Unit 18 Software Design

8/13/2019 Unit 18 Software Design

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While not every IT practitioner is a programmer, an understanding of

the process by which programs are written is important. Developing

software is a complex process and so the design phase is very

important. This unit looks at what is involved in the development

process and the tools and techniques which can be used to createa design.

For anyone considering a career in software development, understanding softwaredesign is clearly important. A job as a software designer is a senior position requiringmany years of experience, but even a trainee programmer is expected to understandthe principles of software design.

The software designer must first of all understand what the program needs to doand then develop a design which will guide the programmers who will writethe software. The creation of the design is therefore very important thedesign must be correct, otherwise it is unlikely that the software that will beproduced from it will do what is required.

Learning outcomesAfter completing this unit you should:

1. know the features of programming languages

2. know the software development process

3. be able to design software development solutions

18Software

design

Credit value: 10


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Assessment and grading criteriaThis table shows you what you must do in order to achieve a pass, merit or distinction grade, awhere you can find activities in this book to help you.

To achieve a passgrade theevidence must show that youare able to:

To achieve a meritgrade theevidence must show that, inaddition to the pass criteria,you are able to:

To achieve a distinctiongradethe evidence must show that, inaddition to the pass and meritcriteria, you are able to:

P1 describe the characteristics ofprogramming paradigms

See Assessment activity18.1 on page 18

M1describe the features ofprogramming languages;

including how sequence,

selection and iteration

are usedSee Assessment activity18.1 on page 18

P2 identify the factors influencingchoice of programming

language


P3 describe the stages of thesoftware developmentlife cycle

See Assessment activity

18.2 on page 24

P4 outline a specification for abusiness requirement


M2select and justify theprogramming language

to be usedSee Assessment activity18.2 on page 24

P5 design a software solution toa business requirement using

appropriate design tools


M3describe the data types andsoftware structures used in a

design solution


D1 develop algorithms torepresent a design solution

See Assessment activity

18.3 on page 36

P6 review the design against theoriginal requirementSee Assessment activity18.3 on page 36

D2 evaluate the designtools usedSee Assessment activity18.3 on page 36

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How you will be assessedThis unit is internally assessed. You will provide a portfolio of evidence to showthat you have achieved the learning outcomes.Your portfolio of evidence canbe supplied in many formats including electronically as well as paper-based. Thegrading grid in the specification for this unit lists what you must do to obtain pass,merit and distinction grades. The Assessment activities in this unit will guide youthrough tasks that will help you to be successful in this unit.

Your tutor will tell you exactly what form your assessments will take, but you couldbe asked to produce:

presentations written documentation: o supporting documents o specification o design documentation an evaluation of your work.

Unit 18Software

Amir Khalid, a 17-year-oldLevel 3 learner

I studied this unit last year, and now Im on the Level 3 course. I was

really interested in this unit, as software development is what I want

to do. To be honest, design isnt the most interesting part of software

development but now I realise how important it is.

I think one thing that we found quite hard was relating the fairly simpleexamples of programs and designs that we looked at with real commercial software.

The part I found most difficult was creating the structure and dataflow diagrams. To start with,

I found this very hard and needed to practise it. I discovered that the best way to produce

these diagrams was to have a very good understanding of what the software is supposed to

do and then to follow the whole process through, using action lists and storyboards initially.

As far as the assessments were concerned, the toughest parts were justifying my choice

of programming language and evaluating the design tools I used. Of course, I also found

producing the design hard too!

Overall I think this was the most challenging unit we did, but I found it really interesting.

Over to you

Do you have an interest in software development? Have you ever wonderedabout how computer games are designed and created, for example? Whatsort of attitude or personality do you think makes a good programmer?

Do you know anyone who has done programming, perhaps on a higher levelcourse or at university, or as a job? Speak to them and find out howthey find writing programs (easy or difficult, fun or challenging?).


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1. Know the features ofprogramming languages

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Warm upStart up Programming a robotImagine you have to write a program that controls a robot. It might be an industrial

robot that paints a car in a factory or a robot the army would use to search for

roadside bombs.

Working in a group, think about how you would design the program to control

the robot.

The first thing you would need to decide is what sort of robot it is and exactly

what it will do.

How would you split up the design and creation of the control program

between the members of your group?

What techniques could you use to design the way the robot works?

You dont need to define this in a lot of technical detail, just think in general terms

about how you could create a design.

Case study: Working as a specialist software developer

I run a small software

company that writesspecialist software forthe building trade. Wehave created a programwhich is used to helpbuilders estimate howmuch a particular jobwill cost. Over the yearswe have developedand added to the

program so it now includes many different modules.It is used by over 250 different building companiesaround the country.

We wrote the original version in Visual Basicbecausethats the language my partner and I knew best.Also, we wanted to use MicrosoftAccessas thedatabase and these two link together well. We havebuilt up expertise in these products in the company,so it would be difficult to change the programminglanguage we use now, although we have consideredit because it does restrict our customer base to thosewho run MicrosoftWindows.

The software is always being developed and, because

we have lots of customers, we have to survey themall on the features they would like to see in the nextversion. In that way we create a wish list ofnew features.

Its very important to understand the building tradeand what they need from the program. Once weare clear on what new features we will add in thenext version, we begin the design. Over the yearsweve found that, although the design stage can bedifficult and not as interesting or as exciting as writingthe programs and getting them working, it is veryimportant to get the design right. If its not right, wehit problems when we come to write the programs.

1. What influenced the companys choice ofprogramming language?

2. What must the company do before they startwork on a new version of their software?

3. Why is the design stage so important?


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What is a computer program?A computer program is a series of instructions that tells the computers

processor what to do. Programs are what make computers powerful. Unlike

typewriters or fax machines, which have only one function, computers must

carry out many different tasks. Programs make this possible.

Computers, however, can understand only machine codeinstructions

in the form of binary codes. Programs for the very first computers were

written using these binary codes, but they are very difficult for people

to understand. Because of this, programminglanguages(sometimes

called high-level languages) were developed.

Programs consist of two important parts:

instructions, which tell the computer what to do

data, which the instructions act on.

Data is often input by the user of the program. In the Windows

Calculator program, the user enters the numbers that they want to do

the calculation with this is input data. The program then carries outthe instructions to do the calculation and displays the result this is the

output data. The data varies depending on what the user inputs, but the

instructions for a given calculation stay the same.

1.1 Programming paradigmsComputer programs can be very complex indeed. Over the years a

number of different paradigmshave been developed to deal with

this complexity and encourage the development of efficient and

error-free programs.

Procedural programmingIn this approach the programmer writes the instructions as a series of

logical steps to produce the desired result. Different functions within

the program are provided by different procedures (or subroutines). In

the WindowsCalculator, different procedures might be provided to do

addition, subtraction, multiplication, etc.

Procedures can be run from the main program to carry out their specified

function or from within other procedures. One of the benefits of this

approach is that where tasks need to be carried out several times within

the program, the instructions do not need to be duplicated. Instead, the

procedure runs as many times as required. This provides a better way

to organise or structure a large, complex program. With the procedural

approach the data in the program is shared between the procedures.

Object-orientedObject-oriented (or OO) methods were developed as a response to the

increasing complexity of software, which was difficult to handle with the

procedural approach.

