bsse lecture 2

8/4/2019 BSSE Lecture 2

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Fundamentals ofAlgorithmds

Fundamentals of Algorithms

LECTURE 02

IIT UST BANNU


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Review of the Previous Lecture

What is computer Science?

Some Misconception about Computer Science.

What is an algorithm and how it is related toComputer Science?

Types of Operations in Algorithms.

Example of Algorithms in Real Life e.g. HumanGenome project, Robots etc.


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The Parts of an Algorithm

It is worthwhile to think about these algorithmrelated terms because we can rely on thesewhen we write new algorithms. Common parts ofalgorithms include

1.Variables: Named Values 2.Parameters: Named Inputs 3.Conditionals: Handling Different

Conditions

4.Repetition 5.Subroutines: Named Helper Algorithms 6.Recursion: Helping Yourself
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Variables: Named Values

As we write algorithms, we like to name things.

Sometimes we use long names, Sometimes, we use shorter names,

As we start to write more formal algorithms, we

may explicitly say that we're using names.


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Parameters: Named Inputs

Many algorithms work on data that are passed tothe algorithm.

Similarly, an algorithm to find a name in thephone book might take the name and the phonebook as input values.

We call such input valuesparameters.


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Conditionals: Handling Different

Conditions

At times, our algorithms have to account fordifferent conditions, doing different thingsdepending on those conditions.

We call such operations conditionals. Theytypically take either the form


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Conditionals: Handling Different

Conditions

ifsome condition holdsthen do something

of the more complex form

ifsome condition holdsthen do somethingotherwisedo something else

At times, we need to decide between more than

two possibilities.


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Repetition

At times, our algorithms require us to dosomething again and again.

. We call this technique repetition. Repetitiontakes many forms.

We might do work until we've reached a desiredstate.


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Repetition

Repeat some actionuntil some condition holds

We might continue work as long as we're insome state.

repeat some actionwhile some condition holds

We might repeat an action a fixed number oftimes.

repeat some actionNtimesYou can probably think of other forms of

repetition.


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Subroutines: Named Helper

Algorithms

Many algorithms require common actions fortheir operation.

We can write additional algorithms for thesecommon actions and use them as part of ourbroader algorithm.

We can also use them in other algorithms. Wecall these helper algorithms subroutines.

Just like functions in C++


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Recursion: Helping Yourself

As strange as it may seem, we sometimes find ituseful to define an algorithm in terms of itself.

In particular, we can have an algorithm deal witha large or complex input by using itself as ahelper with a smaller or less complex input.


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Consider the following example

To make N peanut butter and jelly sandwiches

If N is 0 then conditional Celebrate, you're done!

Otherwise

Make one PBJ sandwich subroutine Make N-1 PBJ sandwiches recursion


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We call this technique recursion. Masteringrecursion is one of the key steps on your path tobecoming a computer scientist.


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Characteristics of an algorithm

Let us try to present the scenario of a man brushing his ownteeth(natural denture) as an algorithm as follows.

Step 1. Take the brush

Step 2. Apply the paste

Step 3. Start brushing

Step 4. Rinse

Step 5. Wash

Step 6. Stop


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Characteristics of Algorithms

An Algorithm should have the following fivecharacteristics:

1. Input

2. Output

3. Definiteness

4. Effectiveness 5. Termination


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Finiteness:

It means that the algorithm terminates after afinite number of steps


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Definiteness:

Should have well defined and unambiguouslyspecified steps.


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Input:

The algorithm should be given valid inputs andclearly specified.

E.g. Student Record Search Algorithm


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Output:

It means that it can be proved to produce thecorrect output given a valid input


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Effectiveness:

steps are sufficiently simple and basic.

It should effectively solve the problem within

limited resources.


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Termination:

It means that the algorithm should terminateafter completion of its task or failure (optional).


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Ways of Expressing Algorithms

Pseudo code

Flowchart


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Whats Pseudocode ?

Artificial and Informal language

Helps programmers to plan an algorithm Similar to everyday English

Not an actual programming language


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E.g : Pseudocode

Read A, B

Calculate C = A*B

Display CStop


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Comparative Between Flowchart vs

Pseudocode

Flowchart

A graphical way of writing pseudocode

Rounded rectangleterminal

Parallelograminput / output

Rectangleactions

Diamondsdecision / conditional

Circlesconnector


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E.g : Flowchart

Start

Stop

Read ARead B

Display theResult C

Calculate ResutC=A*B

Start Terminal.

