a level computing 9691 paper 2 notes

7/31/2019 A Level Computing 9691 Paper 2 Notes

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A Level. Computing (9691/ 2)

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CPT2: Programming Techniques and Logical Methods

DJM 7

ADDITIONAL NOTES:

2.1 DESIGNING SOLUTIONS TO PROBLEMS

Candidates should be able to:

1. Discuss the importance of good interface design;

2. Design and document data capture forms, screen layouts,report layouts or other forms of input and output (eg sound) fora given problem.

Purpose of a human-computer interface

A human-computer interface must allow effective and efficient communication between thecomputer and the human user.

Humans need to communicate with computers to: give instructions Print, Save, Open, Delete, Copy, Paste etc.

enter information file names, number of pages to print;

make choices Yes, No, Cancel etc..

Computers need to communicate with humans to:

inform of errors illegal operation, printer out of paper, wrong password;

tell on progress copying, deleting,installing, downloading;

display the results of processing ask for options eg number of pages to be

printed, which file to open.

provide help with performing tasks thishelp could be in the form of status bar text,

yellow Tip boxes, an Office Assistant or even a full-blown help file accessed via the Helpmenu.

Failure to create an interface that allows effective dialogue would mean that:

only the highly trained would be able to use the system and even they may need some

training; the number of errors would increase if there is no feedback to tell the user that the correct

instruction was being carried out (or had been carried out);

working on such a system would usually be very slow.



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ADDITIONAL NOTES:

Good interface design

A good interface is needed to make it easier for the end-user. It will allow them to:

learn to use the system quickly;

enter data quickly;

enter data without errors.

A good interface is also needed to ensure that the output is clear and understandable.

Learn to use the system quickly

A good interface will have a layout with which is familiar to the user. It will use familiar controls(input boxes, buttons, etc.) and place these controls in familiar places. There will be consistency

between the different screens.

Enter data quickly

To help the user to enter data quickly, the interface will contain. Some of these will have the

predicted response set as a default value.

Enter data without errors

Accurate data entry can be helped by the use of drop-down lists, option buttons and check-boxes.

Dialogue boxes can also be used to alert the user to validation errors.



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DJM 9

ADDITIONAL NOTES:

Considerations when designing an interface

A user interface must match both the abilities and the needs of the intended users. It must take intoaccount:

the potential user expert/novice, young child/older adult, specialist professional/general

public; partially sighted or physically disabled. For example, a novice user should beprovided with command buttons that are large enough to select easily; an expert user should

be provided with keyboard short-cuts in order to select commands quickly and without

having to take her hands from the keyboard. A young childs interface should be bright and

possibly include pictures within command buttons particularly for the pre-reading agegroup; a specialist professional such as a graphics design artist may need a graphics tablet

and a very high resolution VDU. It is also appropriate to vary the input and output devices

according to the potential users concept keyboards for children; speakers for the visuallyimpaired; microphones for voice recognition if another form of input is not possible.

tasks to be performed are they repetitive, do they require special skills, are the taskssimilar or different. Note that an office worker who regularly switches between word

processing and spreadsheets will probably require a Windows like interface while a travelagent who only uses the computer to book holidays will require an interface consisting of

forms and dialogues.

environment if the computing device will be used in a noisy atmosphere then a naturallanguage recognition interface is useless. If the environment is dirty, such as in a car repair

shop, then an interface that requires the use of a mouse cannot be used.

available technology interfaces change as technology develops. Only the most recentlydeveloped interfaces are able to make use of natural language recognition or touch screens.

Interface design tips

Interface windows and dialogues should be given meaningful titles to identify them:

labels, text boxes, buttons etc should be appropriately laid out not too cluttered, nor toospaced. The type and format of data entry should be indicated for each text box;

items should be arranged in a logical sequence;

colours should be used very carefully each colour should be used for a specific reason andnot just because the designer likes it;

default values should be used within text boxes, combos, option-groups and check-boxes;

on-screen help facilities should be provided where necessary;

the user should be able to correct/confirm entries before final submission (e.g. useconfirmation prompts such as Yes/No buttons).

error messages should be clear they should inform the user of the error and tell them howto proceed.



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10 DJM

ADDITIONAL NOTES:

Designing forms for input and output

A typical question for this section may be:

An electronic general knowledge game displays, on the screen, the following:

a question

four possible answers

a clock to allow a set amount of time to answer the question

a score (which is added to if the question is answered correctly)

the score required in order to win the game.

The player touches the answer that they want to input as their choice.

You will be expected to use good interface design techniques and draw the interface. For example:

Note that the context of this question implies a full-screen video game if the interface was not full-screen, then a title to the window would be expected.



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DJM 11

ADDITIONAL NOTES:

Data requirements


3. Determine the data requirements of a program (relating to

3.2.3: Data types and data structures).

This is a skills section in which you may need to state the details of data that a program will use.For each data item you will need to include the following details:

The identifier name of the variables (or fields) that will hold the data;

The data type;

The size of the data;

The validation that needs to be carried out on the data.

A possible question may be:

A gaming system stores the 100 highest scores. Describe the fields that will be necessary in terms oftheir name, data type and size.

Solution:

Field name Data type Size of field (bytes)

Name String 15

Score Integer 2

Date Date 8

Note that in the context of this question, it is a game name that is required and so 15 bytes issufficient if the players full name (first and last) was required then 25 or more bytes wouldprobably be needed.



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ADDITIONAL NOTES:

Sub-dividing a problem


4. Explain the advantages of designing a solution to a problem by

splitting it up into smaller problems (top-down/modulardesign);

5. Produce and describe top-down/modular designs usingappropriate techniques including structure diagrams, showingstepwise refinement.

Top down/modular design

Top-down design is when a problem is split into smaller sub-problems, which themselves are splitinto even smaller sub-problems until each is just one element of the final program.

Benefits and drawbacks of modular programs

Benefits Drawbacks

Smaller problems are easier to understand Modules must be linked and additional testingmust be carried out to make sure that the linkswork correctly

Smaller problems are easier to test and debug Programmers must ensure that cross-

referencing is done

Development can be shared between a team ofprogrammers each person programmingdifferent modules according to their individualstrengths.

Interfaces between modules must be planned

Code from previously developed modules canbe re-used



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DJM 13

ADDITIONAL NOTES:

Structure diagrams

A structure diagram is a pictorial representation of a modular system.

Stepwise refinementStepwise refinement is the process of developing a modular design by splitting a problem intosmaller sub-tasks, which themselves are repeatedly split into even smaller sub-tasks until each is

just one element of the final program



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ADDITIONAL NOTES:

Algorithms


6. Produce algorithms to solve problems;

7. Describe the steps of an algorithm using a program flowchart;

8. Describe the steps of an algorithm using pseudo-code.

Algorithm

An algorithm is a sequence of steps, which perform a specific task.

In computing, algorithms are usually represented as a program flowchart, or in pseudo-code.

Program flowchart

A program flowchart is a pictorial representation of an algorithm.