Unit 18Software

Machine code the instructions that

are executed directly by the computer

CPU, sometimes called the instruction

set. These instructions are in the form o

binary numbers.

Programming language a defined

set of English-like instructions which

are used to create a program. These

instructions are easier for people to

understand than binary codes. The

instructions are converted into the bina

codes that the computer understands,

using a piece of software called

a compiler.

Paradigm an approach, framework o

way of doing something.

Key terms


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Object

With the OO approach to programming, the things a procedure does

(its functions) are put together with the data it needs in something

called an object. The idea of an object is based on the real things the

program is dealing with, for example:

in a library system, the objects might include books, members

and loans in a mail order system, the objects might include customers, orders

and products.

Objects in a software system can represent physical things like

customers or books, or conceptual things like an order or a loan.

Class

A class is a template for an object and defines the data (or attributes in

OO terminology) that each object of that class will have. For example, think

of a library system which has a class for books. The data that the book class

would define includes things such as the book title, author, ISBN number,

etc. An object of the book class is created using the class template and hasactual values for the data that the class defines, such as Harry Potter and

the Goblet of Fireas the title and J. K. Rowling as the author.

Methods

Classes also have functions (things the class can do), which in OO

terminology are called methods.

A customer class might have a change of address method.

A book class in a library system might have a go on loan method.

Remember that a class is like a template for objects and defines the data

and methods (functions) of the class. All the objects of that class have

the same attributes and methods. Classes can be defined by a classdiagram. This lists the name of the class, its attributes and methods.

Figure 18.1 shows the class diagram for a learner class in a system for

recording learners assignment marks. This simple class has just two

attributes (name and marks) and three methods (set name, show marks

and update marks).

An OO system is made up of objects (created using the class as a

template). These objects work together by one object requesting a

service from another (using the methods of that class). For example,

if there is also a Tutor class, then an object of the Tutor class might

request an object of the Learner class to run its showMarks method, soit could find out the marks that the learner currently has.

It is important to understand that the only way in which other objects

can gain access to the attributes of a class is by the methods of that

class. This is one of the key differences between the procedural

approach to programming (where data is shared between procedures)

and the OO approach (where the data is contained within the object

and is only accessible via the objects methods). This concept is known

as encapsulation.

Attributes the name given to the data

used within a class.

Methods the functions of a class, i.e.

the things a class can do.

Encapsulation the technical term for

the way in which the attributes (data) and

the methods (functions) of a class are

packaged together within a class, and

the data of a class can only be accessed

by the methods.

Key terms

Figure 18.1:Class diagram for the

Learner Class.

LearnerName

LearnerMarks

Learner

setName

showMarks

updateMarks


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Event drivenThe event driven approach to programming refers to how the program

responds to user input, not to how the program is structured. Therefore,

event driven programs can be written using a procedural or an object-

orientated approach.

Event driven programming is used with the Windowsuser interface.

Windowsprograms respond to user input for example, when you

click a button or choose a menu option a particular function is carried

out. Clicking buttons or choosing menu options are all considered

events. Event driven programming therefore involves writing event

procedures which respond to the actions the user takes.

Events inputs to the program that

control what the program does. Events

are often user inputs, such as mouse

clicks or menu selections.

Key term

What different events can a Windowsprogram respond to? As wellas left-clicking on a button or menu on the screen, there are severalother events that popular Windowsapplications respond to. List asmany as you can.

Activity: Events in a Windowsprogram

1.2 Program featuresTo help you understand the features that programs have, well look

at some simple examples of programs written using the Visual Basic

programming language. This is not intended to be a tutorial on writingprograms in Visual Basic the programs are shown to help you

understand the features and structures of programs.

As mentioned on page 5, a program consists of instructions and data.

The data in a program is usually input by the user. It is then stored in

the program in a memory area that the programmer creates called a

variable.Variables have three properties:

A name, which is given by the programmer. Variable names should

give some clue as to what the variable is used for, but they cannot

contain spaces.

A data type, also defined by the programmer. This defines the sortof data the variable can hold. Typically a programming language will

have data types for text and various numerical formats.

A value, which is held in the variable. This may often be set as a

result of user input.

Lets look at a simple program, as shown in Figure 18.2 on page 8.

Here we are using a console application, which does away with the

visual side of the language and uses simple text input and output via a

console window. Using a console application helps us to concentrate on

Variable a storage area in a program

for data. This data might come from us

input or from a calculation done within

the program.

Console application a text-based

application that runs in a command

prompt window.

Key terms

Did you know?

You can download MicrosoftVisu

Basic2010 Express for free by

going to Hotlinks and clicking on

this unit.


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the key features of programming, rather than having to worry about the

complexity that using a Windowsapplication adds.

This program takes two numbers input by the user, adds them together

and outputs the result. Figure 18.3 shows the program running.

In this simple example, the instructions in the program are followed

in the order they appear in the program. However, programs can also

select to carry out one set of instructions instead of another. The choice

(selection) of which set of instructions to follow is usually made by

the user.

There are other sections of code which may be repeated a certain

number of times or until some condition is met. These different

situations give us three ways that the flow of program instructions can

be arranged:

Sequence: Instructions are followed one after the other, as in the

example above.

Selection: A choice is made as to which set of instructions to follow.

Figure 18.2:A simple Visual Basicconsole application.

These instructions define 3 variables,called input1, input2 and result all with adata type of integer (whole numbers).

These output instructions display theresult variable and wait for the user topress any key before the console

window closes.

This instruction adds together the valuesin Input1 and Input2 and assigns theresult to the variable called result.

These instructions output messages tothe user and then assign the input theuser makes to the variables input1 andinput2.


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Iteration: A set of instructions is repeated in what is sometimes

called a loop.

We will demonstrate selection and iteration by making some

modifications to the simple program we looked at earlier.

SequenceWhen the program needs to do things like calculations, acceptinguser input or outputting data, the program instructions are followed in

sequence from top to bottom. But in many situations, a program needs

to do different things, perhaps because of the input the user has made.

In these situations selection is used.

SelectionIn most programming languages selection is done using an if

instruction. The instruction compares one value or variable with another

and if the outcome is true, the instructions following the if statement

are executed. If the outcome of the comparison is false, the followinginstructions are skipped and the program continues at the end of

the if instructions.

Several different comparisons can be done to take into account various

different possibilities. Figure 18.4 (on page 10) shows a modified version

of the program we looked at in Figure 18.2 (on page 8). The new

instructions are in red.

Figure 18.3:The adding program running.


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This type of instruction is sometimes called if... then... else. Notice

how the instructions are indented following the if instruction. This

shows which instruction is executed depending on the result of the if

comparison. For example, the instruction in figure 18.5 is only executed

if the choice variable contains an A.

Where there are a number of different values to consider in a selection

instruction block, the if instruction can be rather long and complex. A

slightly simpler alternative is the Case instruction. Look at the modified

version of the previous program shown in Figure 18.6. This allows the

Sub Main()

Dim input1 As Integer


Dim result As Integer

Dim choice As String

Console.WriteLine("Enter a number")

input1 = Console.ReadLine()

Console.WriteLine("Enter another number")


Console.WriteLine("Enter A to add or M to multiply")

choice = Console.ReadLine()

If choice = "A" Then

result = input1 + input2

ElseIf choice = "M" Then

result = input1 * input2

Else

Console.WriteLine("Invalid choice")

End If

Console.WriteLine("The result is " & result)

Console.WriteLine("Press any key to continue")

Console.ReadLine()

End Sub

This new variable is namedchoice and has a data typeof String (text).

