1 CS 501 Spring 2006

CS 501: Software Engineering

Lecture 10

Requirements 4

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Course Administration

Presentations, March 7-9

Read the instructions on the Assignments Web page.

Reserve a time slot by sending email to moskwa@cs.cornell.edu. Time slots are listed on the home page of the Web site. First-come-first-served.

Quiz 3

This quiz may be moved to March 16.

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Planning for the Presentation

How will you use the time?

This is a presentation to the client, with the Instructor as a secondary audience. Possible topics:

• Overview of project and progress against plan

• Presentation of assumptions, decisions

• Summary of requirements in moderate detail

• What has been learned since feasibility study. Changes in plans

Allow 15 minutes for questions. Expect interruptions.

"This is our understanding of your requirements."

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Planning for the Presentation

Logistics

Have a rehearsal, check visual aids and demonstrations. Then change nothing.

Check out the equipment in the meeting room. What network will you use (if any). How will you connect a computer (if you do)? What about firewalls?

Will one person act as chair and call on other members of the team? Never interrupt your colleagues.

Not everybody is a great presenter, but everybody can be well-prepared.

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Formal Specification

Why?

• Precise standard to define and validate software.

Why not?

• May be time consuming

• Methods are not suitable for all applications

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Remember

Formal specification does not prescribe the implementation

With formal specification it is possible, at least theoretically, to generate code automatically from the specification, but this may not be the most effective way:

• Writing the generator may be a very large programming task.

• The resulting code may perform badly.

Formal specification does not guarantee correctness

• If the specification is wrong, the system will be wrong.

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Formal Specification using Mathematical Notation

Mathematical requirements can be specified formally.

Example: requirements from a mathematical package:

B1, B2, ... Bk is a sequence of m x m matrices

1, 2, ... k is a sequence of m x m elementary matrices

B1-1 = 1

B2-1 = 21

Bk-1 = k ... 21

The numerical accuracy must be such that, for all k,

BkBk-1 - I <

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Formal Specification Using Diagrams

digitunsigned integer

digit. E

+

-

unsigned integerunsigned integer

unsigned number

Example: Pascal number syntax

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Formal Specification of Programming Languages

<unsigned number> ::= <unsigned integer> | <unsigned real>

<unsigned integer> ::= <digit> {<digit>}

<unsigned real> ::= <unsigned integer> . <digit> {<digit>} | <unsigned integer> . <digit> {<digit>} E <scale factor> | <unsigned integer> E <scale factor>

<scale factor> ::= <unsigned integer> | <sign> <unsigned integer>

<sign> ::= + | -

Example: Pascal number syntax

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Formal Specification using Z ("Zed")

Z is a specification language developed by the Programming Research Group at Oxford University around 1980. Z is used for describing and modeling computing systems. It is based on axiomatic set theory and first order predicate logic.

Ben Potter, Jane Sinclair, David Till,

An Introduction to Formal Specification and Z

(Prentice Hall) 1991

Jonathan Jacky

The Way of Z

(Cambridge University Press) 1997

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Informal: The function intrt(a) returns the largest integer whose square is less than or equal to a.

Formal (Z):

intrt: N N

a : N •

intrt(a) * intrt(a) < a < (intrt(a) + 1) * (intrt(a) + 1)

Example: Specification using Z

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Example: Implementation of intrt

1 + 3 + 5 + ... (2n - 1) = n2

Static specification does not describe the design of the system.

A possible algorithm uses the mathematical identity:

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Example: Program for intrt

int intrt (int a)/* Calculate integer square root */{ int i, term, sum; term = 1; sum = 1; for (i = 0; sum <= a; i++) { term = term + 2; sum = sum + term; } return i;}

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Formal Specification of Finite State Machine Using Z

A finite state machine is a broadly used method of formal specification:

• Event driven systems (e.g., games)

• User interfaces

• Protocol specification

etc., etc., ...

