COM111 Lecture
Schedule 11
Introduction to
Computer Programming, Algorithm, Flowchart, Decision Tables, Pseudo code and
Program Control Structures
Computer programming and languages
It can perform a variety of tasks like
receiving data, processing it, and producing useful results. A computer needs
to be instructed to perform even a simple task like adding two numbers.
Computers work on a set of instructions called computer program, which clearly
specify the ways to carry out a task. Computer programs are developed using
computer or programming languages. Programming involves many stages such as
task analysis, flowcharting, coding, program testing, implementation,
documentation and maintenance.
Developing
a program
A program consists of a series of
instructions that a computer processes to perform the required operation. In
order to design a program, a programmer must determine three basic rudiments:
1. The instructions to be performed.
2. The order in which those instructions
are to be performed.
3. The data required to perform those
instructions.
Program
Development Cycle
Development cycle of a program includes the following phases:
1.
Analyse/Define
the Problem: The problem is analysed precisely
and completely. Based on understanding, the developer knows about the scope
within which the problem needs to be developed.
2.
Task
analysis: The developer needs to develope various
solutions to solve the given problem. The optimum solution is chosen, which can
solve the problem comfortably and economically.
3.
Developing
Algorithm: After selecting the appropriate
solution, algorithm is developed to depict the basic logic of the selected
solution. An algorithm depicts the solution in logical steps. Algorithm is
developed by flowcharts and pseudocodes.
4.
Testing
the Algorithm for Accuracy: Before converting the
algorithms into actual code, it should be checked for accuracy. The main
purpose of checking algorithm is to identify major logical errors at an early
stage.
5.
Coding:
The actual coding of the programs takes place in the chosen programming
language. A program can be written by using computer languages of different
levels such as machine, assembly or high-level languages.
6.
Test
and Debug the program: A program compiler and
programmer-designed test data machine tests the code for syntax errors.
Depending upon the complexity of the program, several rounds of testing may be
required.
7.
Documentation:
Once the program is free from all the errors, it is the duty of the program
developers to ensure that the program is supported by suitable documentation.
Documenting a program enables the user to operate the program correctly. It
also enables other persons to understand the program clearly.
8.
Implementation:
After performing all the above mentioned steps, the program is installed on the
end user’s machine.
9.
Maintenance
and Enhancement: After the program is implemented,
it should be properly maintained taking care of the changing requirements of
its users and system. The program should be regularly enhanced by adding
additional capabilities.
Program
Development Cycle
ALGORITHM
Algorithm is a precise rule which
specifies how to solve some problem. An algorithm must be unambiguous and it
should reach a result after a finite number of steps. Algorithm can be
represented into programs, flowcharts, and pseudocodes.
To determine the largest
number out of three numbers A,B,C, the following algorithm may be used.
Step1: Start
Step2: Read three numbers say A,B,C
Step3: Find the larger number between A
and B and store it in MAX_AB
Step4: Find the larger number between
MAX_AB and C and store it in MAX
Step5: Display MAX
Step6: Stop
Example 1: Write an
algorithm to add two numbers
Example 2: Write an
algorithm to find simple arithmetic calculation
Example 3: Write an algorithm to find
area of circle
FLOWCHART
Flowchart is a means of visually
presenting the flow of data through an information processing system, the
operations performed within the system and the sequence.
SYMBOL
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SYMBOL NAME
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DESCRIPTION
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Flow Lines
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To connect symbols. These lines indicate the sequence of steps
and direction of flow.
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Terminal
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To represent the beginning the termination or halt in the program
logic.
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Input / Output
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Represents information entering or leaving the system, such as
customer order and servicing.
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Processing
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Process symbol is used for representing arithmetic and data
movement instructions.
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Decision
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Decision symbol denotes a decision to be made. This symbol has
one entry and two exit paths.
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Connector
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To join different flow lines.
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Off-page Connector
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To indicate that the flowchart continues on the next page.
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Document
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To represent a paper document produced during the flowchart
process.
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Annotation
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To provide additional information about another flowchart symbol.
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Manual Input
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Represents input to be given by a developer/programmer.
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Manual Operation
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Shows that the process has to be done by a developer/programmer.
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Online Storage
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Represents the online data storage such as hard disks, magnetic drums,
or other storage devices.