The user is requested to

enter an A or an M andtheir input is placed inthe choice variable.

An if instruction checks thechoice variable and carriesout add or multiply basedon what the user enters.

If the user enters somethingother than an A or an M this

part of the if instructionis executed.

Figure 18.4:Program demonstrating selection.

Console.WriteLine("Enter A to add, M to multiply, ")

Console.WriteLine("S to subtract or D to divide


Select choice

Case "A"


Case "M"


Case "S"

result = input1 - input2

Case "D"

result = input1 / input2

End Select



Figure 18.5:This instruction is only executed

if the choice variable contains an A.

Figure 18.6:Program demonstrating the use of the Case instruction.


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user to carry out all four arithmetic operations (add, subtract, multiply

and divide). Note that not all the program code has been included.

The case instruction starts with the instruction Select, followed by the

name of the variable which contains the value to be inspected. Then for

each possible value you have a Case instruction followed by the value

you are expecting. This is followed by the instructions which will be run

if the variable contains that value.

IterationIteration is when instructions are repeated. Lets look at a version of

our program that includes a loop. As it stands the program will just

run once, then finish. By putting the main part of the instructions in an

iteration loop, it can run until the user ends it.

Sub Main()


Dim input2 As Integer Dim result As Integer

Dim choice As String

Dim EndIt As String = "Y"

Do While EndIt = "Y"

Console.WriteLine("Enter a number")


Console.WriteLine("Enter another number")


Console.WriteLine("Enter A to add or M to multiply")


If choice = "A" Then


ElseIf choice = "M" Then


Else

Console.WriteLine("Invalid choice")

End If


Console.WriteLine("Press Y to continue or any key to exit")

EndIt = Console.ReadLine()

Loop

End sub

Another new string variable,set to Y so the loop runsat least once.

This is the start of the loop.It will keep looping while theEndIt variable is set to Y.

End of the lo

At the end of the calculationthe user inputs a Y, whichis placed in the EndIt variableto keep the program looping.Any other input will endthe loop.

Figure 18.7:Program demonstrating a loop.

In this example, a Do while loop continues while the EndIt variable

contains a Y. If the user enters any other value, the loop (and therefore

the whole program) ends.


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Do while loops are not the only types of loop there are also forloops and other types. Find out how these loops are coded and whatthey can be used for.

Activity: Loops

Logical operatorsThe logical operators AND and OR are used to create more complex

comparisons in if, Do while and other selection and loop instructions.

With these operators you can create comparisons which look at more

than one value. For example, imagine you are writing a fantasy game

program. You have decided that if a dragon character in the game has

collected two swords and a golden key, they are awarded 200 extra

bonus points. The instructions you write might look something like

those shown in Figure 18.8.

Figure 18.8:Instruction using the logical operator AND.

If Swords = 2 AND Gkey = 1 Then

BonusP = BonusP + 200

End if

Swords, Gkey and BonusP are all variables with a data type of integer

(whole number). One problem with this code is if the dragon has

already collected three swords when it gets the golden key. With this if

instruction the bonus points would not be awarded.

As well as using equals signs in comparisons, you can also use the

symbols for greater than and less than. In this case we need to modify

the code so if swords is equal to or greater than 2 then the bonus points

are awarded, as shown in Figure 18.9.

Figure 18.9:Instruction using the symbol for greater than or equal to.

If Swords> = 2 AND Gkey = 1 Then

BonusP = BonusP + 200

End if

The list of comparison operators are shown below in Table 18.1.

Table 18.1: Comparison operators.

Operator Meaning Operator Meaning

> greater than = greater than or equal to


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Using the AND logical operator means both values that you compare

must be true, but there may be situations where you want to make

comparisons where either one OR the other can be true. In these

situations we use the OR logical operator. For example, using the same

imaginary game, if a dragon has 400 bonus points or more OR they

have two golden keys then they get an extra life. The if instruction

would look something like that shown in Figure 18.10.

Figure 18.10: Instruction using the logical operator OR.

If BonusP >= 400 OR Gkey = 2 Then

Lives = Lives + 1

End if

Local and global variablesThe simple programs we have looked at so far only contain a single

procedure, so all the variables we have created are localto that

procedure in other words, the variables are only used in thatprocedure. In more complex programs with lots of different procedures,

some variables may be created which are globaland can be used in all

the procedures of the system.

Global variables are generally created at the start of the instructions for

the whole system, whereas local variables are created at the start of the

individual procedure where they can be used, like the variables we have

seen so far.

It is good programming practice to avoid global variables where

possible and stick to local variables, although sometimes the use of

global variables may be unavoidable. The reason for this is that globalvariables can cause program errors if they are used improperly. For

example, two different procedures may use the same global variable in

a slightly different way or one procedure may modify a global variable

that another procedure is using, and this may cause an unexpected

result or even cause a program to crash.

Assignment statementsWe have already seen a number of assignment statements.

For example, lets start with the following instruction:

Assignment statements these carry

out assignment operations, which cons

of taking the value on the right side of

the assignment operator (=) and storin

it in the variable on the left.

Key term

BonusP = 100

This can also be done as part of an arithmetic operation, such as:


You can also assign an initial value to a variable when you create it,

such as:

Dim EndIt As String = "Y"


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Input and output statementsIn the example programs weve looked at so far, input and output with a

console (text-based) application can be done with instructions, such as

ReadLine and WriteLine. Input and output with Windowsprograms are

a little different. These programs normally use a form in Visual Basic

Express this is created by using a forms editor which allows you to add

controls, such as labels, text boxes and buttons to your form. Figure18.11 shows the Visual Basicforms editor.

Figure 18.11:The Visual Basicforms editor.

Visual languages languages designed

to create Windowsprograms.

Key term

Windows

applications like this are event driven, so input can be madevia the mouse by clicking on controls such as buttons and check boxes.

Text input can be made using text boxes, and text output can be made

using a text box or label.

1.3 Types of languagesThere are a number of different general types of programming

languages best known are visual, script and markup.

Visual languagesBecause the Windowsuser interface is so widely adopted, many of theprogramming languages utilised today are known as visual languages.

Visual languages come with many of the standard Windowsfeatures

(such as dialog boxes, menus, buttons and scroll bars) already built into

them, so the programmer does not have to create these from scratch.

It obviously saves a lot of program development time if all these

visual elements are already made for you. Visual languages normally

support event driven procedures. As we have seen, the forms editor in


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MicrosoftVisual Basicallows you to create buttons and other controls

that the user can interact with. Event procedures are written to deal with

these user interactions.

Script languagesTwo common examples of script languagesare macros and web

page scripts. A macrois a small program that is written or recorded to carry

out some task in another application. The best known examples

of macros are the ones you can create in MicrosoftOffice

applications, such as Wordor Excel, to carry out a complex or

repetitive task within a document or spreadsheet. You can learn

more about macros in Unit 9: Customising software(page 165).

Web page scriptsare small programs, embedded within web

pages, which are run by an Internet browser, such as Microsoft

Internet Explorerand carry out a task related to the web page.