Program flowcharts use special symbols:

Start/stop symbol

Process symbol

Input/output symbol

Decision symbol



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DJM 15

ADDITIONAL NOTES:

Flowchart to output the first five square numbers



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ADDITIONAL NOTES:

Pseudo-code

Pseudo-code is a simplified form of programming code that uses common programming keywords,but does not use the strict syntax rules of a programming language.

An example of a pseudo-code algorithm:

BEGIN

INPUT CardNumber

REPEAT

INPUT PIN

IF PIN is wrong for this CardNumber THEN

OUTPUT Wrong Pin

END IF

UNTIL Pin is correct

INPUT Amount

IF there are enough funds THEN

Dispense Cash

Update customers balance

ELSE

OUTPUT Sorry, insufficient funds

END IF

END



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DJM 17

ADDITIONAL NOTES:

Evaluating algorithms


9. Understand, and implement algorithms and evaluate them by

commenting on their efficiency, correctness andappropriateness for the problem to be solved.

Algorithms should be evaluated using the following criteria:

Efficiency

Correctness

Appropriateness

Efficiency

An algorithms efficiency can be judged in terms of:

speed how quick the algorithm produces the required output;

memory requirements how many lines of code the algorithm contains.

Correctness

Although an algorithm is expected to produce the correct outputs, correctness can still be measuredin terms of:

accuracy how many decimal places Produce output with greater accuracy e.g. moredecimal places;

range will the algorithm work with the complete range of inputs, or can it only deal withpositive numbers, whole numbers, numbers below 1 million, etc.

reliability will the algorithm always produce correct output within the range that it isdesigned to work, or are there values which it will not accept (zero, for example).

Appropriateness

Appropriateness can be measured in terms of:

length if the problem is simple then a short algorithm would normally be expected; speed if the output needs to be generated quickly, then the algorithm must be able to

complete quickly;

memory requirements an algorithm controlling a washing machine must not have the needfor a lot of memory!



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ADDITIONAL NOTES:

Rapid Application Development


10. Describe the use of Rapid Application Development (RAD) as a

design strategy, including prototyping and iterativedevelopment, and state its advantages and disadvantages.

Rapid Application Development (RAD) is a method of designing and writing software by

repeatedly producing prototypes, which are evaluated by the end-user and repeatedly refined until

the final version is finished.

RAD is an example of iterative development.

During the RAD process: the prototypes are tested and evaluated by the end-user;

the outcome of this evaluation is used in the development of the next prototype;

this evaluation and development of successive prototypes is repeated until the finishedsoftware is produced.

RAD usually uses CASE tools (Computer Aided Software Engineering). This allows prototypeswith reduced functionality to be produced quickly each prototype should be produced within two

or three weeks.

Advantages Disadvantages

The end-user can see a working prototype soonerthan in the Systems Life Cycle approach.

RAD focuses on the end result rather than on theprocessing - this means that the final solution is likely

to be inefficient in the way that it uses the availableresources (e.g. memory and processor time).

The user is involved with the design and caninfluence the direction the software is taking (e.g.what changes need to be made; what key featuresneed to be included in the next prototype).

RAD is not time-efficient when developing largescale applications the initial prototypes are likely tobe so different to the final product that a lot of re-thinking in necessary between prototype versions.

The overall development time is usually shorter andso costs are reduced.



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DJM 19

ADDITIONAL NOTES:

2.2 THE STRUCTURE OF PROCEDURAL PROGRAMS


11. Define and correctly use the following terms as they apply to

procedural programming: statement, subroutine, procedure,function, parameter/argument, sequence, selection,iteration/repetition, loop;

12. Identify the three basic programming constructs used tocontrol the flow of execution, i.e. sequence, selection anditeration.

Procedural programs are ones in which instructions are executed in the order defined by the

programmer.

Procedural languages are often referred to as third generation languages and include FORTRAN,ALGOL, COBOL, BASIC, and PASCAL.

Statement

A statement is a single instruction in a program, which can be converted into machine code andexecuted.

In most languages a statement is written on a single line, but some languages allow multiple lines

for single statements.

Examples of statements are:

DIM name As String

A=X*X

While x < 10



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DJM 21

ADDITIONAL NOTES:

Control structures

Computer programs execute statements one line after the next, unless there is a command thatinstructs the program to jump backwards or forwards to an alternative line.

SequenceSequence is when the programming statements are executed one after the other, in the order inwhich they appear in the program.

Selection

Selection is a control structure in which there is a test to decide if certain instructions are executed.

If x < 10 then

Do this line

And this lineElse

Do this line

End if

Iteration

Iteration is a control structure in which a group of statements is executed repeatedly either a set

number of times, or until a specific condition is True.

RepeatDo this line

And this line

Until n=5



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ADDITIONAL NOTES:

Flowcharts showing the control structures

Sequence Selection Iteration



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DJM 23

ADDITIONAL NOTES:

Selection


13. Understand and use selection in pseudo-code and a procedural

programming language, including the use of IF statements andCASE/SELECT statements.

IF-THEN-ELSE

This selection method is used if there are two possible outcomes to a test:

IF x < 0 THEN

OUTPUT Sorry, you cant have negative values

ELSE

a = x*x

OUTPUT a

END

SELECT-CASE

This selection method is used if there are more than two possible outcomes to a test:

SELECT CASE KeyPress

CASE LeftArrow

Move one character backwards

CASE RightArrow

Move one character forwards

CASE UpArrow

Move one character up

CASE DownArrow

Move one character down

END SELECT



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24 DJM

ADDITIONAL NOTES:

Iteration


14. Understand and use iteration in pseudo-code and a procedural

programming language, including the use of count-controlledloops (FOR-NEXT loops) and condition-controlled loops(WHILE-ENDWHILE and REPEAT-UNTIL loops).

FOR-NEXT

This is an unconditional loop in which the number of repetitions is set at the beginning.

FOR X = 1 TO 5

Answer = X*3

OUTPUT X, AnswerNEXT

WHILE-ENDWHILE

This is a conditional loop, which has a test at the start and repeats until the condition is false:

X = 0

WHILE X < 6 DO

X = X + 1

Answer = X*3

OUTPUT X, Answer

END

REPEAT-UNTIL

This is a conditional loop, which has a test at the end and repeats until the condition is true:

X = 0

REPEAT

X = X + 1

Answer = X*3OUTPUT X, Answer

UNTIL X > 4



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DJM 25

ADDITIONAL NOTES:

Comparison of the different iterations

FOR-NEXT WHILE-WEND REPEAT-UNTIL

An unconditional loop it willrepeat itself a set number oftimes.

A conditional loop it willrepeat until a variable hasreached a stated value.

A conditional loop it willrepeat until a variable hasreached a stated value.

The condition is tested at thestart of the loop.

The condition is tested at theend of the loop.

Iteration statements will alwaysexecute at least once.

Iteration statements may notactually be executed.

Iteration statements will alwaysexecute at least once.

For x= 1 To 10

a=x*5

Output x, a

Next x

x=1

While x < 11

a=x*5

Output x, a

x=x+1

End While

x=1

Repeat

a=x*5

Output x, a

x=x+1

Until x > 10



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26 DJM

ADDITIONAL NOTES:

Nested selection and iteration


15. Understand and use nested selection and iteration statements.

Nested selection

This is where there is an IF statement contained within an IF statement.

