Course: Computing
Science
Software Design and
Development Units
Level: National 4/5
April 2013
This advice and guidance has been produced for teachers and other staff who
provide learning, teaching and support as learners work towards qualifications.
These materials have been designed to assist teachers and others with the
delivery of programmes of learning within the new qualifications framework.
These support materials, which are neither prescriptive nor exhaustive,
provide suggestions on approaches to teaching and learning which will
promote development of the necessary knowledge, understanding and skills.
Staff are encouraged to draw on these materials, and existing materials, to
develop their own programmes of learning which are appropriate to the needs
of learners within their own context.
Staff should also refer to the course and unit specifications and support notes
which have been issued by the Scottish Qualifications Authority.
http://www.sqa.org.uk
Acknowledgements
© Crown copyright 2013. You may re-use this information (excluding logos) free of
charge in any format or medium, under the terms of the Open Government Licence.
To view this licence, visit http://www.nationalarchives.gov.uk/doc/open-governmentlicence/ or e-mail: psi@nationalarchives.gsi.gov.uk.
Where we have identified any third party copyright information you will need to obtain
permission from the copyright holders concerned.
Any enquiries regarding this document/publication should be sent to us at
enquiries@educationscotland.gov.uk.
This document is also available from our website at www.educationscotland.gov.uk.
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Contents
Introduction ..................................................................................................... 4
Approaches to learning, teaching and assessment ......................................... 8
Subject content ............................................................................................. 13
Staff notes ................................................................................................ 15
Advice on practical evidence .................................................................... 18
Introduction .............................................................................................. 23
Data representation .................................................................................. 23
Computer architecture .............................................................................. 28
Programming languages .......................................................................... 31
Designing a program ................................................................................ 36
Writing programs ...................................................................................... 38
Programming constructs ........................................................................... 42
Errors in programs .................................................................................... 46
Glossary ................................................................................................... 50
Qualifications ............................................................................................ 55
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INTRODUCTION
Introduction
This resource provides advice and guidance for staff delivering Software
Design and Development units at National 4 and 5.
Before embarking on these units, staff should be familiar with the contents of
the technologies principles and practice paper:
http://www.educationscotland.gov.uk/learningteachingandassessment/curricul
umareas/technologies/index.asp
Staff should also read the computing science professional focus paper
produced by Education Scotland to ensure that they have familiarised
themselves with the key areas of significant change and potential approaches
to teaching and learning.
http://www.educationscotland.gov.uk/nationalqualifications/professionalfocusp
apers/index.asp?bFilter=1&bSortCD=0&11_subjecttopics=New National
Qualifications\|Technologies\|Computing Science (NQ subject)
The general unit information that follows has been taken, summarised and
adapted from the Computing Science National 4 and 5 course support notes.
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INTRODUCTION
Guidance for staff
Key skills, knowledge and understanding
A broad overview of the subject skills, knowledge and understanding that will
be assessed in the Software Design and Development unit is given below.
Mandatory skills and knowledge
National 4
Applying, with guidance, aspects of computational
thinking across a range of straightforward contexts.

Applying aspects of computational thinking across
a range of contexts.
Designing, implementing and testing, with
guidance, digital solutions (including computer
programs) to straightforward problems across a
range of contemporary contexts.


Designing, implementing and testing, digital
solutions (including computer programs) to
problems across a range of contemporary contexts

Developing skills in computer programming and the
ability to communicate how a program works by
being able to read and interpret code.

Communicating basic understanding of key
concepts related to software design and
development clearly and concisely using
appropriate terminology.

Communicating understanding of key concepts
related to software design and development clearly
and concisely using appropriate terminology.
Basic knowledge of the impact of contemporary
technologies on the environment and society.
Applying computing science concepts and
techniques to create solutions across a range of
contexts.
National 5



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INTRODUCTION
Links to prior learning
Entry to this unit is at the discretion of the centre. However, learners would
normally be expected to have attained some relevant skills and knowledge
through prior experience.
Skills and knowledge developed through any of the following, while not
mandatory, are likely to be helpful as a basis for further learning for this
course.
Other SQA qualifications
For National 4:
 Computing Science National 3 or relevant component units
 Numeracy National 3.
For National 5:
 Computing Science National 4 or relevant component units
 Numeracy National 4.
Experiences and outcomes
In this course, any of the following Experiences and Outcomes may be
relevant.
Having investigated a current trend of technological advance in Scotland or
beyond, I can debate the short- and long-term possibilities of the technological
development becoming a reality.
(TCH 4-01a)
I can debate the possible future impact of new and emerging technologies on
economic prosperity and the environment.
(TCH 4-01c)
By discussing the business, environmental, ethical and social implications of
computer technology, I can begin to gain an understanding of the need for
sustainability and accessibility.
(TCH 4-05a)
I can compare different forms of security software to gain knowledge and
understanding of their functions in protecting contemporary technologies.
(TCH 4-08b)
I can integrate different media to create a digital solution which allows
interaction and collaboration with others.
(TCH 4-08c)
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INTRODUCTION
Through research, I can gain knowledge of computer systems or emerging
technologies to understand their differing features and consider their suitability
for the world of work.
(TCH 4-08d)
By learning the basic principles of a programming language or control
technology, I can design a solution to a scenario, implement it and evaluate its
success.
(TCH 4-09a)
I can create graphics and animations using appropriate software which utilise
my skills and knowledge of the application.
(TCH 4-09b)
I can use features of software to create my own animation which can then be
used to create an animated sequence.
(TCH 4-09c)
Progression from this unit
From National 4 progression may be possible to:




Computing Science National 5
National Certificate Group Awards in computing, IT and related disciplines
other technological courses at National 4
employment, apprenticeships and/or training in IT and related fields.
From National 5 progression may be possible to:




Higher Computing Science
National Certificate Group Awards in computing, IT and related disciplines
other technological courses at National 5
employment, apprenticeships and/or training in IT and related fields.
Ultimately, for some, progression may be possible to:




Advanced Higher Computing Science
a range of computing-related Higher National Diplomas (HNDs)
degrees in computing, IT and related disciplines
careers in computing, IT and related disciplines.