Typical tasks a web page script might perform are validating a users

input into a web page form or opening a new browser window orpop-up window. One of the most common scripting languages is

JavaScript, which is related to the Javaprogramming language.

Markup languagesHTML is the best known example of a markup language it is used to

format web pages. In many ways HTML is not really a true programming

language, as it does not support many of the features associated with

programs, such as selection structures and loops. HTML is really just a

way of formatting text by using tags. These tags indicate how the text

following the tag should be formatted when it is displayed by the web

browser. HTML allows you to include graphic images, links and many

other elements. You can learn more about HTML in Unit 17: Website

development (pages 292293).

Other types of languagesIt may seem that all the programs you use are Windowsprograms

and that there isnt really any other type of program, but this is not the

case. Windowsprograms are the ones that users interact with directly,

but in the background there are many other programs running. A

device driveris a good example of this. Every new piece of hardware,

such as a printer or digital camera, needs to have a device driverprogram written for it which interfaces between the hardware and the

Windowsoperating system. Device drivers are programs that run in

the background. Because they dont interface directly with the user

they would not be written using a visual or event driven programming

language. Device drivers are often written using languages such as C.

Script language a type of

programming language which, rather

than being standalone programs that

run on their own, runs within some othe

application to carry out a specific task.

Device driver a program that provid

an interface between a piece of hardwa

(such as a graphics card or network

interface) and the operating system.

Key terms


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1.4 Choice of languageThere are a very large number of different programming languages

available. Most programs are written using a piece of software called

an IDE (integrated development environment). This includes all the

things a programmer needs to write the program, such as:

aneditor this often provides additional tools to help write theinstructions, such as pop-up help

acompiler this converts the instructions into machine code that

the computer can understand

aWindowsforms editor to create Windowsforms and add all

the required controls (buttons, text boxes, check boxes, etc.)

adebugger to help identify bugsin the program.

Visual BasicExpress edition is an example of a sophisticated IDE which

includes all these tools.

Integrated development environment

(IDE) a piece of software which lets

you write code, test it and build it into an

application or game.

Bugs errors or mistakes in the program

which prevent it from working properly.

Key terms

There are many different programming languages. Find examples onthe Internet of code written in the most popular ones, such as C++,Javaor Pascal. Can you spot any similarities between them?

Activity: Programming languages

With such a wide range of languages, the choice of which one to use

to write a program may seem impossibly complex. In reality the

choice is often quite simple and will depend on one or more of the

following factors.

Organisational policyAn organisation that develops a lot of computer software will often have

a policy to use a particular language. This may be for reasons such as

consistency or to maintain compatibility with other software they have

developed. It will also enable a company to develop expertise in that

particular language.

Suitability in terms of available features

and toolsCertain languages are better suited, in terms of the features andtools they have available, to particular tasks or hardware and software

environments. For example, as discussed earlier, visual programming

languages, such as MicrosoftVisual Basic, may be best suited to a

project to develop a Windowsapplication. Visual Basicalso has very

good links to other Microsoftproducts, such as the Accessdatabase.

So if you are developing a program which needs to interface to an

Accessdatabase, then this may be a good choice.


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On the other hand, if you are developing a program that needs to be able

to run on a wide range of different hardware and operating systems, then

Visual Basicwould be a poor choice as it only runs on PC hardware with

the MicrosoftWindowsoperating system. Javamight be a much better

choice as it is available for a wide range of hardware and operating systems.

Availability of trained staffIt takes a considerable amount of time to develop a good level of

expertise in a particular programming language. This means that if a

company already has staff who are experts in a particular programming

language, then that language will be the obvious choice for any new

project. Also, if a company is setting out on a new software development

project it may need to employ new programmers. It will be easier for

the company to find programmers with knowledge of the most popular

languages rather than the lesser known ones, even if the less well-known

languages are better suited to the project for other reasons.

ReliabilitySome programming languages are specifically designed for applicationswhere reliability is very important. These are sometimes called mission

critical applications. They include things like aircraft or nuclear power

plant control systems where failure would be catastrophic. Ada is an

example of a language developed for these types of application. It was

originally developed for the United States Department of Defense for

use on military projects.

Development and maintenance costsSome languages and software development approaches have a

reputation for being able to create software systems in a shorter

timescale than other approaches. This means they can provide systems

at a lower cost. For example, rapid application development (RAD) is

one approach which uses prototyping to speed up the development

process. Prototypes are simple versions of the program under

development with limited functionality. The prototypes are used to gain

user feedback and develop improved versions.

Equally, some languages have a reputation for being easier to

maintain. Maintenance can include updates, bug fixes and additions to

the original program. A company may, therefore, choose a particular

language or development approach, because they hope to achievelower development and maintenance costs.

ExpandabilitySome software systems may only be used by a small number of people,

while others many need to be used in a large organisation by thousands

of employees simultaneously. For example, a banking application that

controls ATMs (hole in the wall cash machines) will need to handle

many thousands of simultaneous transactions from machines all around


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the country (and world!). The choice of programming language and

database systems to support such a huge system will need to consider

how well they can cope with a large and growing number of users.

Some programming and database systems are designed with such

expandability in mind (sometimes called scalability), while others are

better suited to single user or small multi-user applications.

What are the most popular programming languages at the moment?Take a look at programming job adverts on the Internet and see whichlanguages are in the most demand. Please go to Hotlinks and click onthis unit to access a good website to start you off on this activity.

Activity: Popular programminglanguages

PLTS

Investigating different programming

approaches will provide evidence that

you are an independent enquirer.

Functional skills

Preparing a presentation will provide

evidence for your functional ICTskills

find and select information and

develop, present and communicate

information.

It will also provide evidence for your

functional Englishskills in writing.

You are working for a mobile phone company which is consideringstarting their first software development project. One of themanagers has asked you to investigate the different approachesto programming (paradigms) and factors that might influence theirchoice of programming language.

1. Create a presentation for the manager describing theapproaches to programming (paradigms) and identifying thefactors influencing the choice of language. P1 P2

2. Add to the presentation you prepared for task 1 by describingthe features of a programming language of your choice,including how sequence, selection and iteration are used. M1

Grading tip

When completing question 2, you will need to describe thefeatures of programming languages, including how sequence,selection and iteration are used. Check the unit content forlearning outcome 1 and make sure you cover the featureslisted there.

P1 P2 M1Assessment activity 18.1

Just checking

1. What features does HTML not have that can be found in most programming languages?

2. What instruction is used in most programming languages to implement (write instructions to

perform) selection?

3. Explain the difference between local and global variables.

4. What three attributes or properties does a variable have?


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2. Know the software developmentprocess

2.1 Software development

life cycleLike any large commercial project, there is a series of steps that must

be completed when developing a software system. This is sometimes

called the software development life cycle. It is represented by the

diagram in Figure 18.12.

Figure 18.12:The software development life cycle.

Gather user

requirements

Write

programs

Create

specification

Create

design

Maintain

system

Test

programs

We refer to this process as a life cycle because, even when the

software is completed and has entered the maintenance phase, new

requirements and developments are likely to be suggested, resulting

in work starting on a new version of the software. In this way the

development cycle begins and repeats again and again.

Lets now look at each stage in detail.

Gather user requirementsMost commercial programs are written to solve some kind of problem

or to take advantage of an opportunity. Clearly, you need to understand

the nature of the problem or opportunity before you can write a

program that will achieve what is required. This involves working with


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the people who understand the problem and who will be using the new

system. Since these people will probably not be computer experts, the

discussions at this stage focus on the business aspects of the problem

or opportunity.

