Software engg lect1

Software Engineering
(LECT 1)

Dr. M. Sarma

1

Organization of this Lecture:
• What is Software Engineering?
• Programs vs. Software Products
• Evolution of Software Engineering
• Notable Changes In Software
Development Practices
• Introduction to Life Cycle Models
• Summary

2

What is Software Engineering?

• Engineering approach to develop
software.
–Building Construction Analogy.
• Systematic collection of past
experience:
–techniques,
–methodologies,
–guidelines.

3

Engineering Practice

Heavy use of past experience:
Past experience is systematically
arranged.
Theoretical basis and quantitative
techniques provided.
Many are just thumb rules.
Tradeoff between alternatives
Pragmatic approach to cost-
effectiveness 4

Technology Development Pattern

Engineering

Technology

Craft
Esoteric Systematic Use of Past
Past Experience and Scientific
Experience Basis
Unorganized Use
of
Art Past Experience
Time

5

Why Study Software Engineering? (1)

• To acquire skills to develop large
programs.
– Exponential growth in complexity and
difficulty level with size.
– The ad hoc approach breaks down when
size of software increases

6


• Ability to solve complex programming
problems:
– How to break large projects into smaller and
manageable parts?
• Learn techniques of:
– specification, design, interface
development, testing, project
management, etc.

7


• To acquire skills to be a
better programmer:
• Higher Productivity
• Better Quality Programs

8

Software Crisis
• Software products:
–fail to meet user requirements.
–frequently crash.
–expensive.
–difficult to alter, debug, and enhance.
–often delivered late.
–use resources non-optimally.

9

Software Crisis (cont.)

Hw cost
Sw cost

1960 Year 1999
Relative Cost of Hardware and Software

10

Factors contributing to the software crisis

• Larger problems,
• Lack of adequate training in
software engineering,
• Increasing skill shortage,
• Low productivity improvements.

11

Programs versus Software Products

• Usually small in size • Large
• Author himself is sole • Large number of users
user • Team of developers
• Single developer • Well-designed interface
• Lacks proper user • Well documented &
interface user-manual prepared
• Lacks proper • Systematic development
documentation
• Ad hoc development.

12

Emergence of Software Engineering

• Early Computer Programming
(1950s):
–Programs were being written in
assembly language.
–Programs were limited to about a
few hundreds of lines of assembly
code.

13

Early Computer Programming (50s)

• Every programmer developed his
own style of writing programs:
–according to his intuition
(exploratory programming).

14

High-Level Language
Programming (Early 60s)

High-level languages such as
FORTRAN, ALGOL, and
COBOL were introduced:
This reduced software
development efforts greatly.

15

High-Level Language
Programming (Early 60s)

Software development style
was still exploratory.
Typical program sizes were limited to a few thousands of lines of
source code.

16

Control Flow-Based Design (late 60s)
• Size and complexity of programs
increased further:
–exploratory programming style
proved to be insufficient.
• Programmers found:
–very difficult to write cost-effective
and correct programs.

17

• Programmers found:
– programs written by others very difficult
to understand and maintain.
• To cope up with this problem,
experienced programmers advised:
``Pay particular attention to the
design of the program's control
structure.'’

18


• A program's control structure indicates:
– the sequence in which the program's
instructions are executed.
• To help design programs having good
control structure:
– flow charting technique was developed.

19


• Using flow charting technique:
–one can represent and design a
program's control structure.
–Usually one understands a
program:
• by mentally simulating the
program's execution sequence.

20

Control Flow-Based Design
(Late 60s)

• A program having a messy
flow chart representation:
–difficult to understand and
debug.

111 21

Control Flow-Based Design (Late 60s)

• It was found:
–GO TO statements makes control
structure of a program messy
–GO TO statements alter the flow of
control arbitrarily.
–The need to restrict use of GO TO
statements was recognized.

22


• Many programmers had extensively
used assembly languages.
– JUMP instructions are frequently used
for program branching in assembly
languages,
– programmers considered use of GO TO
statements inevitable.

