitionCoil

}

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Selection Example – Car Repair (cont)

if (motor turns)

then

{

CheckFuel

CheckSparkPlugs

CheckCarburettor

}

else

{

CheckDistributor

CheckIgnitionCoil

}

Should be a

true or false

condition.

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Selection Example -- Car Repair (cont)

if (motor turns)

then

{

CheckFuel

CheckSparkPlugs

CheckCarburettor

}

else

{

CheckDistributor

CheckIgnitionCoil

}

Sequence if

the condition

is true.

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Selection Example -- Car Repair (cont)

if (motor turns)

then

{

CheckFuel

CheckSparkPlugs

CheckCarburettor

}

else

{

CheckDistributor

CheckIgnitionCoil

}

Sequence if the

condition is

false.

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Selection – Several Conditions

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

then

{

Go to EE134 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 Thursday ) then { if ( the time is 10.00am ) then { Go to EE134 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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Selection – Several Courses of Action

• What if there are several courses of action?

if ( button pressed is 1 )

then

{

CheckAccountBalance

}

else

{


then

{

TransferFunds

}

else

{


then

{

PayBills

}

else

{


then

{

ExitPhoneBanking

}

else

{

say “Invalid option”

}

}

}

} Form 1

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Selection – Several Courses of Action (cont)


then

{

CheckAccountBalance

}

else if ( button pressed is 2 )

then

{

TransferFunds

}


then

{

PayBills

}


then

{

ExitPhoneBanking

}

else

{

say “Invalid option”

}

Form 2 “Cascaded”

selection.

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Selection (between 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 -- 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 ( )

}

}

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Repetition – Example (cont)


{

Phone(name)



ListenToReply ( ) start begging count at zero while ( reply is "No" and begging count < 100 ) { Say("Oh please!")


ListenToReply ( )

}

}

Condition is tested

before sequence

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{

Phone(name)



ListenToReply ( ) start begging count at zero while (reply is "No" and begging count < 100) { Say("Oh please!")


ListenToReply ( ) }

}

Sequence may not

get executed at all

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{

Phone(name)




add 1 to begging count ListenToReply ( )

}

}

Ensure initial

values of variables

used in the

conditions are set

correctly

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{

Phone(name)




add 1 to begging count ListenToReply ( )

}

}

Ensure the variables

used in the conditions

are updated in each

iteration

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{

Phone(name)



ListenToReply ( )

start begging count at zero

while (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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Repetition – Pre-tested Loop



while ( not successfully booked )

{




}

SighWithRelief pre-tested loop

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Repetition – Pre-tested Loop



until ( successfully booked )

{




}

SighWithRelief pre-tested loop

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Repetition – Post-tested Loop



do

{




} while ( not successfully booked )

SighWithRelief post-tested loop

Sequence is

executed at

least once

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Repetition – Post-tested Loop



repeat

{




} until ( successfully booked )

SighWithRelief

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Repetition -- Variations



loop

{




if ( successfully booked )

then

{

break loop

}

}

SighWithRelief

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Selection (between instructions)

Repetition (of 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 -- 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)

{




}

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

SighWithRelief

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CE131: Introduction to Algorithms - egec.yolasite.com · Introduction to Computing Lecture 00: Part 2 Introduction to Algorithms and Flowcharts ... •Algorithms •Flow Charts