The work to gather user requirements is normally carried out by a

systems analyst. As well as being an expert in systems development,

a systems analyst is usually also an expert in the business area that thesystem is being developed for. For example, if the software is being

developed for use in a bank, the systems analyst will need to be an

expert in banking and finance systems.

The gather user requirements stage is when information is collected

about what the exact requirements of the system are. The information

may be collected by a number of different methods including

interviews, observation of current working practices and

reviewing documentation.

Systems analyst someone who

operates at the design stage of systems

development. They work closely with the

people who will use the system beingdeveloped to identify exactly what it

needs to do.

Key term

Case study: Requirements gathering for a learner gradetracking system

A college needs some software to keep track oflearners grades. The grade tracking records arecurrently kept using a paper-based system.

At the requirements gathering stage, the systemanalyst speaks to the tutor who looks after the gradetracking records. He takes copies of the documentscurrently used and discusses with the tutor how thecurrent system works so he is sure he fully understandsit. He also discusses with the tutor the limitations of

the current system and what improvements he hopesthe new computerised system will provide.

1. What improvements do you think anelectronic system is likely to provide over apaper-based one?

2. Is there anyone else you think the systemsanalyst should speak to, other than thetutor, about the new system?

Create specificationAt the end of the requirements investigation stage, the systems analyst

will produce a specificationdocument. This is an important document

as it is often the basis of a contract between the company developing

the software and the company who is paying for the software to be

developed. There are a number of things which need to be described in

the specification, and these are listed below.

What is the scope of the system?

To decide on the scopeyou need to ask the question What will the

system do, and what wont it do? This may be an obvious question,

but it is an important one. Computers are very powerful machines and,

if you had endless time and money, there are many facilities you could

include in the software you are developing. You may need to decide

what the most important features are (referring back to the aims of the

system) and develop the first version of the software with just those

features, putting the rest on a wish list for future versions.

Specification a document describing

what the system that will be developed

will do in non-technical terms.

Scope all the functions and facilities

that are to be included in the system.

Key terms


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Inputs and outputs

The data that will be input to the system and the outputs that it will

provide are clearly important. There are a number of possible sources

of input data; they may come from a human user or from another

computer system, for example. Outputs may also be in a number of

different formats, such as on the computer screen, in a printed report or

to another computer system.

Case study: Identifying the scope of the learner gradetracking system

The scope of the learner grade tracking system isquite straightforward. It will record the grades forlearners on the CTEC Diploma course, which consistsof three units only. Each unit can be graded as:

Incomplete

Pass

Merit

Distinction.

The system should also calculate an overall grade foreach learner. It should allow a tutor to input learnernames and view, add and update their grades. Allinput and output should be via a Windowson-screen form.

Possible future versions of the program might includthe following additional features:

dealing with multiple class groups

dealing with other qualifications with differentnumbers of units

printing class lists of grades.

1. Is writing a program the only solution to thisproblem? Are there other ways that this gradrecording system could be implemented? Whaadvantages and disadvantages would otherapproaches have?

2. Sketch out the main menu form for this system

Processing

This aspect of the specification describes how the input data will be

transformed, for example calculations may be carried out on the inputdata. In a system that calculates employees monthly salaries, the input

data will probably be the number of hours worked by each employee

plus any additional information, such as overtime hours or days off sick.

This data will be used to calculate each employees pay by multiplying

it by their hourly rate and then subtracting things like tax and National

Insurance contributions.

User interface

The specification will give details of how the user will interface with the

system. Most commonly this will be by a standard Windowsinterface

using a monitor, mouse and keyboard, but there may be other methods.

Many applications these days use the Internet and so their user

interface may be via a web browser, such as MicrosoftInternet

Exploreror Mozilla Firefox.

Specialist applications may use a text-based interaction (like the

console applications we looked at on pages 78).

Touch screens are particularly well suited to applications where a

keyboard is not convenient, such as mobile applications or in retail

(for example, point of sale terminals in shops).


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Constraints

Most software is developed to a budget which will constrain (limit) the

amount of time that can be spent developing the software. Estimating

how long it will take to develop a software system and, therefore,

how much it will cost is one of the most difficult aspects of software

development. There are many examples of systems which have taken

far too long and cost much more than originally estimated.

The budget available may restrict the features that can be included in

the system, and may mean some features have to be pushed onto the

wish list for the next version. Other possible constraints include the

hardware platform and software system that must be adopted. The new

system may well have to be used on existing hardware and software,

including a particular operating system.

Case study: Learner grade tracking system processing,user interface and constraints

Processing:The processing requirements of the learner gradetracking system are fairly simple. The systems mainpurpose is the storing and retrieval of learner grades,so there is not much processing involved. However,the calculation of the final grade does requiresome processing and this should be defined in thespecification. The rules for deciding on thefinal grade are:

for a pass a learner gets one point

for a merit a learner gets two points

for a distinction a learner gets three points.There are three units, so if a learner has less than threepoints they have failed. If they get three to five points

they get a pass. If they get six to eight points they geta merit. Nine points achieves a distinction.

User interface:The user interface of the system will be a standardWindowsinterface.

Constraints:The system needs to work with standard PC hardwareand the Windows7 operating system.

1. Is any other information required to fullyunderstand how the grade tracking system

will work?

2. Create several imaginary sets of learner unitgrades and work out what overall gradethey achieve.

Create designThe design stage builds on the requirement and the specification.

The requirement and the specification documents describe whatthe

software will do, whereas the design considers howthe software is

going to work.

The first step may be to break the problem down into components,

particularly for large, complex software.

Once the general structure of the solution has been decided (what

the components are and how they interact), each component can be

designed in detail.

There are many different tools available to software designers: action

lists, storyboards, structure diagrams, data flow diagrams, algorithms,

and so on. These are discussed on pages 2629 of this unit.


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Unit 18Software

Write programs (coding)The coding (or implementation) stage is when the program is written.

Programmers follow the design and refer back to the requirements

to help them. If implementation highlights any problems with the

requirements or design, then these documents should be

updated appropriately.

Test programsAll but the simplest of programs contains bugs. In some ways, writing

software is like writing a book try as you might to avoid spelling mistakes,

poor grammar and factual errors, some inevitably slip through the net.

Software developers use testing (just as publishers use editors and

proofreaders) to minimise the severity and number of errors as much as

possible. If the requirements have been written carefully, then many of

the tests will be taken directly from these.

For a large software project, each component is tested individually

these tests are called unit tests. They may be run manually or writtenas extra software that can be run automatically. Further tests, called

integrationtests, check that the components work properly together.

Documentation

Each stage of the software development process results in the creation

of one or more documents. The requirements investigation stage results

in a formal requirements specification. During the design stage, various

charts and documents are produced to record the design decisions that

have been made. Even the implementation and testing stages need

documentation in the form of program comments and test plans and logs.

Often, additional time must be put aside to finalise the internaldocumentation (for example, to make sure that the design documents

are up to date) and to prepare customer documentation, such as a user

manual or help pages built into the software.