The following algorithm allows a maximum of four attempts to login to a computer system:

INPUT Password

IF NumberOfTries < 5 THEN

IF Password is correct THEN

OUTPUT Successful Login

ELSE

OUTPUT Password was incorrect

END

ELSE

OUTPUT You have made too many attempts

END

Nested iteration

This is where there is a loop within a loop.

A nested iteration is needed to initialise a two-dimensional array:

FOR row = 0 TO 7

FOR column = 0 TO 5

SET MyArray (row, column) = 0

NEXT column

NEXT row



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DJM 27

ADDITIONAL NOTES:

Subroutines


16. Understand, create and use subroutines (procedures and

functions), including the passing of parameters and theappropriate use of the return value of functions.

Subroutine/sub-program

A subroutine is a self-contained section of program code which performs a specific task and isreferenced by a name.

A subroutine resembles a standard program in that it will contain its own local variables, data types,

labels and constant declarations.

There are two types of subroutine. These are procedures and functions.

procedures are subroutines that input, output or manipulate data in some way;

functions are subroutines that return a value to the main program.

A subroutine is executed whenever its name is encountered in the executable part of the main

program. The execution of a subroutine by referencing its name in the main program is termed

calling the subroutine.

The advantages of subroutines.

The advantage of using procedures and functions are that:

the same lines of code are re-used whenever they are needed they do not have to be repeatedin different sections of the program.

a procedure or function can be tested/improved/rewritten independently of other procedures orfunctions.

it is easy to share procedures and functions with other programs they can be incorporatedinto library files which are then linked to the main program;

a programmer can create their own routines that can be called in the same way as any built-incommand.



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ADDITIONAL NOTES:

Recursion


17. Identify and use recursion to solve problems; show an

understanding of the structure of a recursive subroutine,including the necessity of a stopping condition;

18. Trace the execution of a recursive subroutine including calls toitself;

19. Discuss the relative merits of iterative and recursive solutionsto the same problem.

Recursion is when a function or a procedure contains a call to itself.

Recursion is used when calculating factorials:

FUNCTION Factorial(n)

IF n = 1 THEN

RETURN 1

ELSE

RETURN n * Factorial(n 1)

END IF

END

Note that a recursive function needs a stopping condition i.e. a statement that will stop it fromrepeating infinitely.

In the Factorial function above the stopping condition is IF n = 1.



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DJM 29

ADDITIONAL NOTES:

Tracing the execution ofx = Factorial(3)

01 FUNCTION Factorial(n)

02 IF n = 1 THEN

03 RETURN 1

04 ELSE

05 RETURN n * Factorial(n 1)

06 END IF

07 END

Note that in the above algorithm, the recursion happens on line 05.

LineNumber

Function Call ReturnedValue

Condition State of FunctionCall

01 Factorial(3)

02 False

05 3 * Factorial(2) New Call: n=2

n=3: Postponed

01 Factorial(2)

02 False

05 2 * Factorial(1) New Call: n=1

n=2: Postponed

01 Factorial(1)

02 True

03 1

07 n=1: Completed

05 2 * 1 = 2 n=2: Resumed

07 n=2: Completed

05 3 * 2 = 6 n=3: Resumed

07 n=3: Completed



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ADDITIONAL NOTES:

Iteration compared to recursion

Iteration Recursion

01 INPUT n

02 a=1

03 FOR i=1 TO n

04 a=a*n

05 NEXT i

06 OUTPUT a

07 END

01 FUNCTION Factorial(n)

02 IF n=1 THEN

03 RETURN 1

04 ELSE

05 RETURN n*Factorial(n1)

06 END IF

07 END

There is only one set of variable values that areused within the iteration.

A recursive subroutine is called repeatedly andeach call has its own set of variables that aredistinct from the variables in the other function

calls.

Iteration requires less memory than recursion because it only uses one set of variable values.

Iteration variables may need to be initialised andthe value of the tracking variable may need tobe tested for each iteration.

There needs to be a stopping condition thatcauses the subroutine to stop without callingitself.

Recursive subroutines are often more elegantand easier to understand than the equivalent

iterations.



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DJM 31

ADDITIONAL NOTES:

2.3 DATA TYPES AND DATA STRUCTURES

Data types

Candidates should be able to:20. Define different data types, e.g. numeric (integer, real),

Boolean, character and string; select and use themappropriately in their solutions to problems.

A data type is a method of interpreting a pattern of bits.

Intrinsic data types

Intrinsic data types are the data types that are defined within a particular programming language.

There are numerous different data types. They are used to make the storage and processing of dataeasier and more efficient. Different databases and programming systems have there own set of

intrinsic data types, but the main ones are:

Integer;

Real;

Boolean;

String;

Character;

Date;

Container.

Integer

An integer is a positive or negative number that does not contain a fractional part.

Integers are held inpure binary for processing and storage. Note that some programming languagesdifferentiate betweenshortand long integers (more bytes being used to store long integers).

In REALbasic an integer uses 4 bytes of memory this allows it to take a whole number valuebetween 2, 147, 483, 648.



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ADDITIONAL NOTES:

Real

A real is a number that contains a decimal point.

In many systems, real numbers are referred to assingles and doubles, depending upon the number

of bytes in which they are stored.

In REALbasic, a single uses 4 bytes of memory and can have a value between 1.175494 e-38 and

3.402823 e+38; a double uses 8 bytes of memory and can have a value between

2.2250738585072013 e-308 and 1.7976931348623157 e+308.

r.

Boolean

A boolean is a data-type that can store one of only two values usually these values are True orFalse.

Booleans are stored in one byte True being stored as 11111111 and False as 00000000.

Many of the properties of objects (windows, text-boxes, buttons etc.) in REALbasic have Boolean

values e.g. the visibility of an object will be set to either True (visible) or false (Hidden). Thesevalues are often set using check-boxes.

String

A string is a series of alphanumeric characters enclosed in quotation marks.

A string is sometimes just referred to as text. Any type of alphabetic or numeric data can be storedas a string: Birmingham City, 3/10/03 and 36.85 are all examples of strings.

Each character within a string will be stored in one byte using its ASCII code; modern systems

might store each character in two bytes using its Unicode.

The maximum length of a string is limited only by the available memory.

Notes: if dates or numbers are stored as strings then they will not be sorted correctly; they will be

sorted according to the ASCII codes of the characters 23 will be placed before 9;

telephone numbers must be stored as strings or the leading zero will be lost.



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DJM 33

ADDITIONAL NOTES:

Character

A character is any letter, number, punctuation mark or space, which takes up a single unit of storage(usually a byte).

Dates

In most computer systems dates are stored as a serial number that equates to the number ofseconds since January 1st, 1904 (thus they also contain the time).

Although the serial numbers are used for processing purposes, the results are usually presented in

one of several standard date formats for example, dd/mm/yyyy, ordd MonthName, yyyy.

Dates usually take 8 bytes of storage.

ContainerA container is a data-type used for storing images, video, sound or another type of complex file.