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APPROACHES TO LEARNING, TEACHING AND ASSESSMENT
Approaches to learning, teaching and assessment
Computing Science, like all new and revised national courses, has been
developed to reflect Curriculum for Excellence values, purposes and
principles.
The approach to learning and teaching developed by individual centres should
reflect these principles. Learners should be encouraged to participate fully in
active learning and practical activities by working together, talking, listening,
reading or reflecting on a topic while staff act as a facilitator.
An appropriate balance of teaching methodologies should be used in the
delivery of the course and a variety of active learning approaches is
encouraged, including the following.
Activity-based learning
Whole-class, direct teaching opportunities should be balanced by activitybased learning and practical tasks. An investigatory approach is encouraged,
with learners actively involved in developing their skills, knowledge and
understanding by investigating a range of real-life and relevant problems and
solutions related to areas of study. Learning should be supported by
appropriate practical activities, so that skills are developed simultaneously with
knowledge and understanding.
Group work
Practical activities and investigations lend themselves to group work, and this
should be encouraged. Working within a group will encourage learners to
collaborate and work co-operatively with others. Learners engaged in groupwork strategies capitalise on one another’s knowledge, resources and skills in
questioning, investigating, evaluating and presenting ideas to one another.
While working as a team is not specifically identified as one of the skills for life,
learning and work for this course, and therefore is not assessed, it is a
fundamental aspect of working in the IT and related industries and so should
be encouraged and developed by staff.
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APPROACHES TO LEARNING, TEACHING AND ASSESSMENT
Problem-based learning
Problem-based learning (PBL) is another strategy that will support a learner’s
progress through this course. This method may be best utilised at the end of
an outcome or a topic where additional challenge is required to ensure
learners are secure in their knowledge and understanding, and to develop the
ability to apply knowledge and skills in less familiar contexts. Learning through
PBL develops a learner’s problem-solving, decision-making, investigative,
creative thinking, team working and evaluative skills.
Computational thinking
Computational thinking is recognised as a key skill set for all 21st-century
learners – whether they intend to continue with computing science or not. It
involves a set of problem-solving skills and techniques used by software
developers to write programs.
There are various ways of defining computational thinking. One useful
structure is to group these problem-solving skills and techniques under five
broad headings:
 Abstraction: seeing a problem and its solution at many levels of detail and
generalising the information that is necessary. Abstraction allows us to
represent an idea or a process in general terms (eg variables) so that we
can use it to solve other problems that are similar in nature.
 Algorithms: the ability to develop a step-by-step strategy for solving a
problem. Algorithm design is often based on the decomposition of a
problem and the identification of patterns that help to solve the problem. In
computing science as well as in mathematics, algorithms are often written
abstractly, utilising variables in place of specific numbers.
 Decomposition: breaking down a task so that we can clearly explain a
process to another person – or to a computer. Decomposing a problem
frequently leads to pattern recognition and generalisation/abstraction, and
thus the ability to design an algorithm.
 Pattern recognition: the ability to notice similarities or common differences
that will help us make predictions or lead us to shortcuts. Pattern
recognition is frequently the basis for solving problems and designing
algorithms.
 Generalisation: realising that a solution to one problem may be used to
solve a whole range of related problems.
Underpinning all of these concepts is the idea that computers are
deterministic: they do exactly what you tell them to do. The corollary of this, of
course, is that they can be understood.
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APPROACHES TO LEARNING, TEACHING AND ASSESSMENT
Whilst computational thinking can be a component of many subjects,
computing science is particularly well-placed to deliver it. Staff are encouraged
to emphasise, exemplify and make these aspects of computational thinking
explicit (at an appropriate level) wherever there are opportunities to do so
throughout the teaching and learning of this course and its units.
Using online and outside resources
Throughout the teaching of this course, the stimulation of learners’ interest
and curiosity should be a prime objective. Engagement with outside agencies
or industry professionals can greatly enhance the learning process. Online
resources, such as those listed in the individual unit support notes, may
provide a valuable addition to teaching and learning activities, encouraging
research, collation and storage of information and evaluation of these
materials. The use of interactive multimedia learning resources, online quizzes
and web-based software can also be used to support activity-based
approaches.
Assessment activities, used to support learning, may be usefully blended with
learning activities throughout the course, for example by:





sharing learning intentions/success criteria
using assessment information to set learning targets and next steps
adapting teaching and learning activities based on assessment information
boosting learners’ confidence by providing supportive feedback
encouraging self- and peer-assessment techniques wherever appropriate.
Working towards units and course
Learning and teaching activities should be designed to develop:
 skills and knowledge to the standard required by each unit and to the level
defined by the associated outcomes and assessment standards
 the ability to apply the breadth of knowledge, understanding and skills
required to complete the Added Value Unit at National 4 or the course
assessment at National 5 successfully.
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APPROACHES TO LEARNING, TEACHING AND ASSESSMENT
Meeting the needs of all learners
Within any class, each learner will have individual strengths and areas for
improvement.
For example, within a National 4 class, there may be learners capable of
achieving National 5 standards in some aspects of the course. Where
possible, they should be given the opportunity to do so.
Staff need to consider both the outcomes and assessment standards, and the
table of content in Appendix 2 of the SQA unit support notes, to identify the
differences between National 4 and National 5.
In some aspects of the course the difference between National 4 and National
5 is defined in terms of a higher level of skill. For example, in the Information
System Design and Development unit, Outcome 1 requires National 4 learners
to ‘develop simple information systems using appropriate development tools’,
by creating a structure with links, integrating different media types and
identifying and rectifying errors, while at National 5 in the similar Outcome 1
learners are also required to create a user interface and write or edit simple
code.
In other aspects of the course, the difference between National 4 and National
5 is defined by additional or more in-depth knowledge. For example, in the
Software Design and Development unit, National 4 learners need to report on
a contemporary software-based application, while National 5 learners need to
compare two different software languages or environments.
When delivering this course to a group of learners, with some learners working
towards National 4 and others towards National 5, it may be useful for staff to
identify activities covering common knowledge and skills for all learners, and
additional activities for National 5 learners. This is particularly appropriate
where the National 5 learners have come directly from the Broad General
Education without previously studying National 4.
Where National 5 learners have studied National 4 in a previous year, it is
important to provide them with new and different contexts for learning to avoid
de-motivation.