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State Transition Diagram

Patients Fields Setup ReadyBeam

on

Enter Enter Start

Stop

Select field

Select patient(lock on)

(lock off)

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State Transition Table

SelectPatient

SelectField

Enter lock off Start Stop lock on

Patients

Fields

Setup

Ready

Beamon

Fields

Fields

Fields

Patients

Patients

Patients

Setup

Setup

Setup

Ready

Beamon

Ready

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Z Specification

STATE ::= patients | fields | setup | ready | beam_on

EVENT ::= select_patient | select_field | enter | start | stop | lock_off | lock_on

FSM == (STATE X EVENT) STATE

no_change, transitions, control : FSM

Continued on next slide

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Z Specification (continued)

control = no_change transitions

no_change = { s : STATE; e : EVENT • (s, e) s }

transitions = { (patients, enter) fields,

(fields, select_patient) patients, (fields, enter) setup,

(setup, select_patient) patients, (setup, select_field) fields, (setup, lock_off) ready,

(ready, select_patient) patients, (ready, select_field) fields, (ready, start) beam_on, (ready, lock_on) setup,

(beam_on, stop) ready, (beam_on, lock_on) setup }

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Schemas

Schema:

• The basic unit of formal specification.

• Enables complex system to be specified as subsystems

• Describes admissible states and operations of a system.

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Z in Practice

In carefully monitored industrial use, Z has been shown to improve the timeliness and accuracy of software development, yet it is not widely used in practice.

Complexity of notation makes communication with client difficult.

Few software developers are comfortable with the underlying axiomatic approach.

Heavy notation is awkward to manipulate with conventional tools, such as word processors.

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User Interface Design: Requirements and Refinement

It is very difficult to specify and comprehend an interactive interface in a textual documents

• Requirement documents benefit from sketches, comparison with existing systems, etc.

• Design documents should definitely include graphical elements and often benefit from a mock-up or other form of prototype.

• Implementation plans should include evaluation of user factors and time to make changes.

User interfaces must be tested with users

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The Design/Evaluate Loop

Evaluate

?Design

Build

Analyze requirements

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Requirements: Mock-up

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Methods for Specifying Usability Requirements and Evaluation of Usability

Initial Mock-up Prototype Production

Client's opinions

Competitive analysis

Expert opinion

Focus groups

Observing users

Measurements

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Focus Group

A focus group is a group interview

• Interviewer

• Potential users

Typically 5 to 12

Similar characteristics (e.g., same viewpoint)

• Structured set of questions

May show mock-ups

Group discussions

• Repeated with contrasting user groups

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Special Considerations: Disabilities

• What if the user:

is visually impaired or color blind?does not speak English?is a poor typist?

• There is a tradition of blind programmers

• Navigation of web sites need not be only visual

You may have a legal requirement to support people with disabilities

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Special Considerations: Computer Systems and Networks

The performance, reliability and predictability of computer systems and networks is crucial to usability

• Response timeinstantaneous for mouse tracking and echo of key stroke5 seconds for simple transactions

• Example: Pipelined algorithm for the Mercury page turner

• Quality of Service for real time information

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Special Considerations: Design Tensions in Networked Systems

• Client computers and network connections vary greatly in capacity

• Client software may run on various operating systems; it may be current or an earlier version

• System designers wish to control clients; users wish to configure their own environments

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Special Considerations: Device-Aware User Interfaces

• Examples of devices:

desk-top computer, fast network connectionlaptop computer, intermittent connectivityPalmPilot, synchronizationsmart telephonedigital camera, camcorder

• Device-aware user interfaces are aware of:

=> performance of device=> limited form factor (display, keyboard)=> connectivity

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Special Considerations: Usability and Cost

• Good usability may be expensive in hardware or special software development

• User interface development may be a major part of a software development project

Programming environments provide powerful user interface toolkits

• Costs are multiplied if a user interface has to be used on different computers or migrate to different versions of systems

Web browsers provide a general purpose user interface where

others maintain the user interface software