Note that in MySQL databases the term blob (binary large object) is used rather than container

Comparison of the common data types

Data Type Example value Storage Required

Integer 42 4 bytes

Real 23.1 4 or 8 bytes

Boolean True 1 byte

String Hello World 1 byte for eachcharacter (if usingUnicode, then 2 bytesare required for eachcharacter)

Character X 1 byte

Date 12/11/2009 8 bytes

Container Image, video or sound Lots!



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ADDITIONAL NOTES:

User-defined data types

At times it is useful for programmers to be able to define their own data types. In such cases it iscommon for these user-defined data types to contain items from several of the different intrinsic

types mentioned above.

Some programming languages allow the programmer to do this by defining Records.

Benefits of defined data-types

Whether intrinsic or user-defined, the use of data-types within a programming language:

enable the compiler to reserve the correct amount of memory for the data e.g. 4 bytes foran integer;

trap errors that a programmer has made and errors that a user of a program can make avariable defined as an integer cannot be given a fractional value;

restrict the values that can be given to the data a Boolean cannot be given the valuemaybe;

restrict the operations that can be performed on the data a string cannot be divided by 10.



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DJM 35

ADDITIONAL NOTES:

Data structures


21. Define and use arrays (one- and two-dimensional) for solving

simple problems, including initialising arrays, reading data intoarrays and performing a simple serial search on a one-dimensional array.

A data structure is a collection of different data items that are stored together in a clearly defined

way. Two common data structures are arrays and records.

Array

An array is a data structure, which allows a set of items of identical data type to be stored togetherusing the same identifier name.

Arrays are declared in a similar way to standard variables, except that the array size and dimensions

are included. For example, the following declares an array that reserves five locations in memoryand labels these as Names:

DIM Names(4) As String

The five individual locations areNames(0), Names(1), Names(2), Names(3), Names(4).

Each data item is called an elementof the array. To reference a particular element a programmer

must use the appropriate index. For example, the following statement assigns data to the 5th

element:Names(4) = Johal

Arrays simplify the processing of similar data. An algorithm for getting five names from the user

and storing them in the array Names is shown below:

Dim Names(4) As String

For i=0 to 4

Input Value

Names(i)=Value

Next i



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ADDITIONAL NOTES:

One-dimensional arrays

A one-dimensional array is a data structure in which the array is declared using a single index andcan be visually represented as a list.

The following diagram shows the visual representation of the array Names(4):

Two-dimensional arrays

A two-dimensional array is a data structure in which the array is declared using two indices and can

be visually represented as a table.

The following diagram shows the visual representation of an array Students(4,2):

Each individual element can be referenced by its row and column indices. For example:

Students(0,0) is the data item JONES

Students(2,1) is the item M

Students(1,2) is the item LAMBOURNE



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DJM 37

ADDITIONAL NOTES:

Initialising an array

Initialising an array is a procedure in which every value in the array is set with starter values this

starting value would typically be for a string array, or 0 for a numeric array.

Initialisation needs to be done to ensure that the array does not contain results from a previous use,elsewhere in the program.

Algorithm for initialising a one-dimensional numeric array:

DIM TestScores(9) As Integer

DIM Index As Integer

FOR Index = 0 TO 9

TestScores(Index) = 0

NEXT

Algorithm for initialising a two-dimensional string array:

DIM Students(4,2) As String

DIM RowIndex, ColumnIndex As Integer

FOR RowIndex = 0 TO 4

FOR ColumnIndex = 0 TO 2

Students(RowIndex,ColumnIndex) =

NEXT

NEXT



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ADDITIONAL NOTES:

Serial search on an array

The following pseudo-code can be used to search an array to see if an itemXexists:

01 DIM Index As Integer

02 DIM Flag As Boolean

03 Index = 0

04 Flag = False

05 Input X

06 REPEAT

07 IF TheArray(Index) = X THEN

08 Output Index

09 Flag = True

10 END IF

11 Index = Index + 1

12 UNTIL Flag = True OR Index > Maximum Size Of TheArray

Note that the variableFlag(line 04 and 09) is used to indicate when the item has been found andstop the loop repeating unnecessarily (line 12 ends the loop ifFlaghas been set to True).

To complete the search algorithm some lines should be added, after the loop, to detect the timeswhen the itemXwas not found in the array:

13 IF Flag = False THEN

14 Show Message Item not found

15 END IF



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DJM 39

ADDITIONAL NOTES:

Using an array with differing data types

If an array is to be used to store data of different data types, then:

1. The different data must be defined within a Record:

RECORD Student

Name: String

Gender: Character

Age: Integer

END RECORD

2. The array may now be declared to contain items of data type Record:

DIM MyArray(6) As Record

3. Each record can now be stored as a single item within the array.



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ADDITIONAL NOTES:

Relative advantages of the different data types


22. Explain the advantages and disadvantages of different data

types and data structures for solving a given problem.

COMMENTS TO BE ADDED HERE



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DJM 41

ADDITIONAL NOTES:

Records


23. Design and implement a record format.

Key terms

The following terms are used to describe parts of a database:

Field

A field is a single category of data within a database, which appears in all the records of a table itis a column within a table.

Record

A record is a collection of fields that contains data about a single item or person it is a row withina table.

Table

A table is a collection of related records.



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ADDITIONAL NOTES:

Key fields

A key field is used to identify the records within a database. There are two types of keys:

primary key;

secondary key.

Primary key

The Primary key is a unique field that identifies a single record.

Some natural primary keys are:

CarRegistrationNumber;

ISBN a 10-digit code that uniquely identifies a book;

MAC number a 6-part number that uniquely identifies a network card

NationalInsuranceNumber can uniquely identify employees of a company (not usable forunder 16s or for non-British nationalists!)

Secondary key

A Secondary key is a non-unique field, used in a search, that does not always produce only one

matching record.

Some typical secondary keys are:

LastName; PostCode; DateOfBirth



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DJM 43

ADDITIONAL NOTES:

Modes of file access


24. Define different modes of file access: serial, sequential,

indexed sequential and random; and justify a suitable mode offile access for a given example.

Serial file

A serial file is one in which records are stored, one after the other, in the order in which they areadded not in order of a key field.

This means that new records are stored at the end of the file.

The following shows a serial file that is used to store the number of entries for EdExcel GCSE

Mathematics. The entries were received in the order: Kettlewood, Queens Park, St Marys, WiltonHigh, West Orling.

Centre

Number

Centre

Name

No of

Candidates

27102 Kettlewood 85

38240 Queens Park 103

64715 St Marys 121

30446 Wilton High 156

12304 West Orling 105

Note that the key field in this file would be Centre Number (it uniquely identifies each school)

Both disks and tapes can be used to store a file serially.



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44 DJM

ADDITIONAL NOTES:

Sequential file

A sequential file is one in which the records are stored, one after the other, in the order of the keyfield.

The following shows a sequential file that is used to store the number of entries for EdExcel GCSEMathematics. The entries were added in the order: Kettlewood, Queens Park, St Marys, WiltonHigh, West Orling but they are stored in the order of the key field Centre Number:

CentreNumber

CentreName

No ofCandidates

12304 West Orling 105

27102 Kettlewood 85

30446 Wilton High 156

38240 Queens Park 103

64715 St Marys 121

As with a serial file, both tape and disks can be used to store a file sequentially and access to the

records must take place from the beginning of the file.