Learning about Scotland and Scottish culture will enrich the learners' learning
experience and help them to develop the skills for learning, life and work they
will need to prepare them for taking their place in a diverse, inclusive and
participative Scotland and beyond. Where there are opportunities to
contextualise approaches to learning and teaching to Scottish contexts, staff
should consider this.
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APPROACHES TO LEARNING, TEACHING AND ASSESSMENT
Gathering evidence
Assessment should be an ongoing process that permeates effective learning
and teaching. Staff are encouraged throughout the Curriculum for Excellence
documentation to adopt imaginative and creative approaches to assessment
that take account of learners’ experience in terms of challenge, breadth and
application. Staff should refer to the principles of assessment as laid out in
Building the Curriculum 5:
http://www.educationscotland.gov.uk/thecurriculum/howdoyoubuildyourcurricul
um/curriculumplanning/whatisbuildingyourcurriculum/btc/btc5.asp
For unit assessment purposes, a variety of methods of assessment should be
used to gather a rich range of evidence such as creation of media, written
work, role-play and oral presentations. More detail on gathering evidence can
be found on the SQA secure website in the unit assessment support notes for
National 4 and 5.
Staff should share learning and assessment criteria with learners, provide
effective feedback, encourage peer- and self-assessment and use effective
questioning techniques.
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SUBJECT CONTENT
Subject content
Below is a table summarising the content for the Software Design and
Development unit taken from the Computing Science National 4 and 5 course
support notes:
Software Design and Development
Exemplification and implementation of the following in an application development environment
Topic
National 4
National 5
Computational
constructs
Exemplification and
implementation of the following
constructs:
 expressions to assign values
to variables
 expressions to return values
using arithmetic operations (+,
-, *, /, ^)
 execution of lines of code in
sequence demonstrating input
– process – output
 use of selection constructs
including simple conditional
statements
 iteration using fixed and
conditional loops
String
Numeric (integer) variables
Exemplification and
implementation of the following
constructs:
 expressions to assign values
to variables
 expressions to return values
using arithmetic operations (+,
-, *, /, ^)
 use of selection constructs
including simple and complex
conditional statements and
logical operators (AND, OR,
NOT)
 iteration using fixed and
conditional loops
 pre-defined functions
String
Numeric (integer and real).
variables
Boolean variables
1D arrays
Normal, extreme and exceptional
test data
Syntax, execution and logic
errors in programs
Readability of code (internal
commentary, meaningful
identifiers, indentation)
Data types and
structures
Testing and
documenting
solutions
Normal, extreme and exceptional
test data
Readability of code (internal
commentary, meaningful variable
names)
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SUBJECT CONTENT
Algorithm
specification
Design notations
(for both software
development and
information system
development)
Low-level operations
and computer
architecture
Input validation
Contemporary design notations.
Graphical to illustrate selection and
iteration.
Contemporary design
notations
Pseudocode to exemplify
programming constructs
Use of binary to represent and store:
 positive integers
 characters
 instructions (machine code)
Translation of high-level
program code to binary
(machine code): interpreters
and compilers
Use of binary to represent and
store:
 integers and real numbers
 characters
 instructions (machine code)
 graphics (bit-mapped and
vector)
Units of storage (bit, byte, Kb, Mb,
Gb, Tb, Pb)
Basic computer architecture:
processor, memory, buses,
interfaces
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SUBJECT CONTENT
Staff notes
The main purpose of this text is to provide a single document with the basic
theory for the Software Design and Development unit. It does not provide a
step-by-step practical guide in a particular programming language on how to
cover the practical parts of the unit because there are a variety of excellent
resources available already for this purpose. The unit assessment support
notes for Computing Science National 4 and 5 provide examples of the sort of
work learners are expected to be doing to prove that they are working at the
correct level. The section below breaks down the core programming skills
required in the National 4 and 5 courses and directs staff to some excellent
resources that cover the skills required in a particular programming language.
The list is by no means exhaustive and there are new documents and guides
being shared everyday on the CAS forum
(http://community.computingatschool.org.uk/), which is an excellent
community for computing staff to share ideas and resources.
Useful websites
A short and by no means exhaustive list of useful computing websites.
Back issues of Computer & Information Sciences & Engineering newsletters
http://www.nsf.gov/cise/csbytes/pastissues.jsp
Computer Science Unplugged
http://csunplugged.org/activities
The Roles of Variables home page
http://www.cs.joensuu.fi/~saja/var_roles/
Wikibooks on the topic of computing
http://en.wikibooks.org/wiki/Subject:Computing
Peer Instruction for Computer Science
http://www.peerinstruction4cs.org/
A site which allows staff to create free games, quizzes, activities and diagrams
http://www.classtools.net/
BBC site with a collection of excellent video clips to illustrate course theory
with real-life examples
http://www.bbc.co.uk/learningzone/clips/
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SUBJECT CONTENT
Harvard’s main page of resources for the Scratch programming environment
http://cs.harvard.edu/malan/scratch/index.php
Computer Science for Fun magazine
http://www.cs4fn.org/teachers/activities/
Online interactive tool for learning programming
http://www.codecademy.com/#!/exercises/0
National forum for computing staff to support one another and share
resources, experiences and ideas
http://community.computingatschool.org.uk/
Home page for Education Scotland computing resources and documentation
http://www.educationscotland.gov.uk/nationalqualifications/subjects/computing
.asp
Recommended books
Below are some books that may help learners and staff in aspects of the
Computing Science National 4 and 5 courses.
Beginning Programming For Dummies (paperback), Wallace Wang. ISBN-13:
978-0470088708
Higher Computing (paperback), John Walsh. ISBN-13: 978-0340905630
How to Pass Intermediate 2 Computing (paperback), Frank Frame ISBN-13:
978-1444108361
How to Pass Standard Grade Computing (paperback), Frank Frame. ISBN-13:
978-0340973912
Information Systems for You: Student's Book (paperback), Stephen Doyle.
ISBN-13: 978-0748763672
Megabyte (paperback), Simon Cotton, Graham Teager and Robert Austin.
ISBN-13: 978-0198328254
Pathways to Excellence: Level 3: Computing and ICT (paperback), Frank
Frame and John Mason. ISBN-13: 978-1444110791
Pathways to Excellence: Level 4: Computing and ICT (paperback), Frank
Frame and John Mason. ISBN-13: 978-1444110807
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PCs For Dummies (paperback), Dan Gookin. ISBN-13: 978-0470465424
Raspberry Pi User Guide (paperback), Eben Upton and Gareth Halfacree.
