Software development

Data Structures and
Algorithm Analysis
February- June
2008, EUL
Instructor: Dr. Pasko
Galina Ivanovna
Based on lectures by Shaoying Liu
Data Structures and Algorithms
Course topics
• Software development
• Linear structures (lists, stacks and
queues)
• Nonlinear structures (trees and graphs)
• Elementary sorting and searching
methods
• Basics of algorithm analysis
Data Structure and Algorithms
Software development
The University of Queensland's Systems and Software Engineering research division
Software Contents
development
process
Problems of
software quality
Formal Methods
Program design
www.primeris.com
Software development
process
Abstractly, a software development process can
be perceived as a black-box:
Software life cycle
D e l i v e r y
Software life cycle
• Requirements analysis and specification is a study aiming to
discover and document the exact requirements for the software
system to be constructed.
• Design is an activity to construct system meeting requirements
• Implementation is where the design specification is
transformed into a program written in a specific programming
language, such as Pascal, C, or Java.
• Testing is a way to detect potential faults in the program by
running the program with test cases.
• Delivery and maintenance is where the ultimate system is
delivered to the customer for operation, and is modified either
to fix the existing faults when they occur during operation or to
meet the new requirements.
Software design
• Software design is a meaningful engineering
representation of some software product that
is to be built.
• Design can be
traced to the customer's requirements
assessed for quality against predefined
criteria.
Design focuses on four major areas of
concern: data, architecture, interfaces and
components.
Problems of software quality
1. The overall problem is that software systems often
do not operate as expected. Software systems
usually (or correctly speaking, always) contain
faults (or bugs), which can be roughly classified
into three categories:
Implementation does not satisfy the user
requirements.
Incorrect design (architecture problem)
Implementation bugs (in type, array boundary,
and file operations).
Examples of software
quality problems
National Aerospace Laboratory
of Japan has conducted an
airplane flight test of the
supersonic experimental
airplane (The rocket-powered
experimental plane NEXST-1)
in Woomera, Australia as part
of the Next generation
Supersonic Aircraft Technology
Research and Development,
but failed possibly due to
software malfunction, according
to the newspaper report after
the test flight.
Examples of software quality problems
• Launched in July of 1962, the Mariner 1 Venus
probe veered off course almost immediately and
had to be destroyed
• The problem was traced to the following line of
Fortran code:
DO 5 K = 1. 3
The period should have been a comma.
An $18.5 million space exploration vehicle was
lost because of this typographical error
Examples of software quality problems
• Therac-25
– Therac-25 was a radiation therapy machine produced by
Atomic Energy of Canada Limited. Between June 1985
and January 1987, six known accidents involved
massive overdoses by the Therac-25, leading to deaths
and serious injuries
– There was only a single coding error, but tracking down
the error exposed that the whole design was seriously
flawed
• AT&T Down for Nine Hours
– In January of 1990, AT&T’s long-distance telephone
network came to a screeching halt for nine hours,
because of a software error in the electronic switching
systems
Examples of software quality problems
• UK bank has accidentally transferred 2 billion sterling
pounds to UK and US companies because a
software design flaw allowed payment instructions to
be duplicated.”
Computer Weekly (UK), 19 Oct. 1989
• “American Airlines reckons it has lost $50 million in
passenger bookings due to a software design error in
its own computer reservation system.”
Computer Weekly (UK), 22 Sept. 1988.
• “The …Bank’s five million credit card customers were
yesterday urged to check their statements for
mistakes caused by a computer bug.”
The Times (UK), 11 August 1992
Problems of software quality
2. Software projects are often over budget and
behind schedule.
3. Software requirements are usually not well
understood before systems are implemented.
The user usually has only a rough idea
about the software system they want.
The user requirements are not defined
precisely, and therefore are often misinterpreted
by developers
Problems of software quality
4. Software development process is
usually not well controlled.
– The obligations of system analysts, designers,
and programmers are not precisely defined.
– Documents are often changed without a
rigorous procedure, and such changes are
– often not rigorously verified.
Problems of software quality
5. Program testing is too late and has limited
power in ensuring the correctness of software.
– Testing can only show the presence of bugs, but
never show the absence of bugs.
– Testing is usually costing, around 60% or more
of the total cost of software development.
– What do you do after you find out that the
software cannot be used at all by testing?
Problems of software quality
6. Software systems for safety-critical
applications may result in the loss of life
and/or properties if they are not correct with
respect to their requirements specifications:
• aircraft control systems.
• nuclear power plant control systems.
• medical systems.
• railway control systems.
Important reason for the
above problems
Specifications are written using informal
languages ( English)
• Such specifications can be
ambiguous
offer no foundation for automated
formal transformation and verification.
Informal Specifications
Software system for hotel reservation
is required.
Hotel has the following resources:
single: 100; twin: 50; double: 100
The reservation list must record the following
pieces of information of customers:
•
•
•
•
•
•
•
•
Full name
address
telephone number
passport number (if applicable)
period of stay
type of the room to reserve
check in state
check out state
Following functions must
be provided:
(1) Make a reservation
(2) Cancel a reservation
(3) Change a reservation
(4) Check in
(5) Check out
Program Development
• Three stages of program development
– Develop the algorithm
– Implement the algorithm
– Maintain the program
• When we move from small, well-defined
tasks to large software projects, we need
to add two extra layers on top of these
– Software requirements and specifications
Software requirements
and specifications
• When people talk about a program
specification, they refer to a document
describing the desired functionality of the
program or system using plain English
(or whatever language they speak).
• The specification of a program or system
is simply that what remains if you would
abstract from all implementation details.