Benefits

Sequential files allow the records to be displayed in the order of the key field this makes theprocess of adding a record slower, but significantly speeds up searches.



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DJM 45

ADDITIONAL NOTES:

Indexed sequential file

An indexed sequential file is one in which the records are stored, one after the other, in the order ofthe key field, but which also has an index that enables records to be accessed directly.

Index

An index is a file with two fields, created from the main file, which contains a list of:

the key fields (sorted sequentially);

pointers to where the records can be found in the main file.

Indexed sequential files are useful when:

it is sometimes necessary to process all the records in sequential order; and

it is sometimes necessary to access individual records randomly.

Examples of indexed sequential files

Company employee file

At the end of each month all the records will be processed sequentially, in order to produce

payslips. However, some records will need to be accessed randomly, at other times for example,when an employee changes address.

A schools student file

When an attendance report is printed, the file will be accessed sequentially, but when the details of

an individual student are required the index will be used to find the required record quickly.



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ADDITIONAL NOTES:

Random (direct) access file

A random access file is one in which a record can be written or retrieved without first examiningother records.

A random access file must be stored on disk and the disk address is calculated from the primarykey.

In its simplest form a record with a primary key of 1 will be stored at block 1, a record with aprimary key of 2 will be stored at block 2; a record with primary key 3 will be stored at block 3 etc:

It should be noted that this very simple method where [disk address] = [primary key] is veryinefficient in respect of disk space. For example:

if the lowest primary key is 1001, then all the disk space below block 1001 will be wasted. If there are some values which the primary key never takes (for example odd values) these

storage spaces will be wasted.

In order to be more efficient with the use of disk space, random access files calculate disk addresses

by using a hashing algorithm (also known as just hashing).



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DJM 47

ADDITIONAL NOTES:

Hashing

Hashing is a calculation that is performed on a primary key in order to calculate the storage addressof a record.

A hashing algorithm will typically divide the primary key by the number of disk blocks that areavailable for storage, work out the remainder and add the start address. The answer will be thestorage address of the record.

[disk address] = [primary key] MOD [number of blocks] + [start address]

Example

If a file was to be stored on the first 5000 blocks of a disk then:

[disk address] = [primary key] MOD 5000

That is, the primary key of each of the records would be divided by 5000 and the remainder would

be the disk address for the record.

This means that a record with primary key of 27102 would be stored at the disk address calculatedas follows:

27102

5000= 5 remainder 2102

This means that the disk address for this record will be 2102.

The table shows some other disk addresses calculated using the same hashing algorithm:

CentreNumber

CentreName

No ofCandidates

DiskAddress

27102 Kettlewood 85 2102

38240 Queens Park 103 3240

64715 St Marys 121 471530446 Wilton High 156 446

12304 West Orling 105 2304



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48 DJM

ADDITIONAL NOTES:

Problems with hashing

One problem that could occur with hashing is that a block may already contain a record and be full.

For example records with key fields of 38240 and 43240 will both be assigned a disk address of3240.

If this happens then the new record will need be written somewhere else. Two common ways ofdetermining this alternative location are:

the record can be written to the next available block note that if it is the last address blockwhich is full then the search for an available space will start from the first block.

the record could be written to a separate overflow area and a tag is placed in the calculatedlocation to indicate exactly where in this overflow area the record can be found.



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DJM 49

ADDITIONAL NOTES:

Exercise CPT2.2B

1. (a) What is meant by a serially organised file? [2]

(b) State two characteristics of a sequentially organised file. [2]

2. Details of each new loan in a library can only be added to the end of the loans file. What type

of file organisation does the loans file use? [1]

3. If a program is written which searches a sequential file for a match with a given word that

might be recorded in the file, what are the steps for this process? [4]

4. (a) What file organisation is required if the priority is to find a specific record in a file?

Justify your answer. [2](b) What file organisation is required if the priority is to access individual records quickly?

Justify your answer. [2]

5. List the steps required in order to add a record to a sequential file. [4]

6. What is a direct access medium? [2]

7. (a) What is meant by hashing? [2]

(b) Give two properties that a hashing algorithm should have. [2]

(c) A hashing algorithm uses the expression (ASCII Code)Mod 150.

Why is Mod 150 used? [1]



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50 DJM

ADDITIONAL NOTES:

8. (a) What are the major steps that a typical hashing function/algorithm would use to convert

a alphanumeric string into a two-byte integer. [2]

(b) Why is the hashing function used for passwords on a LAN, a one-way (irreversible)

function? [1]

9. List the programming steps required to generate a direct access file by using a hashing

algorithm on a sequential file. Note that details of how a hashing algorithm works are not

required. [3]

10. When a member of staff logs onto a computer in a particular organisation, they enter there

username and password. The password is input to the logon program as an alphanumericstring and converted to a two-byte integer using a hashing function or algorithm before being

sent across a network or authentication.Outline three major steps that a typical hashing function/algorithm would use to convert an

alphanumeric string into a two-byte integer. [3]



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DJM 51

ADDITIONAL NOTES:

Handling data within files


25. Store, retrieve and search for data in files;

26. Use the facilities of a procedural language to perform fileoperations (opening, reading, writing, updating, inserting,appending and closing) on files of different access modes asappropriate.

Before a program can use a file, the file needs to be opened. The program needs to specify whether

the file is to be opened for writing, or opened only for reading. After the data has been read or

written to the file, it then needs to be closed.

All algorithms for handling data within files must have the following lines:

OPEN [New/Existing] File in READ/WRITE MODE

CLOSE All Files

Note that these lines are not necessarily at the beginning and the end of the code, but they must bein place to make sure that the file(s) is opened and closed correctly.

Adding data

Serial file

Adding data is simple it is added to the end of the file:

OPEN File in WRITE MODE

GOTO End of File

WRITE NewData

CLOSE File



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ADDITIONAL NOTES:

Sequential file

The addition of data to a sequential file is more complicated than in a serial file, because the record

must be inserted into the correct position not just at the end of the file.

In practise, when a record is added to a sequential file, it is usual to create a new file and copy all

the records from the old file, but insert the new record in its correct position.

An algorithm for this is shown below:

OPEN a NewFile in WRITE MODE

OPEN ExistingFile in READ MODE

READ First Record in ExistingFile

REPEAT

IF key of SelectedRecord in ExistingFile < key of NewRecord THEN

COPY SelectedRecord into NewFile

ELSE

COPY NewRecord into NewFile

COPY SelectedRecord into new file

END IF

READ Next Record in ExistingFile

END REPEAT when new record has been copied

COPY ALL remaining records from ExistingFile into NewFile

CLOSE NewFile and ExistingFile

Random fileThe hashing algorithm is applied to the Key field in order to find the storage location for the new

data.