ISBN-13: 978-1118464465
Scratch Programming for Teens (paperback), Jerry Ford, Jr. ISBN-13: 9781598635362
Standard Grade Computing Studies (paperback), John Walsh. ISBN-13: 9780340885000
The BCS Glossary of IT and Computing Terms (paperback), BCS British
Computer Society. ISBN-13: 978-0131479579
The Game Maker's Apprentice (paperback), Jacob Habgood and Mark
Overmars. ISBN-13: 978-1430213475
Visual Basic 6 For Dummies (paperback), Wallace Wang. ISBN-13: 9780764503702
Other resources
There are other digital resources which at time of writing are available for
Intermediate 1/2 Computing and Standard Grade and could provide additional
exercises and theory for the Computing Science National 4 and 5 courses.
These are listed below:
Softplanet Intermediate 2 and Standard Grade
http://www.softplanetgroup.com/study-software/businesscomputing/intermediate-2-computing
http://www.softplanetgroup.com/study-software/business-computing/standardgrade-computing
Webcal Intermediate 1/2 and Standard Grade
http://www.webcal.esmartbiz.com/prod0405.htm
Exercises and content
To reduce the costs of reprographics and allow differentiation in a multi-level
class, National 4 and 5 content is included in the same document. National 5
exercises are in bold and National 5 content is highlighted using shaded
blocks.
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SUBJECT CONTENT
Advice on practical evidence
The following section outlines the practical tasks learners are expected to
attempt during the unit and gives suggestions as to documents that might aid
staff in delivering these experiences.
National 4
Exemplars of assessment and learner responses in SQA Unit Assessment
Support: Software Design and Development (National 4) Package 1: Unit-byunit approach
1
Explain how simple programs work, drawing on understanding of
basic concepts in software development by:
1.1
Reading and explaining code
Exemplar questions in Software Design and Development (National 4)
Package 1: Unit-by-unit approach, pp27–29.
1.2 Describing the purpose of a range of programming constructs and how
they work (The range of programming constructs should include expressions,
sequence, selection and iteration.)
Exemplar questions in Software Design and Development (National 4)
Package 1: Unit-by-unit approach, pp27–29.
1.3
Explaining how data and instructions are stored
Exemplar questions in Software Design and Development (National 4)
Package 1: Unit-by-unit approach, pp27–29.
2
Develop short programs using a software development
environment by:
2.1
Selecting and using expressions, sequence, selection and iteration
Iteration (repetition) p 34 Computing Science Mobile App Development Advice
for Practitioners (using AppInventor) MoleMasher Game.
Selection (if statements) p 39 Computing Science Mobile App Development
Advice for Practitioners (using AppInventor) TimesTable helper.
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Iteration (fixed and conditional loops) p 46 Computing Science Mobile App
Development Advice for Practitioners (using AppInventor) HeadsIWin.
Computing Software Development [INTERMEDIATE 2] David Bethune
Expressions pp58–68, Sequence pp67–72, Selection pp85–101
Repetition/Iteration pp102–133.
2.2
Selecting and using appropriate simple data types, such as numeric
(integer) and string
Variables integer and string p 27 Computing Science Mobile App
Development Advice for Practitioners (using AppInventor) FingerPaint
Variables integer pp53–61 Computing Software Development
[INTERMEDIATE 2]David Bethune
Strings pp38–46 Computing Software Development [INTERMEDIATE 2]David
Bethune
2.3
Testing digital solutions using supplied test data
Testing p29 Computing Science Mobile App Development Advice for
Practitioners (using AppInventor) FingerPaint etc.
Computing Software Development [INTERMEDIATE 2]David Bethune pp53–
59
2.4
Identifying and rectifying errors in programs
p39 Computing Science Mobile App Development Advice for Practitioners
(using AppInventor) TimesTable helper
pp110–112 Computing Software Development [INTERMEDIATE 2]David
Bethune
Programs should include at least one construct and one data type.
3
Produce a short factual report on a contemporary software-based
application by:
3.1
Describing the application
Exemplar report in Software Design and Development (National 4) Package 1:
Unit-by-unit approach, pp17–20.
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SUBJECT CONTENT
3.2
Explaining how its features relate to programming constructs and data
types.
Exemplar report in Software Design and Development (National 4) Package 1:
Unit-by-unit approach, pp17–20.
3.3
Describing its impact on the environment or society
Exemplar report in Software Design and Development (National 4) Package 1:
Unit-by-unit approach, pp17–20.
National 5
1
Explain how programs work, drawing on understanding of
concepts in software development and basic computer architecture
by:
1.1
Reading and explaining code
Exemplar questions in Software Design and Development (National 5)
Package 1: Unit-by-unit approach, pp25–28.
1.2 Describing the purpose of a range of programming constructs and how
they work
Exemplar questions in Software Design and Development (National 5)
Package 1: Unit-by-unit approach, pp25–28.
1.3
Describing the purpose and role of variables
Exemplar questions in Software Design and Development (National 5)
Package 1: Unit-by-unit approach, pp25–28.
1.4
Describing in simple terms how programs relate to low-level operations
and structures
Exemplar questions in Software Design and Development (National 5)
Package 1: Unit-by-unit approach, p29.
The range of programming constructs should include expressions, sequence,
selection, iteration and pre-defined functions.
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SUBJECT CONTENT
2
Develop short programs using one or more software development
environments by:
2.1
Selecting and using a combination of expressions, sequence, selection,
iteration and pre-defined functions
Various examples in Computing Software Development [INTERMEDIATE
2]David Bethune Expressions pp58–68, Sequence pp67–72, Selection pp85–
101 Repetition/Iteration pp102–133, Pre-defined functions pp64–66 & 72–83.
Exemplar programs tasks in Software Design and Development (National 5)
Package 1: Unit-by-unit approach, pp31–33.
2.2
Selecting and using appropriate simple data types, such as numeric
(integer and real), string and Boolean
Variables integer and string p29 Computing Science Mobile App Development
Advice for Practitioners (using AppInventor)
Computing Software Development [INTERMEDIATE 2]David Bethune, pp50–
52.