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Offline Storage
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Represents the offline data storage such as sales on OCR and data
on punched cards.
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Communication Link
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To represent data received or to be transmitted from an external
system.
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Magnetic Disk
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To represent is used to represent data input or output from and
to a magnetic disk.
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Guidelines for Preparing Flowcharts:
The
following guidelines should be used for creating a flowchart:
1.The flowchart should be clear, neat, and
easy to follow.
2.The flowchart must have a logical start
and finish.
3.In drawing a proper flowchart, all
necessary requirements should be listed in logical order.
4.The direction of the flow of a procedure
should always be from left to right or top to bottom.
5.Only one flow line should come out from
a process symbol.
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or
6.Only one flow line should enter a decision symbol. However two or
three flow lines may leave the decision symbol.
<0 >0
=0
7.Only one flow line is used with a terminal symbol. |
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This process will add two numbers , store the value in
9.In case of complex flowcharts, connector symbols are used to
reduce the number of flow lines.
10.Intersection of flow lines should be avoided to make it a more
effective and better wayof representing communication.
1.It is useful to test the validity of the flowchart by passing
through it with normal/unusual test data.
Benefits of
Flowcharts:
v Makes
Logic Clear: To plan a task,
it provides a pictorial representation of the task, which makes the logic
easier to follow.
v Communication: Being a graphical representation of a
problem solving logic, flowcharts are better way of communicating the logic of
a system to all concerned.
v Effective
analysis: With the help
of a flowchart, the problem can be analysed in an effective way.
v Useful
in coding: Once the
flowcharts are ready, the programmers can plan the coding process effectively.
v Proper
Testing and Debugging: A
flowchart helps in detecting the errors in a program, as the developers know
exactly what the logic should do.
v Appropriate
Documentation: Flowcharts
serve as a good program documentation tool.
Limitations of Flowcharts:
Flowchart can be used for designing the basic concept of the program in
pictorial form but cannot be used for programming purposes. Some of the
flowchart limatations are given below:
v Complex: The major disadvantage in using
flowcharts is that when a program is very large, the flowcharts may continue
for many pages, making them hard to
follow.
v Costly: If flowcharts are to be drawn for a
huge application, the time and cost factor of program development may get out
of proportion, making it a costly affair.
v Difficulty
to Modify: Any changes or
modification to a flowchart usually requires redrawing the entire the entire
logic again, and redrawing a complex flowchart is not a simple task.
v No
update: Usually
programs are updated regularly. However, the corresponding update of flowcharts
may not take place, especially in the case of large programs.
DECISION TABLES
A decision table is a way of representing an algorithm in a tabular form.
It contains all the possible conditions for a specific problem and the
corresponding results using condition rules that connect conditions with the
results. It consists of rows and columns separated into four separate
quadrants.
·
Conditions:
Contains a list of all the possible conditions pertaining to a problem.
·
Condition
Alternatives: Contains the condition rules (set of possible values for each
condition) of alternatives.
·
Actions:
Contains actions, which can be a procedure or operation to be performed.
·
Action
Entries: Contains the action rules, which specify the actions to be performed
on the basis of the set of condition alternatives corresponding to that action
entry.
Each column in the quadrant can be interpreted as a processing rule. The
basic four-quadrant structure is common for all decision tables; however, they
may differ in the way of representation of the condition alternatives and
action entries. For example, in some decision tables, Yes/No or numbers (0 or
1) may be used. Similarly, in decision tables, action entries are represented
by placing check marks, cross marks or numbering the actions to be performed.
Steps to develop a decision table
In order to build the decision tables, first the maximum size of the
table is determined. Any impossible situations, inconsistencies or redundancies
are then eliminated from the table. The steps to be followed for developing the
decision table are:
1. Determine the actions that can be associated with a
specific module or procedure.
2. Determine all the conditions affecting the decision.
3. Associate the specified conditions with specific
actions. In the simplest case each condition in the decision table will be
associated with two alternatives (Yes or No)
4. Define rules by indicating actions that occur for
the specified conditions.
