Algorithm and pseudo codes

Prepared By:
Mr. Richard R. Basilio
BSECE – Dip ICT

 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.

 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 that
A = 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.

 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

 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.

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

 “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).

 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.

 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

 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.

 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.

 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.

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.

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

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.

• 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.

 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.

 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.

 To symbolize the arithmetic operators we use
these symbols:
 Note: There is a precedence or hierarchy implied
in this 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:

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

Prepare ½ crosswise yellow paper for
seatwork after the discussion.

Algorithm and pseudo codes

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Algorithm and pseudo codes