Software Engineering
(LECT 1)
Dr. R. Mall
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
Esoteric Past
Craft Systematic Use of Past
Experience Experience and Scientific Basis
Unorganized Use of
Past Experience
Art
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: --- “One thorn
of experience is worth a whole wilderness of
warning.”
6
Why Study Software Engineering?
(2)
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
Why Study Software Engineering?
(3)
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
user users
Single developer Team of developers
Lacks proper user Well-designed
interface interface
Lacks proper Well documented &
documentation user-manual prepared
Ad hoc development. Systematic development
12
Computer Systems
Engineering
Computer systems engineering:
encompasses software
engineering.
Many products require
development of software as well
as specific hardware to run it:
a coffee vending machine,
a mobile communication
product, etc.
13
Computer Systems
Engineering
The high-level problem:
deciding which tasks are to be
solved by software
which ones by hardware.
14
Computer Systems
Engineering (CONT.)
Often, hardware and software are
developed together:
Hardware simulator is used during
software development.
Integration of hardware and
software.
Final system testing
15
Computer Systems
Engineering (CONT.)
Feasibility
Study
Requirements
Analysis and
Specification Hardware
Development
Hardware
Software
Partitioning
Software
Development Integration
and Testing
Project Management
16
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.
17
Early Computer
Programming (50s)
Every programmer developed
his own style of writing
programs:
according to his intuition
(exploratory programming).
18
High-Level Language
Programming (Early 60s)
High-level languages such as
FORTRAN, ALGOL, and COBOL
were introduced:
This reduced software
development efforts greatly.
19
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.
20
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.
21
Control Flow-Based Design
(late 60s)
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.'’
22
Control Flow-Based Design (late 60s)
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.
23
Control Flow-Based Design (late
60s)
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.
24
Control Flow-Based Design
(Late 60s)
A program having a messy
flow chart representation:
difficult to understand and
debug.
111 25
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.
26
Control Flow-Based Design (Late
60s)
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.
27
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.
28
Control Flow-Based Design (Late
60s)
They published several
counter articles:
highlighting the advantages
and inevitability of GO TO
statements.
29
Control Flow-Based Design (Late
60s)
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;)
30
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.
31
Structured Programming
A program is called structured
when it uses only the following
types of constructs:
sequence,
selection,
iteration
32
Structured programs
Unstructured control flows
are avoided.
Consist of a neat set of modules.
Use single-entry, single-exit
program constructs.
33
Structured programs
However, violations to this
feature are permitted:
due to practical considerations
such as:
premature loop exit to support
exception handling.
34
Structured programs
Structured programs are:
Easier to read and understand,
easier to maintain,
require less effort and time for
development.
35
Structured Programming
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.
36
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.
37
Data Structure-Oriented
Design (Early 70s)
Techniques which emphasize
designing the data structure:
derive program structure from it:
are called data structure-
oriented design
techniques.
38
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.
39
Data Structure Oriented
Design (Early 70s)
JSP technique:
program code structure
should correspond to the
data structure.
40
Data Structure Oriented
Design (Early 70s)
In JSP methodology:
a program's data structures are
first designed using notations for
sequence, selection, and iteration.
Then data structure design is
used :
to derive the program structure.
41
Data Structure Oriented
Design (Early 70s)
Several other data structure-
oriented Methodologies also
exist:
e.g., Warnier-Orr
Methodology.
42
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.
43
Data Flow-Oriented Design (Late
70s)
Data flow technique
identifies:
different processing stations
(functions) in a system
the items (data) that flow
between processing stations.
44
Data Flow-Oriented Design (Late
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.
45
Data Flow Model of a Car
Assembly Unit
Engine Store Door Store
Chassis with Partly
Engine Assembled
Fit Fit Car Fit Paint and Car
Engine Doors Wheels Assembled Test
Car
Chassis Store Wheel Store
46
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.
47
Object-Oriented Design (80s)
Relationships among objects:
such as composition, reference,
and inheritance are determined.
Each object essentially acts as
a data hiding (or data abstraction)
entity.
48
Object-Oriented Design (80s)
Object-Oriented Techniques
have gained wide acceptance:
Simplicity
Reuse possibilities
Lower development time and cost
More robust code
Easy maintenance
49
Evolution of Design
Techniques
Object-Oriented
Data flow-based
Data structure-
based
Control flow-
based
Ad hoc
50
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.
51
Evolution of Other Software
Engineering Techniques
life cycle models,
specification techniques,
project management techniques,
testing techniques,
debugging techniques,
quality assurance techniques,
software measurement techniques,
CASE tools, etc.
52
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.
53
Differences between the exploratory
style and modern software
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.
54
Differences between the exploratory
style and modern software
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.
55
Differences between the exploratory
style and modern software
development practices (CONT.)
In exploratory style,
coding is synonymous with
program development.
Now,
coding is considered only a small
part of program development
effort.
56
Differences between the exploratory
style and modern software
development practices (CONT.)
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.
57
Differences between the exploratory
style and modern software
development practices (CONT.)
During all stages of development
process:
Periodic reviews are being carried out
Software testing has become
systematic:
standard testing techniques are
available.
58
Differences between the exploratory
style and modern software
development practices (CONT.)
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.
59
Differences between the exploratory
style and modern software
development practices (CONT.)
Because of good documentation:
fault diagnosis and maintenance are
smoother now.
Several metrics are being used:
help in software project management,
quality assurance, etc.
60
Differences between the exploratory
style and modern software
development practices (CONT.)
Projects are being thoroughly
planned:
estimation,
scheduling,
monitoring mechanisms.
Use of CASE tools.
61
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
maintenance.
62
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.
63
Life Cycle Model (CONT.)
Several different activities may
be carried out in each life cycle
phase.
For example, the design stage might
consist of:
structured analysis activity followed by
structured design activity.
64
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.
65
Why Model Life Cycle ?
Processes are tailored for special
projects.
A documented process model
helps to identify where the
tailoring is to occur.
66
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.
67
Life Cycle Model (CONT.)
When a program is developed
by a single programmer ---
he has the freedom to decide his
exact steps.
68
Life Cycle Model (CONT.)
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.
69
Life Cycle Model (CONT.)
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.
70
Life Cycle Model (CONT.)
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.
71
Life Cycle Model (CONT.)
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.
72
Life Cycle Model (CONT.)
It becomes easier for software
project managers:
to monitor the progress of the
project.
73
Life Cycle Model (CONT.)
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 members.
74
Life Cycle Model (CONT.)
This usually leads to a problem:
known as the 99% complete
syndrome.
75
Life Cycle Model (CONT.)
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
76
Summary
Software engineering is:
systematic collection of decades
of programming experience
together with the innovations
made by researchers.
77
Summary
A fundamental necessity while
developing any large software
product:
adoption of a life cycle model.
78
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.
79
Reference
R. Mall, “Fundamentals of Software
Engineering,” Prentice-Hall of India, 1999,
CHAPTER 1.
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