“The Art of Programming” by Frans Faase
Software requirements
and specifications
Software requirements are broad, but
precise, statements outlining what is
to be provided by the software
product
Software specifications are a detailed
description of the function, inputs,
processing, outputs, and special
features of a software product
Software requirements
and specifications
• Program specification is definition of what
a computer program is expected to do.
• It can be:
- informal - can be considered as a
blueprint or user manual from developer
point of view;
- formal - it has definite meaning defined
in mathematical terms.
Contents
Software development process
Problems of software quality
Formal Methods
Program design
www.primeris.com
Possible solution
Formal Methods
The use of mathematics within
software engineering is often called
formal methods
What are formal methods?
Formal methods = Formal Specification + Formal Verification
• Formal methods can also be understood as
the following three components:
• Formal notation (or language) for writing
specifications
• Logical calculus for formal verification
(or proof)
• Method for developing software system
Formal Verification
• Verification of program correctness, independent of
data testing, is important area of theoretical computer
science research.
• Formal verification is act of proving or disproving
correctness of a system with respect to a certain
formal specification or property, using formal
methods.
• Formal methods have been used successfully in
verifying correctness of computer chips
• It is hoped that success with formal verification
techniques at the hardware level can lead eventually
to success at the software level
Specification refinement
Specification:
squareroot (x: nat0) y:
real
post: y ** 2 = x
Program1:
squareroot(x: nat0)
begin
y: real;
y = Math.sqrt(x);
return y;
end
Program2:
squareroot(x: nat0)
begin
y: real;
y = Math.sqrt(x);
return –y;
end
Specification refinement
Both Program1 and Program2 satisfy the
specification, although the results are different
This means:
Specification: y ** 2 = x
Refinement 1: y ** 2 = x and y >= 0
Refinement 2: y ** 2 = x and y <= 0
Software development process using formal
methods has changed considerably, as shown
next.
The change is at the increase of activities and also at
the cost and the schedule.
For example, formal specification and design may
need more time and cost more money.
Contents
Software development
process
Problems of software quality
Formal methods
Program design
www.primeris.com
Program design
What is design
What is a program
What is program design
Principles of program design
Overview of design methods
What is design
When building any complex products, such
as cars, bridges, houses, computers,
airplanes, trains, … is always necessary to
make a well-documented plan to tell how to
build them, before they are actually built.
The process and activity for making and
documenting such a plan is called design.
Example
1. Study user requirements: build a house
with the following parts:
two sides
one end
one door
two roof panels
one front
2. Build abstract model of house (abstract
description of solution)
2 sides
1
door
1 end
2 roof
panels
1 front
3. Evaluate the model against the user
requirements.
4. Modify and refine the abstract model to
produce a detailed model of the house.
1 back
1 front
Bedroom 3
Living room
toilet
2 ends
2 roofpanels
bathroom
Kitchen
Bedroom 2
Bedroom 1
front door area
Design
Design is a
process of
constructing a
preliminary
plan or model
for
producing an
expected
product or
object.
Use r
re quire ments
Desig n
Abstract mo del
Use r
re quire ments
Desig n
P roduct
Product
Program design
What is design
What is a program
What is program design
Principles of program design
Overview of design methods
What is program?
A program is an algorithm written
in a programming language that provides
behaviors based on input data.
Program = Algorithm +
+ Data structures
Algorithm
Algorithm is a sequence of actions or
commands.
Example: compute the average score of a
class with 40 students.
(1) Input all scores of the class
(2) Compute the total score of the class
(3) Divide the total score of the class by the
number of students in the class
(4) Print out the average score
Abstraction of a program
Mathematical abstraction of a program: a program is
a function, providing mapping P of D to R:
P : D →R
where P denotes a program; D is its domain;
R is its range.
Essential task of constructing a program is to decide:
(1) domain and range
(2) definition of the function P
Abstract and detailed
program design
R e q u ire m e n ts sp e c ific a tio n
A b stra c t d e sign
P ro gra m d e sign
D e ta ile d d e sign
C o d in g
Abstract design
- Identify necessary components, including data
items and operations. This may include definitions
of the data items and the functionalities of the
operations.
- Determine the architecture (or structure) of the
program that indicates the way to integrate the
operations at an abstract level.
- Determine relations between data items and
operations.
For example, what operation processes what data
items.
Detailed design
Define concrete data structures for all the data items
identified in the abstract design. Usually, these data
items are defined using the data structures available in a
specific programming language (e.g., Java, C).
Define the functionality of the operations identified
in the abstract design in detail.
Determine algorithms for implementing the
operations.
Establish precise logical relations between
operations.
For example, under what conditions which
operations can be integrated to provide what behavior.
Characteristics of
program design
Process of program design lacks a definitive formula
to follow. There is no absolute rule for design. This is
different from solving a mathematical problem.
There is a lack of quality measures that can be used
to establish that any program design is the ‘best’ one
possible.
There is no true or false program design, but good or
bad.
Program design is a process of creating
problems and resolving problems.
Program design is an intellectually rigorous
discipline.
Software development
process
How the Customer explained it
How the Project Leader understood it
Software development process
How the Analyst design it
How the Programmer wrote it
Software development process
How the Business Consultant
described it
How the project was documented
Software development process
What was installed
How the Customer was charged
Software development process
How it was supported
What the Customer really needed
http://richardwhitehead.com/jokes.htm
Welcome to software development!
References
• David Budgen “Software Design”, AddisonWesley, 1994.
• Nell Dale “C++ Plus Data Structures”,
4th ed., John and Bartlett pub., Sudbury,
Massachusetts, 2006.
• Frans Faase “The Art of Programming”
• On-line encyclopedia Wikipedia:
www.wikipedia.org
Quiz 1
Answer, please, the following questions.
• What is software life cycle?
• What is a program?
• What is an algorithm?