Algorithm and pseudo codes

Algorithm and Pseudocodes

Prepared By:Mr. Richard R. BasilioBSECE – Dip ICT

Algorithm

What is Algorithm…?

By wikipedia definition:▪ an algorithm is a sequence of finite

instructions, often used for calculation and data processing.

▪ It is formally a type of effective method in which a list of well-defined instructions for completing a task will, when given an initial state, proceed through a well-defined series of successive states, eventually terminating in an end-state.

What is Algorithm…

Euclidean Algorithm

First documented algorithm by Euclid (300 B.C.) to compute greatest common divisor (gcd). Example: gcd(3,21)=3 Condition:

1. Let A and B be integers with A > B 0.

2. If B = 0, then the gcd is A and the algorithm ends.

3. Otherwise, find q and r such thatA = qB + r where 0 r < B

Note that we have 0 r < B < A and gcd(A,B) = gcd(B,r).Replace A by B, B by r. Go to step 2.

Euclidean Algorithm

Example No. 2: Find the greatest common divisor of

A=40, B=15; using Euclidean algorithm;

A = 2B + 10 A = 15 ; B = 10

A = 1B + 5 A = 10 ; B = 5

A = 2B + 0 A = 5 ; B = 0

gcd is 5

Properties of Algorithm

There are three properties of algorithm that must have to consider in solving a certain problem in programming: Finiteness Absence of Ambiguity Sequence Definition Input and Output Definition Effectiveness Scope of Definition


Finiteness The execution of a programmed

algorithm must be complete after a finite number of operations have been performed. Otherwise, we cannot claim that the execution produces a solution.


Absence of Ambiguity The representation of every step of an

algorithm should have a unique interpretation which also understand by the human.

It is convenient to deal with algorithms presented in notational with sparse detail:▪ Example:

▪ Pseudo code▪ Flowcharts


Sequence of Definition The sequence in which the steps of the

algorithm are to carried out should be clearly specified.

In algorithmic specifications, the instructions are performed from top to button, unless the instruction themselves otherwise specified.


Input and Output Definition Inputs – are the data items that is

presented in the algorithm. Outputs – are the data items presented

to the outside world as the result of the execution of a program based on the algorithm.

An algorithm ought to produce at least one output (otherwise, what use is it?...)


Effectiveness it consists of basic instructions that are

realizable. This means that the instructions can be performed by using the given inputs in a finite amount of time.

The instructions of an algorithm may order the computer only to perform tasks that is capable of carrying out.


Scope Definition An algorithm applies to the following:

▪ Specific problem or class of problem▪ The range of inputs has to be predefined▪ The range determines the generality of the

algorithm.

How to express an ALGORITHM?

Algorithms can be expressed in many kinds of notation, including: Natural language Pseudo Code Flowcharts Programming Language

Pseudocode

What is Pseudocode…?

“Pseudo” means imitation or false and “code” refers to the instructions written in a programming language.

Pseudocode is another programming analysis tool that is used for planning a program.

Pseudocode is also called Program Design Language (PDL).

What is Pseudocode…?

By wikipedia definition: Pseudocode is a compact and

informal high-level description of a computer programming algorithm that uses the structural conventions of some programming language, but is intended for human reading rather than machine reading.

Logical Structure of Pseudocode

Pseudocode is made up of the following logic structures that have been proved to be sufficient for writing any computer program: Sequence Logic Selection Logic Iteration Logic

Sequence Logic

It is used to perform instructions in a sequence, that is one after another.

Thus, for sequence logic, pseudocode instructions are written in an order in which they are to be performed.

The logic flow of pseudocode is from top to bottom.

Sequence Logic

Selection Logic

It is used for making decisions and for selecting the proper path out of two or more alternative paths in program logic.

It is also known as decision logic.Selection logic is depicted as either

an IF...THEN or an IF...THEN...ELSE structure.

Selection Logic

Iteration Logic

It is used to produce loops when one or more instructions may be executed several times depending on some of the conditions.

It uses structures called the DO_WHILE, FOR and the REPEAT_UNTIL.

Iteration Logic

Rules for Pseudocode

1. Write only one statement per line. Each statement in your

pseudocode should express just one action for the computer.

If the task list is properly drawn, then in most cases each task will correspond to one line of pseudocode.


Examples


2. Capitalized initial keyword. In the example above, READ and WRITE are in caps.

There are just a few keywords we will use: ▪ READ, WRITE, IF, ELSE, ENDIF, WHILE, ENDWHILE, REPEAT, UNTIL


3. Indent to show hierarchy. We will use a particular indentation

pattern in each of the design structures: ▪ SEQUENCE: keep statements that are

“stacked” in sequence all starting in the same column.

▪ SELECTION: indent the statements that fall inside the selection structure, but not the keywords that form the selection

▪ LOOPING: indent the statements that fall inside the loop, but not the keywords that form the loop


Examples:


4. End multi-line structures.▫All the initial keyword must always in line with

the last or end of the structure.5. Keep statement language independent.▫Resist the urge to write in whatever language

you are most comfortable with. There may be special features available in the language you plan to eventually write the program in; if you are SURE it will be written in that language, then you can use the features. If not, then avoid using the special features.


In summary: Write only one statement per line. Capitalized initial keyword. Indent to show hierarchy. End multi-line structures. Keep statement language

independent.

Standard for good pseudocode…

•These are follows:▫Number each instruction.

This is to enforce the notion, “well-ordered collection of ... operations.”

▫Each instruction should be unambiguous. It means the computing agent, in this case the

reader, should be capable of carrying out the instructions. And also, each instruction should be effectively computable (do-able).

▫Completeness. Nothing should be left out.

Advantage of Pseudocode…

Following are some of the advantages of using pseudocode: Converting a pseudocode to a

programming language is much more easier than converting a flowchart.

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

Limitations of Pseudocode…

It also suffers from some of the limitations. These limitations are as follows: In the cases of pseudocode, a graphic

representation of program logic is not available.

There are no standard rules to follow for using a pseudocode. Different programmers use their own style of writing pseudocode and hence, communication problem occurs due to lack of standardization.

Calculation Symbols

To symbolize the arithmetic operators we use these symbols: Note: There is a precedence or hierarchy

implied in this symbols.

Selection Symbols

When we have to make a choice between actions, we almost always base that choice on a test.

There is a universally accepted set of symbols used to represent these phrases:

Selection Symbols

Logical Operators

For the beginners…

It is more difficult to follow the logic of or write pseudocode as compared to flowcharting.

Examples…Prepare ½ crosswise yellow paper for seatwork after the discussion.

Algorithm and pseudo codes

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