Maintain systemSoftware maintenance is rather like car maintenance. After a car

has been manufactured, it still requires regular tune-ups to keep it

performing efficiently. It may even be taken to a mechanic for an

upgrade, such as fitting satellite navigation. In the same way, software

bugs that are discovered after release will need to be fixed, and users

may request that developers add new features to the software.

One measure of the success of a software product is the length

of time for which it remains in use. Some software in use today

has gone through many versions over the course of years, or even

decades. Because of this, over the lifetime of a software product, the

maintenance stage is likely to last the longest and take the most effort.

If the earlier stages have been rushed or performed carelessly (for

example, if the design documentation is poor) then the maintenance

Did you know?

Software developers often find

writing the code to be the most

enjoyable stage of a project. When

planning for, and working on, your

own software development projec

make sure you allocate a generous

proportion of your time to the

requirements and design stages

(which will save you time overall) a

to the testing and documentation

stages (which will improve the qua

of your software product).

Integration the process of bringing

together software components, perhapwritten by different development teams

into a single piece of software.

Key term


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stage will be heavy going, difficult, inefficient and boring. If care has

been taken, though, the maintenance stage can be as interesting and

productive as the implementation stage.

PLTS

By creating the outline specificationand selecting and justifying the

programming language, you will

demonstrate that you are a

creative thinker.

Functional skills

Creating the outline specification will

provide evidence for your functional

Englishskills in writing.

1. Following the work you have done for Assessment activity18.1, your manager has asked you to add further slides toyour presentation describing the stages of the softwaredevelopment life cycle. P3

2. Your manager has also asked you to produce an outlinespecification for the software they are thinking of developing.The program will be used in their shops so customers can seehow much different mobile phone tariffs would cost them eachmonth. You only need to produce a specification for the basicversion of the system, which will allow customers to enter thenumber of minutes they think they will use each month at

off-peak and peak times. They can choose three different tariffs: low usage 10 monthly fee, peak calls 5p a minute, off

peak calls 1p a minute

high usage 15 monthly fee, peak calls 3p a minute, offpeak calls 1p a minute

business usage 25 monthly fee, peak calls 2p a minute,off peak calls 1p a minute.

Produce an outline specification for this mobile phonecost-estimator program. P4

3. Select a programming language to be used for the mobilephone cost-estimator program and provide written justificationfor your choice.

M2

Grading tips

Check the unit content for learning outcome 2 to makesure you have covered each stage of the softwaredevelopment life cycle.

When justifying the choice of programming language youneed to give reasons why you chose that language. Youcould also include the reasons why you didnt chooseother languages.

P3 P4 M2Assessment activity 18.2

Just checking

1. List the main steps of the software development life cycle.

2. Explain the term scope of the system.

3. List the typical constraints that a development project might have.

4. What sorts of user interface may a program make use of?


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3. Be able to design softwaredevelopment solutionsOnce the specification for the system has been completed and it has

been agreed by the users, and whoever is paying for the system to be

developed, the next stage is the detailed design.

3.1 DesignThe system design provides more detailed information about the

internal working of the software. The work at this stage involves taking

what has already been decided at the investigation stage and adding

more detail to it. There are a number of things that need to be designed

and planned for example, how the program will look to the user, what

data the program will need to store and how that data will be processed.

As you already know, programs take some kind of data as input,

perform some kind of processing on that data (perhaps a calculation)and produce some kind of output. At the design stage, software

developers must add more detail based on their research about what is

to be input, how it will be processed and what will be output.

Where the problem is large and complex, the developers also need

to consider how it can be split into manageable sections. The key to

this is to choose sections that can be as self-contained as possible.

When dividing a system into different sections (called modulesor

procedures), these modules will need to communicate with each other

to some extent (probably by passing values, often called parameters).

However, this should be kept to a minimum, with each module having a

clearly defined purpose and as little interaction with other modules

as possible.

When several programmers are working on the same software

development project, dividing the system up into modules is necessary

so that each programmer can work on an individual module. Dividing

a program into modules also makes the testing easier, because

developers can test each module as it is completed rather than having

to wait until the whole program is complete.

3.2 Design toolsNumerous different methods have been developed to help model the

complexity that is often involved in a software system. Many of these

involve creating diagrams to show the flow of data or the structure of

the system (such as how it is split into different modules). Many of the

methods also rely on a technique which is sometimes called functional

decomposition. This involves creating a general overview model or

diagram of the system which is then refined by creating more and more

detailed diagrams.

Module or procedure a program tha

is part of a larger system. It normally

carries out some specific function or tas

Parameter data that is passed to a

module or procedure. For example, if

you create a procedure which calculate

the area of a circle, it will need to be sethe radius of the circle as a parameter.

Key terms


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Structure diagramsStructure diagrams are graphical tools which are used to split a system

down into more detailed procedures or modules. As with many design

tools, you can start with an overall structure diagram which shows how

the whole system is split into different procedures and then draw a more

detailed structure diagram for each procedure. With a really complex

system, you may even draw further levels of structure diagrams, eachone describing a smaller part of the system in more detail.

A structure diagram is drawn with a box showing the main program

or procedure name at the top and then each procedure or section

presented in boxes below. Arrows show inputs or outputs for each box.

A structure diagram for the learner grade tracking system is shown in

Figure 18.13. By looking at the specification, we can see what the main

functions of the software should be. These main functions form the

main procedures.

Did you know?

At this stage you dont need to worry

too much about detail, although

the procedures you define should

be as self-contained as possible.

Remember that you can always

refine your design as it develops and

its quite acceptable to make many

revisions to it as you work through.

Later in the design process you may

realise that what you had in mind

at this early stage wont work well.

Thats fine all you need to do is

return to this stage and update

your design.

Figure 18.13:Structure diagram for the learner grade tracking system.

Learner GradeTracking System

Update grades

Learner nameLearner name Learnername

Grades

Grades

View gradesInput new

learner details

Narratives and action listsAnother technique often used at the early stage in developing a design

for a software system is to define the way that the system will interact

with the user. The user requirements should have already defined what

inputs and outputs the system will have in the form of a narrative. This

is really just a written description of what the inputs and outputs are. But

narratives are a rather long-winded way of describing the interaction the

program will have with the user and the sequence of events. A simple

action listis a more convenient way of designing the way the user willinteract with the system.

An example of an action list for the Update Grades procedure of the

learner grade system is:

1. open Update Grades screen

2. enter learner name

3. if not found display an error message

4. if found display learner details and current mark

Narrative a written description of how

the system will interact with the user.

Action list a list of user actions and

inputs and the programs responses.

Key terms


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5. choose to update marks or exit

6. if updating marks, enter new mark

7. confirm update

8. exit.

StoryboardsFrom this simple list we can go on to create a more detailed design for

the interaction using a storyboard.

A storyboard is simply a series of drawings (which you can draw by

hand or electronically) which demonstrate the stages of the interaction

between the program and the user. They are like a cartoon story of

the Windowsforms the user will work through to complete a task. An

example of a set of storyboards for the Update Grades procedure is

shown in Figure 18.14.

Did you know?

Storyboards were first used in the

animated film industry and were

developed by the Walt Disney stud

Figure 18.14:Storyboards for the Update Grades procedure.

Storyboards are particularly important when developing software that

has a strong visual element, such as a game.

Data flow diagramsAnother graphical technique you can use is data flow diagrams (DFDs).