Variables Boolean p39 Computing Science Mobile App Development Advice
for Practitioners (using AppInventor).
Exemplar programs tasks in Software Design and Development (National 5)
Package 1: Unit-by-unit approach, pp31–33.
2.3
Testing digital solutions using own test data
Testing p29 Computing Science Mobile App Development Advice for
Practitioners (using AppInventor)
Computing Software Development [INTERMEDIATE 2]David Bethune, p59.
Exemplar programs tasks in Software Design and Development (National 5)
Package 1: Unit-by-unit approach, pp31–33.
2.4
Identifying and rectifying errors in programs
p39 Computing Science Mobile App Development Advice for Practitioners
(using AppInventor) TimesTable helper.
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SUBJECT CONTENT
pp110–112 Computing Software Development [INTERMEDIATE 2]David
Bethune
2.5
Providing internal commentary or documentation
pp42 and 45–46 Computing Software Development [INTERMEDIATE 2]David
Bethune
Exemplar programs tasks in Software Design and Development (National 5)
Package 1: Unit-by-unit approach, pp31–33.
Programs should include at least two constructs and at least two data types.
3
Produce a short detailed report comparing two contemporary
software development languages or environments by:
3.1
Describing how each represents standard constructs
Exemplar report in Software Design and Development (National 5) Package 1:
Unit-by-unit approach, pp17–18.
3.2
Comparing the range of data types provided
Exemplar report in Software Design and Development (National 5) Package 1:
Unit-by-unit approach, pp17–18.
3.3
Comparing their editing features
Exemplar report in Software Design and Development (National 5) Package 1:
Unit-by-unit approach, pp17–18.
3.4
Describing how high-level code is translated and executed
Exemplar report in Software Design and Development (National 5) Package 1:
Unit-by-unit approach, pp17–18.
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Introduction
Computers are just expensive paperweights if no-one tells them what to do.
To be useful, a computer needs instructions. These instructions or commands
come in two sorts:
1.
2.
A program you write yourself telling the computer what to do in a stepby-step list.
A program that someone else has written for you which you buy.
Either way, to get a computer to do what you want, it needs programs.
Programs follow a set format: they generally take in some sort of input,
perform some kind of action or process and then produce an output. Using a
game (type of program) as an example, the game takes input in the form of
key presses, mouse clicks or gamepad movements. The game then
processes the information, for example how far to move a character on
screen, and the output is the character actually moving on the screen.
Game
Input
Process
Output
Key presses, mouse
clicks or gamepad
movements
Deciding how far to
move a character on
screen
Character actually
moving on the screen
Programming is about giving a computer the tools and instructions to solve a
problem. To solve a big problem you generally take a top-down approach and
break the big problem down into smaller chunks to make it more manageable.
Data representation
Units of storage
Bit is short for binary digit and is the smallest unit of measurement on a
computer having either the value 0 or 1. Memory is measured by the byte, and
a byte can store a single character. An example of this is that the word ‘egg’ is
three characters long and therefore requires 3 bytes of computer memory
storage. Modern PCs require billions of bytes to simply load up the operating
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system, so to make these huge quantities of bytes easier to refer to, there are
a number of terms to describe them.
Name
Short form
Byte
Kilobyte
Megabyte
Gigabyte
KB
MB
GB
Terabyte
TB
Petabyte
PB
Precise amount
8 bits
1024 bytes
1024 kilobytes or 1,048,576 bytes
1024 megabytes or 1,073,741,824
bytes
1024 gigabytes or
1,099,511,627,776 bytes
1024 terabytes or
1,125,899,906,842,624 bytes
Approximate
amount
8 bits
1 thousand bytes
1 million bytes
1 billion bytes
1 trillion bytes
1 quadrillion bytes
Storing numbers
Computers count in twos (ones and zeros) this is called binary and contrasts
with the way humans count, which is in decimal.
Power of
2
Decimal
value
Decimal
number
67
(64 + 2 +
1)
27
26
25
24
23
22
21
20
128
64
32
16
8
4
2
1
0
1
0
0
0
0
1
1
Very large numbers are stored using floating point representation. An example
of this is that the number 987654321 would be shown as 0.987654321 × 109.
The 0.987654321 is called the mantissa and the 9 is the exponent.
Storing text
The group of letters, numbers and characters that the computer can work with
is called the character set. At this level of study the character set discussed is
ASCII, which consists of:
Letters both upper case and lower case
Symbols
Non-printing or control characters
Numbers
24
A–Z, a–z
&, *, %, $
<tab>, <escape>, <return>
0–9
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One byte is used to store a single character, for example the ASCII code for
an upper case ‘A’ is 65 in decimal or 01000001 in binary.
Storing graphics
Graphics can be stored in either bitmap or vector format.
In a bitmap graphic each pixel or point on the screen is represented by a
binary number, with an empty dot being represented by a 0 and a filled dot
represented by a 1. Images are then built up from a grid of these pixels or
picture elements.
1
0
0
0
0
0
0
1
0
1
0
0
0
0
1
0
0
0
1
0
0
1
0
0
0
0
0
1
1
0
0
0
0
0
0
1
1
0
0
0
0
0
1
0
0
1
0
0
0
1
0
0
0
0
1
0
1
0
0
0
0
0
0
1
With vector or object-oriented graphics the picture is stored in a mathematical
format with a list of object attributes such as x- and y-coordinates, line width,
length, breadth etc.
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Exercises
1.
1 terabyte is the same as:
A
1024 gigabytes
B
1024 megabytes
C
1000 megabytes
D
1024 bytes
2.
The number 23 represented in binary is:
A
10111000
B
00000111
C
00010111
D
11101000
3.
The binary number 00101001 is what in decimal?
A
82
B
41
C
148
D
255
4.
ASCII can be described as what?
A
A 16 bit code used to store graphics.
B
A 7 bit code used to store graphics.
C
A 16 bit code used to store characters.
D
A 7 bit code used to store characters.
5.
A black and white graphic, 30 pixels by 20 pixels in size, stored as
a bit map, would require how much storage?
A
600 bytes
B
600 KB
C
600 bits
D
50 bits
6.
Each character on the keyboard has its own ASCII code. The ASCII
code for one of the control characters is 13.
(a) Convert the decimal number 13 into a binary number.
(b) Describe the function of ‘control characters’.
