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Chapter 12Systems Design and Development

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Chapter 12Objectives

Describe the process of designing, programming, anddebugging a computer programExplain why there are many different programminglanguages and give examples of several of these languagesExplain why computer languages are built into applications,operating systems, and utilitiesOutline the steps in the life cycle of an information systemand explain the purpose of program maintenance

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Chapter 12Objectives (continued)

Explain the relationship between computer programming

and computer scienceDescribe the problems faced by software engineers intrying to produce reliable large systemsExplain why software companies provide only limitedwarranties for their products

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Chapter 12

Grace Murray Hopper Sails on SoftwareG race Murray Hopper helped chart the course of the computer industry from its earliest daysHopper earned a Ph.D. from Yale University in1928 and taught math for ten years at Vassar College before joining the U.S. Naval Reserve in1943The Navy assigned her to the Bureau of Ordnance Computation at Harvard University,where she worked with Howard Aikens Mark I,

the first large-scale digital computer Hopper wrote programs and operating manuals for theMark I, Mark II, and Mark III

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Chapter 12

Grace Murray Hopper Sails on SoftwareAfter World War II, Hopper left Harvard to work on the UNIVAC I, thefirst general purpose commercial computer, as well as other commercialcomputers

She played central roles in the development of the first compiler (a type of

computer language translator that makes most of todays software possible)and COBOL, the first computer language designed for developing businesssoftware

Hoppers greatest impact was probably the result of her tireless crusadeagainst the Weve always done it that way mind-set

In the early days of computing, she worked to persuade businesses toembrace new technology

In later years, she campaigned to shift the Pentagon and industry away frommainframes and toward networks of smaller computers

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Chapter 12

How People Make ProgramsProgramming is a specialized form of problem solvingTypically involves four steps:

U nderstand the problem. The most important step in the problem-solving process

D evise a plan for solving the problem . What resources are available and how might they be put to work

to solve the problem? Carry out the plan.

Often overlaps the previous step

E valuate the solution. Is the problem solved correctly? Is this solution applicable to other problems?

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Chapter 12

How People Make ProgramsThe programming process can also be described as another four-step process, although in practice these steps oftenoverlap:

Define the problem Devise, refine, and test the algorithm Write the program Test and debug the program

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Chapter 12

How People Make ProgramsF rom Idea to Algorithm

Start with a statement of the problem:

A teacher needs a program that is a number-guessing gameso students can learn to develop logical strategies and

practice their arithmetic. In this game, the computer picks anumber between one and 100 and gives the player seventurns to guess the number. After each incorrect try, thecomputer tells the player whether the guess is too high or toolow.

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Chapter 12

How People Make ProgramsStepwise refinement

Initially, a problem can be divided into three parts: a beginning, amiddle, and an end

Each of these parts represents a smaller programming problem to

solve

Begin

Game

Repeat Returnuntil Number is

Guessed

End

Game

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Chapter 12

How People Make Programs The next refinement fills in a few details for each part:

1. Begin Game

Display instructionsPick a number between one and 100

2. Repeat Turn until Number is Guessed

Input guess from user Respond to guessEnd repeat

3.E

nd Game

Display end message

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Chapter 12

How People Make Programs Control structures

Control structures logical structures that control the order in whichinstructions are carried outThree basic control structures:

Sequence: group of instructions followed in order from first to lastSelection: choosing between alternative courses of action dependingon certain conditionsRepetition: allowing a group of steps to be repeated several times,usually until some condition is satisfied

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Chapter 12

How People Make Programsdisplay instructionspick a number between 1 and 100set counter to 0

if guess < number, then say guess is too small;else say guess is too big

repeat turn until number is guessed or counter = 7input guess from useradd 1 to counterend repeat

A sequence control structure

A selection control structure

A repetition control structure

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Chapter 12

How People Make ProgramsTesting the algorithm

This round of testing is designed to check the logic of the algorithm Test the algorithm by following the instructions using different sets

of numbers

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Chapter 12

How People Make ProgramsF rom Algorithm to Program

A simple program contains: The program heading The declarations and definition The body

The programmer defines the words number, guess, and counter Each of these words represents a variable a named portion of the

computers memory Variables become part of the programs vocabulary Program can examine and change variables

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Chapter 12

How People Make ProgramsInto the Computer

A text editor is an application used to enter and save a program Either a translator or a compiler is used to translate a program into

machine language Translation software (or a translator ), called an interpreter , translates a

high-level program to machine language one statement at a time duringexecution

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Chapter 12

How People Make Programs Syntax errors violations of the grammar rules of a programming

languageOften flagged automatically as soon as theyre typed into the editor

L ogic errors problems with the logical structure of a program Cause differences between what the program is supposed to do andwhat it actually does

Not always as easy to detect

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Chapter 12

How People Make Programs A compiler translates an entire high-level program to machine language

before executing the program A typical compiler is an integrated programming environment, containing

