Writing Pseudo Code

MSIT

Agenda History of Algorithm Definition of Algorithm Algorithm criteria Different ways to write Algorithm What is Pseudo Code? Need for Pseudo Code. Pseudo Code Standards. Different Constructs.

History Of Algorithm The word algorithm comes from the name

of a Persian author ,Abu Ja’far Mohammed bin Musa al Khowarazimi(825 A.D.).

This word has taken on a special significance in computer science, where “algorithm” has come to refer to a method that can be used by a computer for the solution of a problem.

That is what makes algorithm different from words such as process, technique, or method.

Definition of Algorithm An algorithm is a finite set of instructions that, if

followed ,accomplishes a particular task.

All algorithms must satisfy following criteria.1. Input2. Output3. Definiteness4. Finiteness5. Effectiveness

Algorithm criteria’s..1. Input: Zero or more inputs externally supplied.2. Output: At least one quantity is produced.3. Definiteness: Each instruction is clear and

unambiguous.4. Finiteness: If we trace out the instructions of

an algorithm , then for all the cases, the algorithm terminates after a finite number of steps.

5. Effectiveness: Every instruction must be very basic so that it can be carried out, in principle, by a person using only pencil and paper.

Contd.. According to criteria 1 and 2 algorithm

needs to produce one or more outputs for the given zero or more inputs.

According to criteria 3 each operation should be definite , that it must be very clear about what should be done. For examples: Directions such as “add 6 or 7 to

x” or “compute 5/0” are not permitted because it is not clear which of the two possibilities should be done or what the result is.

Contd.. Criteria 4 says that they terminate after finite

number of operations. A reasonable consideration is that time for

termination should be reasonably short. Consider an algorithm that decides whether any given

position in game of chess is a winning position. This algorithm works by examining all possible moves

and countermoves that could be made from starting position.

The problem with this algorithm is that even using the most sophisticated modern computers, it takes billions of years to make the decision.

Contd.. According to criteria 5, each operation should be

effective.

That it can be done by a person using a pencil and paper.

For example: Performing arithmetic operations on integers is an

example of effective operation. But arithmetic operations on real numbers violates

effectiveness. As each value may be expressible only by infinitely long

decimal expansions. Adding such values violates effectiveness.

Contd.. We can describe an algorithm in many

ways .

We can use natural language like English, if we choose this operation , the resulting operations instructions are definite.

Graphical representations called Flow Charts are another possibility, but they work well only if the algorithm is small and simple.

Difference between Pseudo Code & Flowchart Algorithm Lamp: Begin: If Lamp_plugged in then,

If Bulb_burned out then,Replace bulb

Else buy new bulb.Else plug in lamp.End:

Flow ChartPseudo Code

Pseudo Code Pseudo code consists of short, English phrases

used to explain specific tasks within a program's algorithm .

Pseudo code is a kind of structured English for describing algorithms.

It allows the designer to focus on the logic of the algorithm without being distracted by details of language syntax. 

At the same time, the pseudo code needs to be complete. 

It describe the entire logic of the algorithm so that implementation becomes a rote mechanical task of translating line by line into source code.

Contd.. In general the vocabulary used in the pseudo code should

be the vocabulary of the problem domain, not of the implementation domain.

The pseudo code is a narrative for someone who knows the requirements (problem domain) and is trying to learn how the solution is organized.  E.g. Extract the next word from the line (good)

set word to get next token (poor) Append the file extension to the name (good)

name = name + extension (poor) FOR all the characters in the name (good)

FOR character = first to last (ok)

Need for pseudo code The programming process is a complicated one.

You must first understand the program specifications, then you need to organize your thoughts and create the program.

This is a difficult task when the program is not easy.

You must break the main tasks that must be accomplished into smaller ones in order to be able to eventually write fully developed code.

Writing pseudo code will save you time later during the construction & testing phase of a program's development.

Pseudo code standards Each textbook and each individual designer may

have their own personal style of pseudo code.

Pseudo code is not a rigorous notation, since it is read by other people, not by the computer.

There is no universal "standard" for the industry, but for instructional purposes it is helpful if we all follow a similar style.

The following format is recommended for expressing your solutions.

standard constructsThese are the basic constructs for flow of control are

sufficient to implement any "proper" algorithm.

SEQUENCE is a linear progression where one task is performed sequentially after another.

WHILE is a loop (repetition) with a simple conditional test at its beginning.

IF-THEN-ELSE is a decision (selection) in which a choice is made between two alternative courses of action.

REPEAT-UNTIL is a loop with a simple conditional test at the bottom.

CASE is a multiway branch (decision) based on the value of an expression. CASE is a generalization of IF-THEN-ELSE.

FOR is a "counting" loop.

Sequence Sequential control is indicated by writing one action after another,

each action on a line by itself. The actions are performed in the sequence (top to bottom) that they

are written. Example (non-computer)

Brush teeth Wash face Comb hair Smile in mirror

Example (computer) READ height of rectangle

READ width of rectangle COMPUTE area as height times width

Several keywords are often used to indicate common input, output, and processing operations.