7.
How much storage space is required to store a black and white
image that is 4 inches long and 3 inches tall at a resolution of 200
dpi? Give your answer in kilobytes and show all your working.
8.
A bit-mapped image measures 11 by 8 inches at 600 dpi in 24-bit
colour. Calculate the file size of the uncompressed image. Show all
your working and give your answer in MB.
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9.
Explain the meaning of the term 'resolution' when referring to a
bitmapped image.
10.
What is the relationship between resolution and file size in a
bitmapped graphic?
11.
Using a bitmapped graphics package create a portrait of your
learning partner or neighbour and save the file. After completing
the portrait in a bitmapped package try to recreate a similar portrait
using a vector drawing package and save the file. Compare the file
size of the two portraits. Which takes up the most disk space and
why?
12.
Complete the bit map for the 8 × 8 image below.
1
1
1
1
1
1
1
1
1
0
0
0
0
0
0
1
1
0
1
1
1
1
0
1
1
0
1
0
0
1
0
1
1
0
1
0
0
1
0
1
1
0
1
1
1
1
0
1
1
0
0
0
0
0
0
1
1
1
1
1
1
1
1
1
13.
Calculate the storage requirements for the above image.
14.
Create your own image and complete a bit map for your image.
15.
A program calculates average rainfall for a week. An example of the
results for one week is 19.6 mm. Describe how floating point
representation is used to represent this real number.
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Computer architecture
Processor
The central processing unit (CPU) is essentially the brains of the computer,
organising the computer's inputs, processes and outputs. The CPU takes in
the data that is input to the computer, processes it and then outputs it to, for
example, a printer or monitor.
Examples of this are illustrated in the table below.
Input
Process
Output
Entering values into a
spreadsheet
A character for a
computer game is
drawn using a graphics
tablet
Product details are
entered into a database
Calculating the average
value
Graphic is resized
Printing the results
Products are sorted on
price in ascending
order from cheapest to
most expensive
Graphic is saved to a
USB memory stick
Results displayed on
screen
The CPU consists of three main parts:
 arithmetic and logic unit (ALU)
 control unit
 registers.
The control unit sends out control signals to execute instructions, eg to store
data in memory, to read data from memory and to decode and carry out
instructions.
The ALU carries out both arithmetic and logic tasks. Arithmetic functions
include tasks such as addition, subtraction, multiplication and division. Logic
functions enable the computer to make decisions, for example by comparing
values using operators such as >, <, =, if and else.
Registers are small, temporary memory locations on the actual processor itself
used to store data, instructions and memory addresses, and retrieve them
immediately.
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Memory
There are two categories of memory: main memory and backing storage.
Backing storage is for the permanent storage of a user’s data and applications
on devices like hard disks and pen drives. Main memory is the memory inside
the computer itself. Main memory on a computer is divided into random
access memory (RAM) and read-only memory (ROM). RAM holds data and
programs as they are being used by the processor – data can be exchanged
and updated quickly but is lost when the power is switched off. ROM is used to
hold essential core programs and data and therefore cannot be changed and
does not lose its contents when the power goes off.
Buses
In simple terms buses are the wires that connect the processors to the
memory and devices attached to the computer. There are a number of
different kinds of buses and they can be unidirectional (only allow data to go
one way) or bidirectional (allow signals to go in both directions). More is
learned about the various types of buses and their functions in Higher
Computing.
Interfaces
When talking about computer architecture the interface is the technology (both
hardware and software) that lets the processor communicate with the
hardware devices (peripherals) connected to it. Interfaces are the ‘middle man’
between the computer and the device, making up for things like differences in
speed, signal and voltage between them and carrying status messages such
as when a printer is out of paper. More is learned about the various types of
buses and their functions in Higher Computing.
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Exercises
1.
The main parts of a processor are:
A
RAM, ROM and registers
B
input devices, output devices and backing storage
C
control unit, ALU and registers
D
main memory, backing storage and registers
2.
What is the purpose of a computer processor?
3.
Draw a four-box diagram representing the main components of a
computer system.
4.
What is the name of the part of the processor that is responsible
for the timing of operations?
5.
What is the name of the part of the processor that carries out
calculations and makes logical decisions?
6.
If a document is in RAM and power is suddenly lost, what would
happen to the document?
7.
A printer is connected to a computer using an interface. Describe
one function of an interface.
8.
A computer system makes use of both main memory and backing
storage. Explain the difference between these.
9.
State one feature of RAM that differs from ROM.
10.
What is the name given to the wires that connect the processor to
the memory and devices attached to the computer
30
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Programming languages
Low-level languages
Computers are two-state machines made up of many electronic switches that
are either off or on. This is represented by 0 for off and 1 for on, and this
system called binary or machine language. A program written in machine
language might look like this:
1101 1010 1100 0000 0011 0101 0101 0011 1101 0011 1110 0001
To write a program only using 1s and 0s is difficult, as it is easy to make
mistakes and it takes a long time to write. To make programming easier
something called assembly language was invented, which uses short phrases
like MOV and JMP instead of lines of 1s and 0s. Although these phrases or
mnemonics are easier for humans to read, understand and write in, they still
have drawbacks, for example it is difficult to transfer programs from one type
of computer to another (something called portability) and the programs are still
time-consuming to write. Given these drawbacks, programming languages
closer to the English language have been developed. These languages, which
use English-like phrases such as print, do, if, then and else, are called highlevel languages.
High-level languages
Advantages of high-level languages:
 allow programs to be written much faster than in assembly language
 quicker to learn than machine or assembly language
 reading, fixing and changing high-level language code is easier than for
lower level code
 high-level language programs can run on a number of different computers
(they are more portable).
There are a wide variety of high-level languages available, each with pros and
cons, and each suited to solving a particular type of problem. Depending on
what type of computer a programmer wants to control or what type of problem
a programmer wants to solve, a programmer picks a specific type of
programming language for the job.
Compilers and interpreters
Once the program has been written in a high-level language using an editor, it
might make sense to a human but, as mentioned earlier, computers only
understand 1s and 0s. To fix this, a program is required to translate the source
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code into machine code. There are two types of program to do this translation:
interpreters and compilers. Interpreters convert each line of code one line at a
time. This translation is only temporary and every time the program is run it
has to be interpreted again. Compilers take the programmer’s source code
and convert it in one go; these machine code instructions are stored in another
file called an executable. This conversion only has to happen once unless the
source code is updated.