This is a series of diagrams drawn at different levels each level adds

further detail to the system. Data flow diagrams, as the name suggests,

are designed to help you figure out how data will flow between the

various procedures within the system as such, they are better suited

to a system which works in conjunction with a database. DFDs would

not be of much use for designing a system such as a game, but wouldbe more useful with a system like the learner grade tracking system we

have been considering.

The first step in creating a data flow diagram is to create the Level 0

diagram, which simply shows the system as a box. Oval shapes are

drawn outside the box representing external people or systems (known

as external entities) that have inputs or outputs to or from the system.

Arrows are drawn and labelled indicating what those inputs or

outputs are.

Entities real world things, such as

customers, products or books, that nee

to be represented in the system that yo

are designing.

Key term


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Figure 18.15 shows an example Level 0 DFD for the Update Grades

procedure. The tutor entity is shown twice only to make the diagram

easier to read the same tutor would complete all the interactions

involved in updating a learners grade.

Figure 18.16:Level 1 DFD for the Update Grades procedure.

Figure 18.15:Level 0 DFD for the Update Grades procedure.

Learner class

Tutor TutorLearner name

Current mark Confirm update

Input new mark

Having completed the Level 0 DFD, the next step is to create the Level

1 DFD. This keeps the same external entities as the Level 0 DFD and

the same inputs and output, but includes details of the processes and

data stores that exist within the system. As a general rule you will need

a process to deal with each input and output you identified in the

Level 0 DFD.

Processes are drawn as a numbered box with the description of the

process inside it. Where the system or procedure needs to store data

(to be read or written to by the processes) it is shown as an open-ended

rectangle, which is also given a number, starting with a letter D. Figure

18.16 shows the Level 1 DFD for the Update Grades procedure.

Tutor Tutor

1.1

D1 Learner name, grade

Find learner

1.2

Find learner

Learner name

Learner name New grade

Grade

Input new grade

Confirm updateConfirm update


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There are some rules for drawing the Level 1 DFD:

1. you must have the same external entities and data flows in the Level

1 diagram as in the Level 0 diagram

2. in a Level 1 diagram only processes can read or write to data stores

3. each input or output must be handled by a process in the

Level 1 diagram.

Like many of the design techniques we have looked at, creating DFDs

is not easy and needs practice. It is also difficult to figure out the

processing steps involved unless you have a good understanding of

how programs are written.

Entity relationship modelsEntity relationship models (ERMs) show the relationships between

data items.

When an analyst has used the DFDs to identify the processes and data

stores in the system, the ERMs can be produced. There is no strict order

in producing these diagrams; often the design evolves over some time,with DFDs, ERMs and other diagrams being refined through several

versions. ERMs are used to model the relationships that exist between

different entities in the system.

Entities have attributes these are the elements that define a particular

entity. Some simple examples are shown in Table 18.2.

An occurrenceof the entity Customermight be:

Name: John Smith Office Supplies

Address: 10 Main Street, Watford

Credit limit: 1,500

Primary and foreign keys

One (or more) of the attributes of a particular entity is normally defined

as the primary keyattribute.

To guarantee uniqueness, numbers are normally used for primary

key attributes. So, for example, on a database recording details of

the entity Learner, the attribute Learnernumberwould be the primary

key. The system would need to ensure that each learner received a

unique number.

Most systems have a number of entities within them, and entities often

have relationships between them. For example, the entity Customers

and the entity Ordersare related, because every order a company

receives comes from a customer. The relationship between two entities

can normally be described by a verb. In this example, the verb is place,

because customers place orders see Figure 18.17.

Table 18.2: Examples of entitiesand attributes.

Entity Possible attributes

Customer Name

Address

Credit limit

Product Description

Type

Price

Book Title

Publisher

Price

Occurrence one complete set of valu

for each attribute for a particular entity

is sometimes called a record.

Primary key the attribute used to

uniquely identify a particular occurrenc

of an entity.

Key terms

Figure 18.17: The ERM showing the relationship Customers place orders.

Customers OrdersPlace


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How could you know which orders were placed by a particular

customer? One of the attributes of the entity Customerwill be the

customer number. This unique number identifies each customer and so

is the primary key of that entity. When that customer places an order, an

occurrence of the orders entity is created. We need to be able to tell

which customer placed the order, so the customer number is inserted

into that occurrence of the orders entity. This is known as a foreignkey

(foreign because the key value belongs to another entity); an example is

shown in Figure 18.18.

Foreign key an attribute that is a

primary key in a different entity.

Foreign keys are used to create links

between entities.

One-to-many relationship where one

occurrence in an entity can be related to

many occurrences in another entity.

Key terms

Figure 18.18: Primary and foreign keys.

Entity: CUSTOMER

Customer number

Primary key

Attributes:

Name

AddressCredit limit

Etc.

Entity: ORDERS

Order number

Primary key

Customer number

Foreign key

Attributes:

DateTotal value

Etc.

Note that the fields used to create the relationship between the two

entities must have the same data type in both entities. This is one of the

reasons why you must choose data types with care.

One-to-many relationships

Each customer will have at least one order that they placed, and

probably many more. So, for one customer there will be many

occurrences of the orders entity. This type of relationship is called a

one-to-many relationship. In an ERM, this is shown by a fork at the

many end of the line that joins the two entities, as shown in Figure 18.19.

Figure 18.19:A one-to-many relationship.

Customers Orders

One-to-one relationshipsOne-to-many relationships are the most common type of relationship

between entities, but they are not the only type. Imagine a company

that has a system to store data about its employees. The entity

Employeewill have attributes, such as name, address and salary, and

the primary key will probably be Employee number.

Some employees who work for the company are provided with a company

car. That gives us another entity, Company car, which has attributes such

as registration number, make, model, etc. There is a relationship between


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the two entities Employeeand Companycar, which can be described

by the verb uses because an employee uses a company car. As each

employee is assigned only one company car at a time, this is not a one-to-

many relationship, but a one-to-one relationship.The ERM for this type

of relationship is shown in Figure 18.20.

One-to-one relationship where one

occurrence in an entity is related to onl

one occurrence in a different entity.

Structured English or pseudo code

method of representing programming

code using English language rather

than the strict syntax of a programming

language.

Key terms

Figure 18.20: A one-to-one relationship.

Salesperson Company carUses

DFDs and ERMs are techniques used most often when designing

software that works with a database. When designing programs which

dont use a database, such as a game, then DFDs and ERMs would not

be used.

Create an ERM design for a doctors surgery appointments system.The surgery has several doctors and, of course, lots of patients.

Activity: ERMs

AlgorithmsAn algorithm is a mathematical expression which defines how a

complex calculation is done. As part of the design for a program,

algorithms may be defined using techniques like structured Englishor

pseudo code.

Both these methods are a type of half-way house between writing a

description of the calculation or processing and the actual program

instructions. Rather than having to worry about the exact syntax (rules)

of the programming language instructions, these methods allow you

to concentrate on the workings of the algorithm. Structured English

and pseudo code dont really have any rules as such; you just write the

calculation in a way that resembles program code. But if you dont have

much experience of writing program code, this is quite hard.

Lets look at an example. The specification for the learner grade tracking

system includes an explanation of how the overall grade is calculated.Written in pseudo code it might look like Figure 18.21.

This looks a bit like the code that will eventually be written, but it does

not follow all the rules of if instructions in Visual Basic.