23

Control-flow Based Design (Late 60s)

• At that time, Dijkstra published his
article:
–“Goto Statement Considered
Harmful” Comm. of ACM, 1969.
• Many programmers were unhappy
to read his article.

24


• They published several counter
articles:
–highlighting the advantages and
inevitability of GO TO statements.

25


• But, soon it was conclusively proved:
– only three programming constructs are
sufficient to express any programming
logic:
• sequence (e.g. a=0;b=5;)
• selection (e.g.if(c=true) k=5 else m=5;)
• iteration (e.g. while(k>0) k=j-k;)

26

Control-flow Based Design (Late 60s)

• Everyone accepted:
–it is possible to solve any
programming problem without
using GO TO statements.
–This formed the basis of
Structured Programming
methodology.

27

Structured programs

Unstructured control flows
are avoided.
Consist of a neat set of modules.
Use single-entry, single-exit
program constructs.

28


A program is called structured
when it uses only the following
types of constructs:
sequence,
selection,
iteration

29

Structured programs

However, violations to this
feature are permitted:
due to practical considerations
such as:
premature loop exit to support
exception handling.

30

Structured programs

Structured programs are:
Easier to read and understand,
easier to maintain,
require less effort and time for
development.

31


Research experience
shows:
programmers commit less
number of errors
while using structured if-then-
else and do-while statements
compared to test-and-branch
constructs. 32

Data Structure-Oriented
Design (Early 70s)

Soon it was discovered:
it is important to pay more
attention to the design of data
structures of a program
than to the design of its control
structure.

33

Data Structure-Oriented
Design (Early 70s)

Techniques which
emphasize designing the
data structure:
derive program structure from
it:
are called data
structure-oriented
34

Data Structure Oriented
Design (Early 70s)

Example of data structure-
oriented design technique:
Jackson's Structured
Programming(JSP)
methodology
developed by Michael Jackson in
1970s.
35

Design (Early 70s)

JSP technique:
program code structure
should correspond to the
data structure.

36

Design (Early 70s)

Several other data structure-
oriented Methodologies also
exist:
e.g., Warnier-Orr
Methodology.

37

Data Flow-Oriented Design (Late

70s)

Data flow-oriented
techniques advocate:
the data items input to a system
must first be identified,
processing required on the data
items to produce the required
outputs should be determined.
38

70s)

Data flow technique
identifies:
different processing stations
(functions) in a system
the items (data) that flow
between processing stations.
39

70s)

Data flow technique is a generic
technique:
can be used to model the working of
any system
not just software systems.
A major advantage of the data
flow technique is its simplicity.
40

Data Flow Model of a Car
Assembly Unit
Engine Store Door Store

Chassis Partly
with Engine Assembled
Fit Fit Car Fit Paint and Car
Engine Doors Wheels Assembled Test
Car

Chassis Wheel Store
Store

41

Object-Oriented Design (80s)

Object-oriented technique:
an intuitively appealing design
approach:
natural objects (such as
employees, pay-roll-register, etc.)
occurring in a problem are first
identified.
42


Relationships among objects:
such as composition, reference,
and inheritance are determined.
Each object essentially acts as
a data hiding (or data abstraction)
entity.

43


Object-Oriented Techniques
have gained wide acceptance:
Simplicity
Reuse possibilities
Lower development time and cost
More robust code
Easy maintenance

44

Evolution of Design
Techniques
Object-Oriented

Data flow-based

Data structure-
based

Control flow-
based

Ad hoc

45

Evolution of Other Software
Engineering Techniques

The improvements to the
software design methodologies
are indeed very conspicuous.
In additions to the software
design techniques:
several other techniques evolved.

46

Evolution of Other Software
Engineering Techniques
specification techniques,
project management techniques,
testing techniques,
debugging techniques,
quality assurance techniques,
software measurement
techniques,
CASE tools, etc.