OPEN File in WRITE MODE

READ KeyField of NewRecord

Apply Hashing Algorithm to calculate DiskAddress

WRITE NewRecord to calculated DiskAddress

CLOSE File



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DJM 53

ADDITIONAL NOTES:

Searching for/retrieving data

Serial file

To retrieve data from a serial file, a program must examine the first record and then all subsequent

records until the desired one is found or until the end of the file has been reached.The following algorithm does this:

OPEN File in READ MODE

READ First Record

SET Variable Found = False

REPEAT

IF RequiredRecord = SelectedRecord THEN

SET Variable Found = True

ELSE

READ Next Record

END IF

END REPEAT when Found = True OR when EOF is reached

CLOSE File

Note that to be sure that a record does not exist in a serial file, every single record must be

examined.

Sequential file

Searching a sequential file is the same as searching a serial file, except that the search only has to

continue until a record with a higher Key field is reached this would mean that the data is not inthe file.


READ First Record

SET Variables Found = False, Exists = True

REPEAT

IF RequiredRecord = SelectedRecord THEN

SET Variable Found = True

ELSE

READ Next Record

IF Key of RequiredRecord > Key of SelectedRecord THEN

Exists = False

END IF

END IF

END REPEAT when Found = True OR Exists = False OR when EOF is reached

CLOSE File



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54 DJM

ADDITIONAL NOTES:

Random file


READ KeyField of RequiredRecord

Apply Hashing Algorithm to calculate DiskAddress

READ Record at DiskAddress

IF Record does not exist THEN

OUTPUT Record Not Found

END IF

CLOSE File

Deleting data

Serial or sequential file

OPEN a NewFile in WRITE MODE

OPEN ExistingFile in READ MODE

READ First Record in ExistingFile

REPEAT

IF Key of SelectedRecord Key of RecordToDelete THEN

COPY SelectedRecord into NewFile

ELSE

DO NOT COPY SelectedRecord

END IFREAD Next Record in ExistingFile

END REPEAT when EOF is reached

CLOSE NewFile and ExistingFile

DELETE ExistingFile

RENAME NewFile to Name of ExixtingFile



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DJM 55

ADDITIONAL NOTES:

Calculating file size


27. Estimate the size of a file from its structure and the number of

records.

The basic formula for estimating the size of a file is:

Size of file = [size of each record] [number of records] + [a little bit more!]

If we consider a file with 200 records, which stores the details of an organisations customers:

CUSTOMER(RefCode, Name, PostCode, Telephone, DoB, Age)

We can estimate the size of the record as follows:

Attribute Data type Extreme Example Size of field(bytes)

RefCode Integer 99 999 4

Name String Margaret Edwards 20

PostCode String WC12 16AA 9

Telephone String (0203) 9898 1234 16

DoB Date 31-12-76 8

Age Real 104 4

Total 62

Thus 200 records would require:

62 200 = 12400 bytes

=12400

1024Kbytes

= 12.1+1.21(10%)

= 13.3 Kbytes

Note that to determine the maximum field length, an extreme case was considered and several bytes

added to play safe.



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56 DJM

ADDITIONAL NOTES:

2.4 COMMON FACILITIES OF PROCEDURAL LANGUAGES


28. Understand and use assignment statements.

Assignment

An assignment is an instruction in a program that places a value into a specified variable.

Some typical assignments in REALbasic are:

TheLength = 20.5

TheUsersName$ = Charlie

TheArea = TheWidth * TheLength

TotalCost = LabelledCost + 15

Counter = Counter + 1

Note that the last example is a common method used to increment the value of a variable. It couldbe read as:

The new value of Counter is its existing value plus one

Type Mismatch errors

A type Mismatch error occurs in a program when a variable has been declared as one data type, but

it is later assigned a value that is of an incompatible data type.The following code will produce a Type Mismatch error because Charlie is not aninteger:

DIM MyCounter AS Integer

MyCounter = Charlie

Other Type Mismatches will be produced by the following:

DIM RentalDate As Date

MemberRentalDate = September

DIM ShoeSize As Integer

JohnsShoeSize = 10.3

Note that a variable that is declared as a string will never produce a type mismatch error.



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DJM 57

ADDITIONAL NOTES:

Arithmetic operations


29. Understand arithmetic operators including operators for

integer division (+, -, *, /, MOD and DIV) and use these toconstruct expressions.

Addition, subtraction and multiplication

Operator Meaning/purpose Example Result

+ Addition Result=3+5 8

Subtraction Result=37 -4

* Multiplication Result=3*7 21

Powers


^ Power Result=3^2 9

DivisionA result of a division such as 17 4 can be expressed either as a real (4.25) or as two integers(4 remainder 1).

The integer method, in most programming languages, uses the operators DIV and MOD.


/ Division Result=14/5 2.8

DIVInteger Division returns the result of adivision after ignoring the decimal portion of

all numbers

14 DIV 5

7.2 DIV 3.9 (= 7 DIV 3)

2

2

MODRemainder returns the remainder of thedivision of two integers

14 MOD 5

18 MOD 6

3 MOD 7

4

0

3



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58 DJM

ADDITIONAL NOTES:

Relational operators


30. Understand a range of relational operators, eg =, =

and and use these to construct expressions.

Relational operators are used in the format: [Expression] [Operator] [Expression]and will always return a Boolean (True or False) value.

Relational operators are typically used with the IF selection and also within conditional loops(REPEAT-UNTIL or WHILE-WEND).

In the following table, the variables a and name$ have the following assignments:

a=3+5

name$=JAMES


=Equal tests if two expressions areidentical

IF a=8

IF name$=JANET

IF a=14

True

False

False

14

False

False

False

Not Equal tests if two expressionsare different

IF a8

IF name$JANET

IF name$JAMES

IF a14

False

True

False

True

=14

True

False

False



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DJM 59

ADDITIONAL NOTES:

Boolean operators


31. Understand the Boolean operators AND, OR and NOT and use

these to construct expressions.

AND and OR

The AND and OR operators always return a Boolean result and are used in the format:

[Boolean] [Operator] [Boolean]

The following truth table summarises the result of the Boolean operations:

Values Results

X Y X AND Y X OR Y

True True True True

True False False True

False True False True

False False False False

NOT

The NOT operator reverses the result of the Boolean expression and is used in the format:

NOT [Boolean]

The following truth table summarises the NOT operation:

X NOT X

True False

False True



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60 DJM

ADDITIONAL NOTES:

Examples of Boolean logic

Consider the following algorithm, which is used to monitor a printer and display its output via anLCD display in the front panel:

IF NOT(PaperTrayEmpty) AND (FilesWaiting > 0) THENOUTPUT PRINTING

ELSE

OUTPUT PLEASE ADD PAPER

END IF

If the values of the variables are:

PaperTrayEmpty = False

FilesWaiting = 2

Then the output will be PRINTING

The following table shows why:

X Y X AND Y

PaperTrayEmpty FilesWaiting NOT(PaperTrayEmpty) FilesWaiting > 0

False 2 True True True

Other options are shown in this table:

X Y X AND Y

PaperTrayEmpty FilesWaiting NOT(PaperTrayEmpty) FilesWaiting > 0

True 2 False False False

True 0 False False False

False 0 True False False

Note that in all three of these cases the output will be PLEASE ADD PAPER even the last one

in which PaperTrayEmpty = False.