Program start

here

Stop Terminal

Program end

here

Input.

Enter values for

A and B

Process

Output


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Comparative Between Flowchart vs

Pseudocode (..Cont.)

Pseudocode

No syntax ruleIndependent from anyprogramming language

Write in an ordinary language

Uses a structure resembling computer structure

No connector between pages


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E.g : Pseudocode

Read A, BInput

Calculate C = A*B -Action

Display C - OutputStop -Terminal


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Example 2 (Selection)

Read A, BIF A is less than B

BIG = B

SMALL = AELSE

BIG = A

SMALL = BWrite / Display BIG, SMALLStop


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Example 2 (Selection)

Read A, B - InputIF A is less than B - Selection

BIG = B -Action

SMALL = A -ActionELSE - Selection

BIG = A - Action

SMALL = B -ActionWrite / Display BIG, SMALL - OutputStop -Terminal


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Example 3 (Repetition)

Set count to zeroSet total to zero

Read number

WHILE ( not end-of-data )

increment count by 1

total = total + number

read number

IF ( count > 0 ) thenaverage = total / count

Display average

Stop


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Advantages

Converting a pseudocode to a programminglanguage is much more easier than converting aflowchart.

As compared to flowchart, it is easier to modifya pseudocode of a program logic when programmodifications are necessary.

Easily modifiedImplements structured concepts

Done easily on Word Processor


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Limitations

In the cases of pseudocode, a graphicrepresentation of program logic is not available.

There are no standard rules to follow for using apseudocode.

Different programmers use their own style of writingpseudocode; and hence,

Communication problem occurs due to lack ofstandardization.


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Basic Algorithm Writing Techniques


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Rules for Pseudocode

Write only one statement per line Capitalize initial keyword Indent to show hierarchy End multiline structures Keep statements language independent


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One Statement Per Line

Each statement in pseudocode should express justone action for the computer. If the task list isproperly drawn, then in most cases each task willcorrespond to one line of pseudocode.

Task List

Read name, hours worked, rate of pay

Perform calculations

gross = hours worked * rate of pay

Write name, hours worked, gross

Pseudocode

READ name, hoursWorked, payRate

gross = hoursWorked * payRate

WRITE name, hoursWorked, gross


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Capitalize Initial Keyword

In the example below note the words: READ and WRITE.These are just a few of the keywords to use, others include:READ, WRITE, IF, ELSE, ENDIF, WHILE, ENDWHILE

Pseudocode

READ name, hoursWorked, payRate

gross = hoursWorked * payRate

WRITE name, hoursWorked, gross


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Indent to Show Hierarchy

Sequence:Keep statements in sequence all starting in the same column

Selection:Indent statements that fall inside selection structure, but not the keywords that form theselection Loop:

Indent statements that fall inside the loop but not keywords that form the loop

Each design structure uses a particular indentationpattern

READ name, grossPay, taxes

IF taxes > 0

net = grossPay taxes

ELSE

net = grossPay

ENDIF

WRITE name, net


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End Multiline Structures

See the IF/ELSE/ENDIF as constructed above,the ENDIF is in line with the IF.The same applies for WHILE/ENDWHILE etc

READ name, grossPay, taxes

IF taxes > 0

net = grossPaytaxes

ELSE

net = grossPay

ENDIF

WRITE name, net


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Language Independence

Resist the urge to write in whatever language you are mostcomfortable with, in the long run you will save time.

Remember you are describing a logic plan to develop a

program, you are not programming!


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The Selection Structureamount < 100

interestRate = .06 interestRate = .10

yes no

IF amount < 100

interestRate = .06

ELSE

Interest Rate = .10ENDIF

Pseudocode


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The Looping StructureIn flowcharting one of the more confusing things is to separate selection

from looping.

This is because each structure use the diamond as their control symbol. Inpseudocode we avoid this by using specific keywords to designate

looping

WHILE/ENDWHILEREPEAT/UNTIL

Ti W iti G d P d C d


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Fundamentals of

Tips on Writing Good Pseudo Code Use indention for improved clarity

Do not put code in pseudo code make yourpseudo code language independent

Dont write pseudo code for yourselfwrite it in anunambiguous fashion so that anyone with areasonable knowledge can understand and implementit

Be consistent

Prefer formulas over English language descriptions

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