A text editor A compiler A debugger to simplify the process of locating and correcting errors A variety of other programming utilities

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Chapter 12

ProgrammingL

anguages and MethodologiesMachine Language and Assembly Language

Machine language: the native language of a computer Instructions for the four basic arithmetic operations, for comparing pairs

of numbers, for repeating instructions, etc. are all binary Instructions, memory locations, numbers, and characters are all

represented by strings of zeros and ones

ssembly language: functionally equivalent to machine

language but easier for people to read, write, and understand

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Chapter 12

ProgrammingL

anguages and MethodologiesAn assembler translates each statement of assembly languageinto a corresponding machine language statement

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Chapter 12

ProgrammingL

anguages and MethodologiesHigh-Level Languages

High-level languages fall somewhere between natural humanlanguages and precise machine languagesExamples: C++, Java, Basic, F ORTRAN, COBOL, Python,Pascal, LISP, ADA, PROLO G These languages are easier to write and debug and aretransportable between machines

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Chapter 12

ProgrammingL

anguages and MethodologiesStructured Programming

Structured programming is a technique that makes the

programming process easier and more productiveA program is well structured if:

Its made up of logically cohesive modules The modules are arranged in a hierarchy Its straightforward and readable

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Chapter 12

ProgrammingL

anguages and MethodologiesObject-Oriented Programming

In object-oriented programming (OOP ), a program is not just acollection of step-by-step instructions or procedures, but acollection of objectsObjects contain both data and instructions and can send andreceive messagesC++ and Java are todays most popular object-orientedlanguages

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Chapter 12Programming L anguages and Methodologies

With OOP technology, programmers can build programs from prefabricated objects in the same way builders construct housesfrom prefabricated walls

Example: An object that sorts addresses in alphabetical order in a mailinglist database can also be used in a program that sorts hotel reservationsalphabetically

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Chapter 12

ProgrammingL

anguages and MethodologiesVisual Programming

V isual programming tools enable programmers to create large portions of

their programs by drawing pictures and pointing to on-screen objectsEliminates much of the tedious coding of traditional programmingpples HyperCard was probably the first popular example of a visual

programming environment Includes a programming language called HyperTalk

A HyperCard programmer doesnt need to know HyperTalk to create workingapplications

Microsofts V isual BASIC includes many of the ideas and tools of object-oriented programming

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Chapter 12

Programming L anguages and MethodologiesLanguages for Users

Macro languages (also called scripting languages ) allow users to create programs, called macros , that automate repetitive tasks

Microsoft Office includes a scripting variation of Visual Basic called Visual Basicfor Applications (VBA)

F ourth-generation languages (4G L ) use English-like phrases and sentences to issue instructions

Called nonprocedural languages F ocus on what needs to be done, not how to do it One type of 4 G L is the query language that enables a user to request informationfrom a database

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Component Software

Component software makes it possible to construct smallcustom applications from software componentsThe logical extension of object-oriented languages

Customization is possible only if applications are programmed toallow it

More and more software programs, including operating systems, aredesigned with extensibility in mind

May soon reach a level where users and managers can build their own applications

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Extreme Programming

Programmers use a variety of languages, including C and C++, towrite Web applications

Some programming languages are particularly useful for developing Web applications:

HTML JavaScript Java Perl XML

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Many experts see a future in which PC applications willtake a backseat to Web-based applicationsWeb-based personal information managers, reference tools,and games are growing steadily in popularityBecause of the distributed nature of the Web and thelimited bandwidth of many Internet connections, Web-

based applications present several challenges for users

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Chapter 12Programs in Perspective: Systems Analysis

and the Systems L ife CycleSystems Development

Systems development: a problem-solving process of: Investigating a situation Designing a system solution to improve the situation Acquiring the resources to implement the solution Evaluating the success of the solution

A steering committee may be formed to decide what projectsshould be considered first

Made up of people from each functional area in the organization

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and the Systems L ife Cycle

A business organization may choose to contract, or outsource , asystems analyst from an outside consulting firm

A systems analyst is an IT professional primarily responsible for developing and managing a system

Avoids the need for permanent in-house staff Allows an organization to hire talent for selected activities on a contract basis

E nd-user development allows end users to create applications End-users need access to and training in the use of Web site development

tools, spreadsheet and database management packages, and fourth-generation languages

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Investigation

Analysis

D esign

D evelopment

Implementation

Maintenance

Retirement

The Systems Development Life Cycle (SDLC)

The graphical waterfallmodel of the SDLC shows

a basic sequential flow fromidentifying the right things

to do to making sure thatthings are done right

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Investigation

Define the problem:

Identify the information needs of the organizationExamine the current systemDetermine how well it meets the needs of the organization

Study the feasibility of changing or replacing the currentsystem

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Analysis

During the analysis phase, the systems analyst :

G athers documentsInterviews users of the current systemObserves the system in actionG enerally gathers and analyzes data to understandthe current system and identify new requirements