Comparing programming languages
Programming languages can be categorised according to functions or
features. A small selection of common categories is shown in the table below:
Category
Web page
Scripting
Database
Rapid application
development
32
Description
Allows the programmer
to create interesting,
interactive web pages
rather than just plain
text on a screen
Allows the programmer
to customise an
existing program with
added functionality or
an altered user
interface
Allows the programmer
to write code that
makes storing,
retrieving and printing
information from a
database more
convenient
Allows the programmer
to create programs with
a graphical user
interface quickly.
Example
Java
Visual Basic for
Applications (VBA)
Structured Query
Language (SQL)
Visual Basic
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SUBJECT CONTENT
At this level of study you will be asked to research and write a report
comparing two different modern programming languages. The table below
gives a quick example comparison of two programming languages:
Purpose
Visual Basic 6.0
Allows programmers to
make Microsoft
Windows applications
Key features
Designed to be easy to
learn
Programmers can create
graphical user interface
applications
Programming Visual
basic combines visually
placing components or
controls on a form,
setting attributes and
actions for these
components, and then
writing extra lines of
code
Interface
Use of variables
Graphical
Three main types: string,
numeric (byte, currency,
double, integer, long,
and single) and special
(Boolean, date, object
and variant)
Use of constructs
Allows loops (repetition)
such as ‘do...while’ and
‘for…next’ and decisionmaking (selection), such
as ‘if …then...else…’
Scratch 1.4
Allows programmers to
make interactive stories,
animations, games,
music and art
Projects contain scripts
and media
Sounds and pictures can
be imported or authored
in scratch using a sound
recorder and paint tool
Programs are created by
snapping together
coloured command
blocks to control 2D
graphical objects called
sprites moving on a
background called the
stage
Projects can then be
saved to be run later in
scratch or uploaded and
shared on the scratch
website
Graphical
Strings for displaying
text on screen or making
decisions
Numeric (whole
numbers of real
numbers) for things like
movement, musical
notes or decision making
Allows loops (repetition)
with blocks such as
‘forever’ and decisionmaking (selection) such
as ‘if …else…’
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Impact on environment
or society
34
BASIC means
‘beginners' all-purpose
cymbolic instruction
code’
Compared to many other
commercial
programming languages
it is easy to learn
The code uses English
language constructs and
allows less experienced
programmers to create
limited but effective
programs quickly, even if
the solutions are not as
neat or efficient as more
powerful as languages
such as C++
Scratch is an excellent
introduction for learners
to take their first steps
into programming as it is
colourful and involves
very little typing and no
complicated syntax or
programming rules
Software is freeware,
with a huge online
community providing
advice, encouragement
and forums to share
achievements
Scratch allows
programmers to expand
into physical computing
using Lego WeDo and
Pico boards, allowing
programs to interact with
the outside world
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Exercises
1.
Machine code:
A
is easier to understand than high-level language code
B
uses normal English words
C
needs to be translated before it can be executed
D
does not need to be translated before it is executed
2.
A program which translates an entire high-level language program
into a machine code file is:
A
a compiler
B
an interpreter
C
a text editor
D
an algorithm
3.
A program which translates high-level language code into machine
code, running each line as soon as it is translated is:
A
a compiler
B
a text editor
C
an interpreter
D
an algorithm
4.
Translators are required to translate high-level language programs
into machine code. Which type of translator translates the whole
program before any execution takes place?
5.
What type of programming language would be most suitable for
automating a complex series of steps in a database package?
6.
In pairs, decide on two different modern programming languages that
you will research. Individually at first, research and write a report
comparing these two programming languages, looking at factors such as
purpose, key features, interface and impact on environment or society.
After completing this report, swap with your neighbour, noting any
information your neighbour has found that you can add to your report
and give your partner feedback on anything they might have missed that
you found out. Be prepared to give feedback to the rest of your class on
the languages you both researched.
7.
Take the report you produced in question 6 and add some further
detail regarding how each language represents standard
constructs and the range of data types provided. You should also
compare the editing features of the languages and describe how
the high-level code is translated and executed.
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Designing a program
Once a language has been chosen to program in, the programmer should
draw up sketches or prototypes of the user interface for the program and a
design of the logic of the program. This design of the program logic can either
be in text format as a list of instructions outlining how the program will run,
called pseudocode, or graphical using diagrams such as structure diagrams.
Pseudocode helps a programmer find mistakes in the logic of the program
design early, before wasting too much time getting stuck into the actual rules
of the programming language being used. Pseudocode maps out each step
your program will need to follow in the programmer’s native spoken language,
eg English. Programmers start with a very general description of the task
involved and then break down (refine) each step into more detail. This is
called top-down design. Eventually the pseudocode should describe every
possible step required of the computer to complete the task or problem. A
simple example of pseudocode is shown below.
1.
Get co-ordinates of target.
1.1. Have drone pilot type target co-ordinates.
1.2. Ensure co-ordinates are valid.
1.2.1. Ensure target co-ordinates are in the correct format.
1.2.2. Make sure drone has enough fuel to reach co-ordinates
and return.
1.3. Store co-ordinates in memory.
2.
Get drone’s current co-ordinates.
3.
Plot course so drone reaches target.
4.
Take photos of target area.
Structure diagrams in basic terms are read from top to bottom and left to right,
describing the steps involved in a task from a bird’s eye view. An example of a
structure diagram is shown below.
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SUBJECT CONTENT
Exercises
1.
Choose one of the tasks below and write pseudocode and draw a
structure diagram for it.
Getting to school.
Going to Spain.
Texting a friend.
Becoming a pop star.
2.
Part of a program design is shown below.
2.1
2.2
2.3
2.4
2.5
Ask for age.
Loop while age < 18.
Display error message.
Ask for age.
End loop.
Name the design notation that has been used above and explain the
purpose of this part of the program.
3.
Name and describe a graphical design notation and explain an
advantage this notation would have in comparison to pseudocode.
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Writing programs
To create the initial program or source code, programmers use an editor. The
editor is a program that looks like a word processor but has special features to
make the process of writing, editing and printing code easier, such as offering
to auto-complete commands, colour coding, highlighting of errors and pop-up
help windows. Below is an example of the editor in a programming language
called Visual Basic 6.