To avoid this incorrect message, the algorithm should be rewritten using a nested IF, as shown onthe next page:



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DJM 61

ADDITIONAL NOTES:

IF PaperTrayEmpty THEN

OUTPUT PLEASE ADD PAPER

ELSE

IF FilesWaiting > 0 THEN

OUTPUT PRINTINGELSE

OUTPUT STATUS OK

END IF

END IF



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62 DJM

ADDITIONAL NOTES:

Precedence of operations


32. Understand the effects of the precedence of standard

operators and the use of parentheses to alter the order ofprecedence.

This is mathematics follow BODMAS!



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DJM 63

ADDITIONAL NOTES:

Evaluating expressions


33. Evaluate expressions containing arithmetic, relational and

Boolean operators and parentheses.

SOME QUESTIONS TO BE ADDED HERE



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64 DJM

ADDITIONAL NOTES:

String manipulation


34. Understand and use a range of operators and built-in functions

for string manipulation, including location (LOCATE),extraction (LEFT, MID, RIGHT), comparison, concatenation,determining the length of a string (LENGTH) and convertingbetween characters and their ASCII code (ASCII and CHAR).

Location

Many programming languages allow the searching for a short string of characters within a longerstring. This would be similar to searching this book for the word computer.

Common Key words for finding the location of one string within another are LOCATE, FIND andPOSITION

Function Purpose Result

LOCATE Returns the starting position of the first string, within thesecond string (returns 0 if the string is not found):

x$=Locate(math, mathematics)

x$=Locate(the, mathematics)

x$=Locate(tea, mathematics)

1

3

0



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DJM 65

ADDITIONAL NOTES:

Extraction

At times, a programmer will only want part of a string. Any part of a string can be obtained usingthe RIGHT, LEFT and MID functions.

The following string manipulation functions are called by typing:

Result=FunctionName(String [,x, y])


RIGHT Returns the number of specified characters from the rightof the string:

x$=Right(Computing, 3) ing

LEFT Returns the number of specified characters from the left ofthe string:

x$=Left(Computing, 3) Com

MID Returns the number of specified characters from the

middle of the string:

x$=Mid(Computing, 5, 2) ut

Concatenation

Concatenation is where two or more strings are joined together to make a single string.

Note that when two strings are added together the second string is added to the end of the first:

Peter + Jones = PeterJones

Peter + + Jones = Peter Jones

36.85 + 47 = 36.8547

47 + 36.85 =4736.85

3/10/03 + 15 = 3/10/0315



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66 DJM

ADDITIONAL NOTES:

Length

Sometimes a programmer will need to know how many characters there are in a string.


LEN Returns the number of characters in the specified string:

x=Len(Computing) 9

Conversion

Strings and numbers

Strings are a sequence of ASCII characters, even if they contain only numbers such as 241, and

so they cannot be used within an arithmetic calculation they need to be evaluated first.

Likewise a number such as 27.3 is not stored as ASCII characters and so cannot be used within anyof the string functions (such as Left, Right, Mid).

The function STR converts a number into a string and VAL converts a string into a number:


STR Returns the string form of the value:

x$=Str(31) 31

VAL Returns the numeric form of the string:

x=Val(42Chig) 42

Characters and ASCII codes

The following two functions are used to either find the ASCII code for a specific character or to

find the character from the ASCII code:


ASC Returns the ASCII code for the specified character:

x=Asc(A) 65

CHR Returns the character whose ASCII code is specified:

x$=Chr(68) D



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DJM 67

ADDITIONAL NOTES:

Relational operators and strings


35. Understand that relational operations on alphanumeric strings

depend on character codes of the characters and explain theresults of this effect (e.g. why XYZ < abc, 2 > 17 and 3 3.0).

Storing strings

Strings are stored as a sequence of character codes - usually ASCII or Unicode.

ASCII character codes

Below is a table showing the most common characters and their ASCII codes:

Note the following:

the codes less than 32 are control codes that are not used in strings;

space (code 32) has the lowest code;

next comes most of the punctuation symbols;

then digits 09

then uppercase letters

then lowercase letters

all other characters (e.g. , , , etc.) have codes higher than these.



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68 DJM

ADDITIONAL NOTES:

Comparing strings

When comparing strings, the codes of each string are compared, character by character, to decidewhich string is greater.

Because it is the ASCII codes that are compared the following applies:

Computing computing

in fact:

Computing < computing

10 < 2

02 < 1

1 120 < 1,120

computing < Computing

Sorting filenames

The following table shows a list of inconsistently names music tracks and the order in which theywould be sorted:

Required order Sorted order

Track 1.mp3 Track 5.mp3

Track 2.mp3 track 4.mp3

Track 3.mp3 Track 06.mp3

track 4.mp3 Track 1.mp3

Track 5.mp3 Track 11.mp3Track 06.mp3 Track 2.mp3

track 7.mp3 Track 22.mp3

Track8.mp3 Track 3.mp3

Track 11.mp3 Track12.mp3

Track12.mp3 Track21.mp3

track 13.mp3 Track8.mp3

Track21.mp3 track 7.mp3

Track 22.mp3 track 13.mp3

In order to get them sorted into the required order, they must be renamed with consistent use of

uppercase letters, spaces and leading zeros.

Recommended naming would be:

Track 01.mp3, Track 02.mp3, , Track 10.mp3, Track 11.mp,



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DJM 69

ADDITIONAL NOTES:

Data input


36. Input and validate data.

Input controls

Input can be via a command line prompt, but in modern graphical user interfaces, input is usually

via a form or dialogue box. Such forms or dialogue boxes have a set of controls, which let the userenter data:

Text boxes;

Drop-down lists;

Check-boxes;

Option buttons.



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ADDITIONAL NOTES:

Validation

The drop-down lists, check-boxes and option buttons are extremely useful in that they provide anautomatic means of validation the user can only input pre-determined values.

Other validation needs to be programmed into the system and is often added to the submit button.

Presence check

Code can be added to check that a particular control has not been left empty or un-checked:

If Username.Text = Then

Msgbox(You have not entered a Username)

End

Range check

Code can be added to check that a particular control has a value between an allowed maximum andminimum:

If val(Age.Text) < 11 OR val(Age.Text) > 70 Then

Msgbox(The age that you have netered is not within the_

permitted range)

End

Length check

Code can be added to check that a particular control has a value between an allowed maximum andminimum:

If Len(Password.Text) > 12 Then

Msgbox(The password that you have entered is too long)

End

Note that all of the above validation code (and any additional validation) could be added to thesubmit button.



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DJM 71

ADDITIONAL NOTES:

Data output


37. Output data onto screen/file/printer, formatting the data for

output as necessary.

Output will be either to the screen, a file or the printer.

Output controls

Output to the screen could be via a dialogue/message box, a text box or a list box.

COMMENTS TO BE ADDED HERE



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72 DJM

ADDITIONAL NOTES:

2.5 WRITING MAINTAINABLE PROGRAMS


38. Define, understand and use the following terms correctly as

they apply to programming: variable, constant, identifier,reserved word/keyword;

39. Declare variables and constants, understanding the effect ofscope and issues concerning the choice of identifier (includingthe need to avoid reserved words/keywords);

40. Initialise variables appropriately, before using them.

Variable

A variable is a value that can change during the execution of a program.

