Transcript Writing PsuedoCode

Writing Pseudo Code
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.
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Definition of Algorithm
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An algorithm is a finite set of instructions that, if
followed ,accomplishes a particular task.
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All algorithms must satisfy following criteria.
1.
2.
3.
4.
5.
Input
Output
Definiteness
Finiteness
Effectiveness
Algorithm criteria’s..
1.
2.
3.
4.
5.
Input: Zero or more inputs externally supplied.
Output: At least one quantity is produced.
Definiteness: Each instruction is clear and
unambiguous.
Finiteness: If we trace out the instructions of
an algorithm , then for all the cases, the
algorithm terminates after a finite number of
steps.
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.

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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..
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Criteria 4 says that they terminate after finite
number of operations.
A reasonable consideration is that time for
termination should be reasonably short.
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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..
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According to criteria 5, each operation should be
effective.

That it can be done by a person using a pencil
and paper.
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For example:
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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
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Algorithm Lamp:
Begin:
If Lamp_plugged in then,
If Bulb_burned out then,
Replace bulb
Else buy new bulb.
Else plug in lamp.
End:
Pseudo Code
Flow Chart
Pseudo Code
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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..
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In general the vocabulary used in the pseudo code should
be the vocabulary of the problem domain, not of the
implementation domain.
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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.
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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.
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This is a difficult task when the program is not easy.
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You must break the main tasks that must be accomplished
into smaller ones in order to be able to eventually write
fully developed code.
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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.
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The following format is recommended for
expressing your solutions.
standard constructs
These are the basic constructs for flow of control are
sufficient to implement any "proper" algorithm.
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SEQUENCE is a linear progression where one task is
performed sequentially after another.
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WHILE is a loop (repetition) with a simple conditional test at
its beginning.
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IF-THEN-ELSE is a decision (selection) in which a choice is
made between two alternative courses of action.
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REPEAT-UNTIL is a loop with a simple conditional test at the
bottom.
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CASE is a multiway branch (decision) based on the value of an
expression. CASE is a generalization of IF-THEN-ELSE.
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FOR is a "counting" loop.
Sequence
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Sequential control is indicated by writing one action after another,
each action on a line by itself.
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Example (non-computer)
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Brush teeth
Wash face
Comb hair
Smile in mirror
Example (computer)
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The actions are performed in the sequence (top to bottom) that they
are written.
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.
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Input: READ, OBTAIN, GET
Output: PRINT, DISPLAY, SHOW
Compute: COMPUTE, CALCULATE, DETERMINE
Initialize: SET, INIT
Add one: INCREMENT, BUMP
IF-THEN-ELSE
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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 2
ENDIF
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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 THEN
Display overtime message
ELSE Display regular time message
ENDIF
WHILE
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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
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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.
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Example
WHILE Population < Limit
Compute Population as Population + Births – Deaths
ENDWHILE
CASE
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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
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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:
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CASE Title
Mr
Mrs
Miss
Ms
Dr
ENDCASE
OF
: Print "Mister"
: Print "Missus"
: Print "Miss"
: Print "Mizz"
: Print "Doctor"
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CASE grade OF
A
: points = 4
B
: points = 3
C
: points = 2
D
: points = 1
F
: points = 0
ENDCASE
REPEAT-UNTIL
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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.
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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.
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At the conclusion of each iteration, the condition is evaluated,
and the loop repeats if the condition is false.
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The loop terminates when the condition becomes true.
For Loop
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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:
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FOR
FOR
FOR
FOR
each month of the year (good)
month = 1 to 12 (ok)
each employee in the list (good)
empno = 1 to listsize (ok)
NESTED CONSTRUCTS
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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. ...
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Use the CALL keyword to invoke sub procedures or subroutines.
Example to invoke subroutines.
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Example:
Algorithm adding
Begin:
Read a;
Read b;
Call sum(a,b);//invoking sub-routine
end
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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
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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:
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Algorithm Fibonacci (n)
//compute the nth Fib`onacci number
Begin:
if n<=1 then
Display n;//write output to console
else
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set n1=0;
set n2=1;
for i=2 to n do
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set res=n1+n2;
n1=n2;
n2=res;
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display res;//writing the result to console
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End:
Example
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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
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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_st
d.html
http://www.wiley.com/college/busin/icmis/oakma
n/outline/chap05/slides/pseudo.htm
http://www.webopedia.com/TERM/P/pseudocode.
html