Input: READ, OBTAIN, GET Output: PRINT, DISPLAY, SHOW Compute: COMPUTE, CALCULATE, DETERMINE Initialize: SET, INIT Add one: INCREMENT, BUMP

IF-THEN-ELSE Binary choice on a given Boolean condition is indicated by the use

of four keywords: IF, THEN, ELSE, and ENDIF. The general form is:

IF condition THEN sequence 1

ELSE sequence 2ENDIF

The ELSE keyword and "sequence 2" are optional. If the condition is true, sequence 1 is performed, otherwise sequence 2 is performed. Example

IF HoursWorked > NormalMax THENDisplay overtime messageELSE Display regular time messageENDIF

WHILE The WHILE construct is used to specify a loop with a test at the

top. The beginning and ending of the loop are indicated by two

keywords WHILE and ENDWHILE. The general form is:

WHILE condition Sequence

ENDWHILE

The loop is entered only if the condition is true. The "sequence" is performed for each iteration. At the conclusion

of each iteration, the condition is evaluated and the loop continues as long as the condition is true.

Example WHILE Population < Limit

Compute Population as Population + Births – Deaths

ENDWHILE

CASE A CASE construct indicates a multiway branch based on

conditions that are mutually exclusive. Four keywords, CASE, OF, OTHERS, and ENDCASE, and

conditions are used to indicate the various alternatives. The general form is:

CASE expression OF condition 1 : sequence 1 condition 2 : sequence 2 ... condition n : sequence n OTHERS: default sequence

ENDCASE

The OTHERS clause with its default sequence is optional. Conditions are normally numbers or characters

Case indicating the value of "expression", but they can be English

statements or some other notation that specifies the condition under which the given sequence is to be performed. Examples:

        CASE  Title  OF                Mr      : Print "Mister"                Mrs     : Print "Missus"                Miss    : Print "Miss"                Ms      : Print "Mizz"                Dr      : Print "Doctor"        ENDCASE

        CASE  grade  OF                A       : points = 4                B       : points = 3                C       : points = 2                D       : points = 1                F       : points = 0        ENDCASE

REPEAT-UNTIL This loop is similar to the WHILE loop except that the test is

performed at the bottom of the loop instead of at the top.

Two keywords, REPEAT and UNTIL are used. The general form is:

REPEAT Sequence

UNTIL condition

The "sequence" in this type of loop is always performed at least once, because the test is performed after the sequence is executed.

At the conclusion of each iteration, the condition is evaluated, and the loop repeats if the condition is false.

The loop terminates when the condition becomes true.  

For Loop This loop is a specialized construct for iterating a specific

number of times, often called a "counting" loop.  Two keywords, FOR and ENDFOR are used. The general

form is:

FOR iteration bounds Sequence

ENDFOR

it is best to describe the loop using problem domain vocabulary. Examples: FOR each month of the year (good) FOR month = 1 to 12 (ok) FOR each employee in the list (good) FOR empno = 1 to listsize (ok)

NESTED CONSTRUCTS The constructs can be embedded within each

other, and this is made clear by use of indenting. Nested constructs should be clearly indented

from their surrounding constructs. Example SET total to zero REPEAT READ Temperature

IF Temperature > Freezing THEN     INCREMENT total END IF

UNTIL Temperature < zero Print total

In the above example, the IF construct is nested within the REPEAT construct, and therefore is indented.

INVOKING SUBPROCEDURES In computer science, a subroutine (function, procedure, or

subprogram) is a sequence of code which performs a specific task, as part of a larger program, and is grouped as one or more statement blocks.

Subroutines can be "called", thus allowing programs to access the subroutine repeatedly without the subroutine's code having been written more than once. ...

Use the CALL keyword to invoke sub procedures or sub-routines.

Example to invoke subroutines. Example: Algorithm adding

Begin:Read a;Read b;Call sum(a,b);//invoking sub-routineend

Algorithm Sum(a,b)Begin:Set c=a +b;Display c;End:

Exception Handling Handling of errors in the code manually is called

Exception Handling.

BEGIN         statements     EXCEPTION         WHEN exception type             statements to handle exception        WHEN another exception type             statements to handle exception    END

Recursive Algorithms Calling a sub-routine within itself is called

recursive sub-routine.

Algorithm RSum(a,n){

If (n<=0) then return 0.0;Else return RSum(a,n-1)+a[n];//calling subroutine within itself.

}

Example: Algorithm Fibonacci (n) //compute the nth Fib`onacci number Begin: if n<=1 then Display n;//write output to console else set n1=0; set n2=1; for i=2 to n do set res=n1+n2; n1=n2; n2=res; display res;//writing the result to console End:

Example Algorithm Fibonacci(n) //compute the nth Fib`onacci number { if(n<=1) then write(n);//write output to console else { n1:=0;n2:=1; for i:=2 to n do { res:=n1+n2; n1:=n2; n2:=res; } write(res);//writing the result to console } }

The same example written in 2 convetnions There is no specific convention to write Pseudo Code. Pseudo Code is meant for the user not for the system

References Text book Fundamentals of Computer

Algorothms(Galgotia) By. ELLIS HOROWITZ SARTAJ SAHNI SANGUTHEVAR RAJASEKARAN

http://en.wikipedia.org/wiki/Pseudocode http://www.csc.calpoly.edu/~jdalbey/SWE/pdl_std.

html http://www.wiley.com/college/busin/icmis/oakman

/outline/chap05/slides/pseudo.htm http://www.webopedia.com/TERM/P/pseudocode.h

tml