There are two types of variables used by programmers and they are categorised according to their

scope.

Scope

Scope indicates whether a variable can be used by all parts of a program or only within limitedsections of the program for example, within a subroutine.

Global variable

A global variable is one that is declared at the start of the main program and is visible (useable)everywhere in the program and exists for the lifetime of the program.

Note that if one procedure changes the value of a global variable, then the next procedure that usesthe variable will be given this changed value this is a common cause of errors.

Local variable

A local variable is one that is only visible inside the procedure or function in which it is declared.

Note that a local variable cannot be referenced from outside the procedure. In fact a local variable

does not exist until the procedure starts executing and it disappears when the procedure stopsexecuting. Thus any value that is held by a local variable is only stored temporarily.

The lifetime of a local variable is the lifetime of the procedure in which the local variable is

declared.



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DJM 73

ADDITIONAL NOTES:

The advantage of using local variables rather than global variables is that the same variable names

can be used in several different procedures without any chance of values being confused.

Initialising variables

Initialising a variable means setting it to a starter value. Initialisation will usually set the value of aninteger to 0 or1 and a string to Empty ().

The following code initialises the variable Counter to zero before it is used in the iteration:

Counter=0 the variable is initialised

REPEAT

Counter=Counter+1

UNTIL Counter=10

Initialising a variable ensures that its value has not be been retained from a previous use of the

routine and that the value has not been accidently set in another part of the program this helps

avoid errors.

Declaration

A declaration is a statement in a program that gives the compiler or the interpreter informationabout a variable or constant that is to be used within a program.

A declaration ensures that sufficient memory is reserved in which to store the values and also states

the variables data type.

Declaration of local variables

DIM MyCounter AS Integer

DIM FirstName, LastName AS String declares two variables

DIM TheLength AS Single

DIM DOB AS Date

DIM OverDueFlag AS Boolean

Intrinsic variable declarations

Some programming languages require intrinsic variable declarations. This means that variablesmust be declared before they are used.



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ADDITIONAL NOTES:

The advantage of intrinsic variable declaration is that it cuts out possible errors due to the

misspelling of a variable name if a variable is used that has not been declared, the programmingtranslator will identify it as an error.

DIM Number As Integer

Number= Nubmer+1 This will be flagged as an Error!

Constant

A constant is a value that is set when the program initialises and does not change during the

programs execution.

Identifier

Identifier is the name given to a variable, constant or subroutine.

Assigning a variable or constant an identifier name means that it can be easily referenced from any

part of the program within its scope and it helps to make the program more readable.

Reserved word/keyword



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DJM 75

ADDITIONAL NOTES:

Good program writing techniques


41. Explain the need for good program-writing techniques to

facilitate the ongoing maintenance of programs;42. Select and use meaningful identifier names and use standard

conventions to show the data types and enhance readability;

43. Use declared constants to improve maintainability.

Maintenance is the updating of a program after it has been released. Maintenance will be helped

when the programmer uses good programming techniques. This means that the programmer shouldwrite code that is self-documenting and split into small sections.

Specifically, the programmers should: use meaningful identifier names for variables, constants and subroutines;

use declared constants;

annotate code with comments;

indent code within iteration and selection;

split the code into modules when possible.

The need for good program-writing techniques

It is important for a programmer to use good programming techniques when writing code so that thecode:

can be easier to understand by other programmers (who are either part of the programmingteam, or who will be responsible for the maintenance of the program in the future);

can be understandable by the programmer themselves in the future;

is split into smaller blocks which are easier to debug individually and update;

is less prone to errors because the meaningful variable names and comments make it self-documenting and easier to read.



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76 DJM

ADDITIONAL NOTES:

Meaningful identifier names

Identifiers are used to give names to constants and variables. They are also used to nameprocedures, functions and the main program.

Naming conventions

REALbasic identifier names must conform to the following rules (other programming languages

may have slightly different rules):

they must be unique;

spaces must not be used;

they must begin with a letter of the alphabet;

the rest of the identifier must not contain punctuation it may only consist of a mixture ofletters and digits (AZ, az and 09) and the underscore character _;

they must not be a reserved word eg Print, Repeat, For, Dim, Loop, etc.

Note that REALbasic names are not case sensitive this means that MyCounter is the same asMYCOUNTER, mycounter, mYcOUNTeR, etc. This is not the case in all languages, however.

Recommended naming policies

Do not use spaces within identifier names even with programming languages where they are

permitted. Instead use the underscore character _ or, better still, type names in lowercase exceptthe first letter of each word, which should be typed in uppercase.

Examples of good identifier names:

FirstName LastName PostCode

TelephoneNumber WeightAtBirth TestScore

AverageHeight

Further clarity can be given to identifier names by including a prefix that identifies the data type.

The above identifiers would be clearer if given the following names:

strFirstName strLastName strPostCode

strTelephoneNumber sglWeightAtBirth intTestScore

sglAverageHeight



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DJM 77

ADDITIONAL NOTES:

Using declared constants

Constants will be declared at the start of the main program. The following shows the declaration ofconstants for Pi and the VAT rate

CONST Pi = 3.142

CONST VatRate = 0.175

Declaring constants at the start of a program means that maintenance is made easier for tworeasons:

if the value of a constant changes, it only has to be changed in the one part of the programwhere it has been declared, rather than in each part of the program in which it is used;

the code is easier to interpret:

Total=VatRate*CostPrice allows a greater understanding than

Total=0.175*CostPrice



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78 DJM

ADDITIONAL NOTES:

Annotating code


45. Annotate the code with comments so that the logic of the

solution can be followed.

Comments/remarks

Comments (originally called remarks) are added to program code to provide useful, or essential,documentation for the programmer and other people who read the code.

All programs/subroutines should contain comments to indicate:

the details of the person who wrote the code;

the date that the code was written; the purpose of the code;

how parts of the code work;

the use of certain variables.

Comments can be used anywhere in a program either as separate lines by themselves or on the

same line as executable code provided they appear to the right of any code that will execute.

Comments are usually indicated by the use of an apostrophe (), or two forward slashes (//).

PROCEDURE Arrange(NumberOfScores)

Procedure to move the largest element in an array to the end

Coded by J Jones, 02/03/09

DIM Ptr, Temp As Integer ptr is the value of the array index

FOR Ptr = 1 TO NumberOfScores go through each element

test the relative values of the elements

IF Scores(Ptr) < Scores(Ptr + 1) THEN

use a temporary store to help swap the elements

Temp = Scores(Ptr)

Scores(Ptr) = Scores(Ptr + 1)

Scores(Ptr + 1) = Temp

END IFNEXT Ptr increment the index to examine the next element

END PROCEDURE

Note that comments are ignored when the program code is compiled and so they are not presentwithin the stand-alone application.



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DJM 79

ADDITIONAL NOTES:

Indenting code


46. Use indentation and formatting to show clearly the control

structures within the code.

Indentation should be used within iteration and selection statements so that it is clear

a level computing 9691 paper 2 notes

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