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Prototyping is an interactive methodologyin which the prototype is continually

modified and improved until it meetsthe needs of the end-userIdentifyrequirements

Develop workingmodel of system

Use prototype

Evaluate featuresof prototype

Develop application,install prototype for evaluation by end-users,

begin new prototype, or abandon application

Makeany

necessarychanges

to prototype

D esign

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The systems analyst mustcarefully plan and scheduleactivities in the developmentphase of the S DL C becausethey can overlap and occur

simultaneously

D evelopment

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Implementation

D irect cutover approachParallel systems approachPhase-in approachPilot approach

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Maintenance

The maintenance phase involves monitoring,evaluating, repairing, and enhancing the systemthroughout the lifetime of the system

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Retirement

Systems are often used for many years, but at some point in the life of a system,ongoing maintenance is not enough

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Systems Development Tools and TechniquesData collection techniques include:

Review Interviews Questionnaires Observation Sampling

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Modeling tools Modeling tools are graphicrepresentations of systems

System flowcharts, data flowdiagrams, data dictionaries,and decision tables are most

widely-used

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Chapter 12Programs in Perspective: Systems Analysis and

the Systems L ife Cycle

Computer-aided systems engineering (CASE) tools include: Charting and diagramming tools to draw system flowcharts and data

flow diagrams A centralized data dictionary containing detailed information about all

the system components A user interface generator to create and evaluate many different

interface designs Code generators that automate much of the computer programming to

create a new system or application

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Chapter 12Programs in Perspective: Systems Analysis and

the Systems L ife Cycle

Some CASE software packages contain tools that apply primarily to theanalysis and design phases of the systems development life cycle

Others contain tools that automate the later phases of systemsdevelopment, implementation, and maintenance

Integrated CASE tools incorporate the whole spectrum of tools to supportthe entire systems development life cycle

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Chapter 12

Rules of ThumbAvoiding Information Technology Project F ailures

Here are six tips for information workers on how to preventthe failure of IT projects:

IT projects need executive sponsorship IT projects need user input IT projects need specifications IT projects need realistic expectations IT projects need cooperative business partners IT projects need open and honest communication

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Chapter 12

The Science of ComputingMany computer scientists prefer to call their field computingscience because it focuses on the process of computing rather than on computer hardwareComputer science includes a number of focus areas:

Computer theory Algorithms Data structures Programming concepts and languages Computer architecture Management information systems Software engineering

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Chapter 12

The State of SoftwareSoftware Problems

Software errors are difficult to locate and more difficult toremove

Errors of omission Syntax errors Logic errors Clerical errors

Capacity errors Judgment errors

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Chapter 12

The State of SoftwareSoftware Solutions

Computer scientists and software engineers are responding toreliability and cost problems on five main fronts:

Programming techniques Programming environments Program verification Clean-room programming Human management

It could well be that by the close of thetwenty-first century, a new form of truly

accessible programming will be the provinceof everyone, and will be viewed like writing,

which was once the province of the ancient scribes but eventually became universally

accessible. Michael Dertouzos, in W hat W ill Be

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Chapter 12

The State of SoftwareSoftware Warranties

In the past, consumer software manufacturers provided no warranties for their products

Today some manufacturers will give money back if the software cannot beinstalled on the computer No software manufacturer will accept liability for harm caused to you or your business by errors in software:

Why? Additional precautions to make software work better would inflate cost and extend

the time needed for development Only large companies would be able to sustain the pressure of such a scenario

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Chapter 12

The State of SoftwareThe F uture of Programming?

Programming languages will continue to evolve in thedirection of natural languages, like EnglishThe line between programmer and user is likely to grow hazyComputers will play an ever-increasing role in programmingthemselvesF uture programming tools will have little in common withtodays languages

When future computer historians look back, theyll marvel at howdifficult it was for us to instruct computers to perform even the simplestactions

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Chapter 12L esson Summary

Computer programming is a specialized form of problemsolving that involves developing an algorithm for solving a

problem

Most programmers use stepwise refinement to repeatedly break a problem into smaller, more easily solvable problemsComputer languages have evolved through several generations,with each generation easier to use and more powerful than theone that came beforeMost modern languages encourage structured programming

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Chapter 12

L esson Summary (continued)Many applications contain built-in macro languages, scriptinglanguages, and query languages that put programming power inthe usersObject-oriented programming (OOP) tools enable programmersto construct programs from objects with properties and providethe ability to send messages back and forth; many believe thatOOP represents the future of programming

Programs are part of larger information systems

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Chapter 12

L esson Summary (continued)Computer scientists are responsible for the software tools andconcepts that make all other software development possibleOne of the most challenging problems facing computer scienceis the problem of software reliabilityAs more and more human institutions rely on computer systems,it is becoming increasingly important for computer scientists tofind ways to make software that people can trust

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LE_ppt_12

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