47

Differences between the exploratory
style and modern software
development practices

Use of Life Cycle Models
Software is developed through
several well-defined stages:
requirements analysis and
specification,
design,
coding,
testing, etc.
48

development practices

Emphasis has shifted
from error correction to error
prevention.
Modern practices emphasize:
detection of errors as close to
their point of introduction as
possible.
49

development practices (CONT.)

In exploratory style,
errors are detected only during
testing,
Now,
focus is on detecting as many
errors as possible in each
phase of development.
50


In exploratory style,
coding is synonymous with
program development.
Now,
coding is considered only a
small part of program
development effort.
51


A lot of effort and attention is now
being paid to:
requirements specification.
Also, now there is a distinct design
phase:
standard design techniques are being
used.

52

During all stages of
development process:
Periodic reviews are being carried
out
Software testing has become
systematic:
standard testing techniques are
available.
53


There is better visibility of design and
code:
visibility means production of good quality,
consistent and standard documents.
In the past, very little attention was being
given to producing good quality and
consistent documents.
We will see later that increased visibility
makes software project management easier.

54


Because of good documentation:
fault diagnosis and maintenance are
smoother now.
Several metrics are being used:
help in software project management,
quality assurance, etc.

55


Projects are being thoroughly
planned:
estimation,
scheduling,
monitoring mechanisms.
Use of CASE tools.

56

Software Life Cycle
Software life cycle (or software
process):
series of identifiable stages
that a software product
undergoes during its life time:
Feasibility study
requirements analysis and specification,
design,
coding,
testing
57

Life Cycle Model
A software life cycle model (or
process model):
a descriptive and diagrammatic model
of software life cycle:
identifies all the activities required for
product development,
establishes a precedence ordering among
the different activities,
Divides life cycle into phases.
58

Why Model Life Cycle ?
A written description:
forms a common understanding of
activities among the software
developers.
helps in identifying inconsistencies,
redundancies, and omissions in the
development process.
Helps in tailoring a process model for
specific projects.
59

Life Cycle Model (CONT.)

The development team must
identify a suitable life cycle model:
and then adhere to it.
Primary advantage of adhering to a
life cycle model:
helps development of software in a
systematic and disciplined manner.

60


When a program is developed
by a single programmer ---
he has the freedom to decide his
exact steps.

61


When a software product is being
developed by a team:
there must be a precise understanding
among team members as to when to
do what,
otherwise it would lead to chaos and
project failure.

62


A software project will never
succeed if:
one engineer starts writing code,
another concentrates on writing the
test document first,
yet another engineer first defines the
file structure
another defines the I/O for his portion
first.
63


A life cycle model:
defines entry and exit criteria for
every phase.
A phase is considered to be
complete:
only when all its exit criteria are
satisfied.

64


The phase exit criteria for the software
requirements specification phase:
Software Requirements Specification (SRS)
document is complete, reviewed, and
approved by the customer.
A phase can start:
only if its phase-entry criteria have been
satisfied.

65


It becomes easier for software
project managers:
to monitor the progress of the
project.

66


When a life cycle model is adhered
to,
the project manager can at any time
fairly accurately tell,
at which stage (e.g., design, code, test,
etc. ) of the project is.
Otherwise, it becomes very difficult to
track the progress of the project
the project manager would have to
depend on the guesses of the team 67


This usually leads to a problem:
known as the 99% complete
syndrome.

68


Many life cycle models have been
proposed.
We will confine our attention to a few
important and commonly used models.
classical waterfall model
iterative waterfall,
evolutionary,
prototyping, and
spiral model
69

Summary

Software engineering is:
systematic collection of decades
of programming experience
together with the innovations
made by researchers.

70

Summary

A fundamental necessity while
developing any large software
product:
adoption of a life cycle model.

71

Summary
Adherence to a software life cycle
model:
helps to do various development
activities in a systematic and
disciplined manner.
also makes it easier to manage a
software development effort.

72

Software engg lect1

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