3.3 Data typesWeve already seen a number of data types used for the variables in the

example programs weve looked at. Choosing the correct data type for

Figure 18.21: Pseudo code for the learngrade tracking system.

If points < 3 then Fail

If points >3 and 6 and


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a variable is important because if you choose an unsuitable data type

your program may not be able to manipulate the data in the variable

in the way it needs to. For example, you cannot do calculations with

variables that have a text data type. Its also very important that you

do not allow your program to assign a text value to a variable with a

numerical data type, as this will cause the program to crash.

Various numerical data types are provided to store integers (wholenumbers) and numbers with a decimal part of various sizes. Again, it

is important that your program should not place a numerical value in

a variable which cannot store a number of that size, otherwise your

program will crash.

The actual data types you have available will depend on the program

language you use. The data types available in Visual Basic Express are

listed in Table 18.3.

Table 18.3: Data types in Visual BasicExpress.

Type Used for Range of values

Boolean Values that can be true or false True or false

Byte Whole numbers 0 to 255

Integer Whole numbers 32,768 to +32,767

Long Very large whole numbers Approximately plus or

minus 2 billion

Currency Decimal numbers with 2 digits

after the decimal

Single Floating point numbers Up to 7 significant digits

Double Large floating point numbers Up to 14 significant digitsDate Date and time

Variant Any type of data

String Any type of text

3.4 Software structures

Functions, procedures, classes

and objectsAs mentioned on page 25, one of the benefits of breaking a large and

complex system into different procedures is that different programmers

can then work on the different procedures. However, a team of

programmers working on the same system needs to follow the same

rules and procedures to avoid problems. They must be aware of the

structure of the system that is, the way it is divided into procedures

and how the procedures interact. They must also follow certain rules

and guidelines about how the code is written.


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Clearly the functions and procedures or classes and objects (if the

object-orientated approach is used) must be well defined to ensure

they can work together, even though they are being written by different

people. The way the data in the system is used must also be clear to

everyone. This point applies in particular to global variables which are

used by more than one procedure.

Abstraction of dataIn a complex software system, the abstraction of datacan help deal

with the complexity.

Abstraction of data works well in large projects with many different

programmers working on the system, each writing their own

procedures. For example, programmer A creates a procedure which

programmer B needs to use. All programmer B needs to know about

is the functions of that procedure; she does not need to know anything

about the internal workings of that procedure. If at some stage of

the development project, programmer A needs to modify or rewrite

his procedure, he can do that and, as long as he keeps the samefunctions, this wont have any impact on programmer Bs use of

that procedure.

This is a concept related to object-orientated programming, where

data and functions are encapsulated together in an object and global

variables are not used.

There are a number of other considerations when working on a large

and complex project with many programmers.

Pre-defined codeMany different software systems share similar requirements and itmay be possible for some functions to use procedures or code from

previous projects. Using pre-defined code can save a lot of time as the

procedures have already been written and tested.

ReadabilityThe people running complex software projects will often set standards

which the programmers must follow. These standards will include things

like adding comments to the programming code they write.

Comments

Comments are ignored by the computer but make it much easierfor other people to read the code. This is important on a large

project, as other people may need to work on the program. In

particular, during the maintenance phase, the person who carries

out bug fixes or modifications will usually not be the programmer

who originally wrote the code. Even on small software development

projects, adding comments to the program instructions is regarded

as good practice. Figure 18.22 (on page 34) shows an example of a

program with comments added.

Abstraction of data when the way

a procedure deals with data is hidden

inside the procedure, but the interface

with other procedures remains the

same. This is useful in situations where

number of different procedures are usi

the same data.

Pre-defined code programming cod

which has already been written and can

be re-used in another software project.

Key terms


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Using appropriate names for variables

Using appropriate names for variables is another example of good

practice. Variables can be given names, such as A or Bor Data1, but this

kind of name gives no clue as to the purpose of the variable. Its a much

better idea to use names which describe what the variable is used for,

such as LearnerNameor Unit1Grade(note that variable names cannot

contain spaces).

Some programmers recommend prefixing the variable name with a

code which indicates its data type. So a text variable called LrnName

(shortened version of LearnerName) might be named TxtLrnName,

while an integer variable called Pointsmight be named IntPoints. Thiscan help prevent the mistake of assigning an incorrect type of data to a

variable, such as a text value to a numeric data type variable.

Indentation

A final example of good practice, which improves readability of code, is

indenting loop or selection blocks of code, which makes it easier to see

where they start and finish. In fact Visual Basicdoes this for you.

Quality of codeIt takes a long time and a lot of practice to become a proficient

programmer there are many pitfalls in writing programs that youreally only discover with experience. Some attributes of a well-written

program are listed below.

Reliable and robust

You might imagine users will exploit the program in the way it was

intended, but sometimes they may misuse it. For example, users may

make a text input where a numeric one is expected. A well-written,

robust program will be able to cope with such a user error and display

Figure 18.22:Program with comments added.


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an error message rather than crashing. The best way to ensure a

program is as robust as possible is by thoroughly testing it to ensure it

works properly and can cope with being used incorrectly.

Usable

The best programs are easy to use and are intuitive. Ease of use is a

subject of its own (sometimes called HCI human computer interface),

but there are some simple things you can do when designing the userinterface to make a program accessible to users. The following features

all make it easier for the user to figure out how your program is to

be used:

clearly labelled controls (buttons, text boxes, etc.)

logically arranged controls (i.e. arranged in the order they are likely

to be used)

controls grouped by function.

Portable

This is the ability of a program to run on different hardware or software

platforms. This may be important for some systems. As mentionedearlier, some languages such as Javasupport many different types

of hardware.

Maintainable

Every commercial software product needs to be maintained with

updates, bug fixes and new features. There are a number of things that

can be done to make software easier to maintain. These include:

good, detailed technical documentation describing the design and

implementation of the software

well-written programs see pages 34 and 35 for discussion of

several good practices, such as adding comments, using meaningfulvariable names and indenting loop and selection instructions.

3.5 ReviewBefore the work on actually writing the program starts, its wise to review

the design produced and compare it to the specification to make sure

it really matches what is required. Also, once the software is complete,

you should review it against the requirements to see if it matches the

original design.

In a commercial software development project, the organisation that is

paying for the development will often want to check that the completedsoftware meets the requirements defined in the specification, using

what is often called acceptance testing. The final payment for the

development work will often not be made until the system passes its

acceptance tests.

Intuitive when describing a person,

intuitive refers to someone who can wo

out how to do something instinctively,

without being told. An intuitive user

interface is easy to use because the

user can easily see how to achieve thedesired effect.

Key term

Acceptance testing a defined set of

tests which a completed software syste

must pass before it will be accepted by

the customer. The tests are often definearly in the project development proce

and may be contractual (i.e. payment

may depend on the software passing th

acceptance tests).

Key term


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PLTS

By designing a software solution for a

business requirement, you will show

that you are an effective participator.

By evaluating your design you will

show that you are a reflective learner.

Functional skills

Developing an algorithm for

calculating the different tariffs could

provide evidence of your functional

Mathsskills (identify the situation

or problem and the mathematical

methods needed to tackle it).

1. Following on from the work you did in Assessment activity 18.2,create a design for the mobile phone cost-estimator program

Documents

Unit 18 Software Design