Decision table for finding the largest of three numbers:
Conditions
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Rules
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1
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2
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3
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A > B
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Y
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N
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-
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A > C
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Y
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-
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N
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B > C
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-
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Y
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N
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Actions
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A is largest
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X
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B is largest
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X
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C is largest
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X
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PSEUDOCODE
Psedocode is a generic way of describing
an algorithm without the use of any specific programming language-related
notations. It is an outline of a program, written in a form, which can easily
be converted into real programming statements.
For Example the pseudocode given below
calculates the area of a rectangle.
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Pseudocode uses some keywords to denote
programming processes. Some of them are:
v Input: READ, OBTAIN, GET, and PROMPT
v Output: PRINT, DISPLAY, and SHOW
v Compute: COMPUTE, CALCULATE, and DETERMINE
v Initialise: SET and INITIALISE
v Add
One: INCREMENT
Psedocode Guidelines:
1. Statements should be written in simple
English.
2. Steps must be understandable.
3. Pseudocodes should be concise.
4. Each instruction should be written in a
separate line.
5. Capitilise keywords such as READ, PRINT,
and so on.
6. Each set of instructions is written from
top to bottom.
7. It should allow for easy transition from
design to implementation.
Benefits of
Pseudocode:
v It allows the developer to express the
design in plain natural language.
v It is easier to develop a program from a
pseudocode than with a flowchart.
v It is easy to translate pseudocode into
a programming language, by less experienced programmers.
v Its simple structure and readability
makes it easier to modify.
v Pseudocodes allow programmers who work
in different computer languages to talk to each other.
Limitations of
Pseudocode:
·
It
does not provide visual representation of the programs’s logic.
·
There
are no accepted standards for writing pseudocodes. Programmers use their style
of writing pseudocode.
·
Pseudo
code cannot be compiled nor executed, and there are no real formatting or
syntax rules.
Program statements
that affect the order in which statements are executed, or that affect whether
statements are executed are called control structures.
Control structures are used
to express algorithms, flowcharts, and pseudocodes as actual computer programs.
There are three control structures:
1. Sequence, where information flows in a straight
line.
2. Selection, Where the decisions are made according
to some predefined condition.
3. Repetition, where the logic is repeated in a loop
until the desired output is obtained.
1. Sequence Control Structure:
In a sequence structure,
the instructions to be computed simply follow one another in a logical
progression. This structure is denoted by writing one action after another,
each action on a line by itself, and all actions aligned with the logical indent.
The actions are performed in the same sequence in which they are written.
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Action
1
Action 2
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Action N
Flowchart for Pseudocode for
Sequence construct Sequence construct
Sequence Control
Structure
Example Flowchart
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Step By Step Instructions
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Begin the flowchart.
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Indicate value for memory variables X and Y.
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Add X and Y store the result in variable Z.
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Print the value of variable Z as output.
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End the flowchart.
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2.Selection Control Structure:
A selection structure allows the program
to make a choice between two alternate paths, whether it is true or false. The
first statement of a selection structure is a conditional statement. Thus the
selection structure has a only a single entry and a single exit.
Flowchart Pseudocode 
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List
of Actions
ELSE THEN ELSE
List of Different Actions
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Flowchart for Pseudocode
for
Selection construct
Selection construct
Selection Control Structure
Example
Flowchart
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Step By Step Instructions
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 NO YES
   
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Begin the
flowchart.
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Read two
numbers and store the value in memory variables A and B respectively.
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Check
whether the value of A is greater than the value of B.
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If A is
greater than B then display A, otherwise display B.
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Connector
Symbols are used to join the flowlines.
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End the
flowchart.
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3.
Repetition Control
Structure:
Repetition or loop
pattern causes an interruption in the normal sequence of processing. It directs
the system to loop back to a previous statement in the program, repeating the
same sequence over and again, usually with new data. Using looping, the
programmer avoids writing the same set of instructions again.
Flowchart
Pseudocode
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REPEAT
False Sequence 1
Sequence 2 
True
Seqence
N
UNTIL Condition is false
Flowchart for Pseudocode for
Repetition construct
Repetition construct
Repetition Control
Structure
Example Flowchart
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Step-by-step Instructions
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True
False
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Begin the
flowchart.
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Initialise
the memory variable COUNT to zero.
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Increment
the value of COUNT by 1.
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Print the
value INDIA.
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Check
whether the value of COUNT is less than or equal to 10. If yes, then loop
back to the third step otherwise go to the next step.
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End the
flowchart.
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