Computational constructs
This section discusses the building blocks of programs; as mentioned
previously there a huge number of programming languages all with different
rules and interfaces. The programming languages covered by a school will
vary and change from year to year and staff to staff, and it would be very
difficult to provide a programming manual in this document for all of them.
Staff will have programming notes and resources already for the languages
used in a particular school, so this section just takes an overview of the theory
behind the building blocks of programs to help learners with general written
assessments on programming.
Variables
Earlier it was mentioned that computers follow an input, process and output
model. Once data is input into a program the computer needs to find a place
to store the data before it is processed and output as information. To make
this data easier to handle in the computer memory it is stored as a variable.
Variables are named or labelled storage locations in the computer memory,
they are used to store one bit of data at a time but can be reused and at any
time what is stored inside a variable can change, hence the name 'variable'.
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Before writing a program the programmer decides whether variables will be
needed and then plans how many variables will be required and what the
variables will hold (text, numbers, etc). This is called declaring variables.
Like homes and mobiles phones, each variable must be able to be uniquely
identified and so is given a distinct name. Depending on the programming
language used, there will be rules for what can and can’t be used as a variable
name. For example, in the language BASIC variable names can’t start with a
number, have full stops in the name or be the same as keywords used within
the language such as ‘else’ or ‘if’.
Variable types
In this course the following variables are covered.
String
String variables are used to hold text, for example names and addresses.
Even though an address such as ’22 Acacia Avenue’ might contain numbers,
the number part of the string isn’t being used in a calculation.
Numbers (integer and real)
Numbers can be either whole numbers (integers) or numbers with decimal
places (real numbers).
Boolean
A data type that only holds one of two values: true or false, yes or no.
1D arrays
A special category of variable. Normal variables store a single piece of data at
a time. Any new data assigned to a variable overwrites the old data. An array
is one variable name that can hold multiple pieces of similar data, for example
a HighScore (number) stores a number of high scores in one place.
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Exercises
1.
When writing a program, what is the name given to the software tool
used to type in the textual program instructions? State two features of
this type of programming tool.
2.
Classify each of the following as integer, string or real:
(a) 151 (b) Billy Bunter (c) 39.99 (d) DD4 1AB (e) 0.25 (f) –200 (g) MR2
(h) 321 (i) Zebra (j) –9.99 (k) 0.00001 (l) 10 Yemen Street
3.
Which one of the following options would be most suitable for
storing the postcodes of 20 customers?
A
B
C
D
4.
A programmer could use a text editor to:
A
B
C
D
5.
365 string variables
365 numeric variables
An array of strings
An array of numbers
Which one of the following options would be the most suitable for
storing the nine scores in a bowling competition?
A
B
C
D
40
translate a program into machine code
copy and paste sections of code
carry out test runs of a program
automatically check a program for logic errors
Which one of the following options would be the most suitable for
storing the average rainfall for each day of the year?
A
B
C
D
6.
A string of arrays
20 numeric variables
20 string variables
An array of strings
Nine string variables
Nine numeric variables
An array of strings
An array of numbers
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SUBJECT CONTENT
7.
A program stores the number of patients in each of six hospital
wards in Bobland Royal Infirmary. Which is the best method of
holding these six data items?
A
B
C
D
An array of real numbers
An array of integers
Six real variables
Six integer variables
8.
What type of variable would be used to store a list of names?
9.
What type of variable would be used to store whether or not an
employee had a driving licence?
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Programming constructs
For programs to be useful, programmers need to use one or more constructs
(building blocks) in the code they produce. There are a number of common
constructs among programming languages.
Assignment
Assignment involves placing a value into a variable within a program. A value
can be assigned in a number of ways: by fixing a value to it within the program
code, by performing a calculation and placing the result into the variable, or by
asking the user for input and then placing that user input into the variable.
Expressions
Expressions can be used in a program to get the computer to perform a
calculation using a mathematical operation such as addition, subtraction,
division and multiplication.
Sequence
Sequence simply means making sure a program executes commands in the
correct order, for example if a program takes in the test scores for a class and
calculates the average score displaying it on screen, the program must first
obtain the results before the average can be calculated, and similarly the
average must be calculated before the answer can be displayed on screen.
Selection
Selection involves putting in place the instructions, conditions or paths to
enable the computer to make a decision as it executes the program code.
Repetition
Programmers try to write the smallest programs possible; smaller programs
mean less code to type, less code to check for errors and less code for the
computer to read through (program runs faster). A key tool a programmer can
use to achieve this is the use of loops or repetition. Loops can be either fixed
or conditional. Fixed loops mean that the programmer decides at the time of
writing the code how many times the code should be repeated, eg displaying a
message on screen five times. Conditional loops are for when the programmer
does not know how many times the code should be repeated, eg asking for
the user to enter a correct password.
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Pre-defined functions
Depending on the programming language being used, the makers of the
programming environment often include some ready-made commands or tools
which perform quite complex tasks without the programmer having to create
the code from scratch themselves. The pre-defined function takes in a single
value and delivers a single value. An example of this would be the INT
function in Visual Basic, which turns a real number into a whole number by
lopping off any decimal places. For example:
INT (grade_average) = formatted_ grade_average
This line of code takes a grade average, for example 16.798, takes off the
.798 and stores the integer 16 into a new variable called formatted_
grade_average.
Pre-defined functions save time when writing the code and testing the code as
they have already been tested by the makers of the programming tool being
used.
Algorithms
An algorithm is simply a set of instructions or steps that the computer follows
to achieve a task or solve a problem. Some problems or tasks are common to
most programs, such as the ability of a program to search for an item, to count
how many times something happens or to find the highest or lowest value in a
list. Programmers will have a set way of solving these tasks or standard
algorithms. At this level of study learners are expecting to know about an
algorithm called input validation. They also need to know about FindMin,
FindMax, Linear Search and Count Occurrence.
Input validation
Input validation is a piece of code that ensures that what the user inputs into
the program is in a suitable format for the program to handle properly. An
example of this might be if a program is taking in a test score and the test is
out of 100, then the programmer will want to ensure that the user doesn’t put
in a test score of 120, or twenty or –15. Input validation algorithms use a loop
(repetition) to take in a value from the user, check whether it is within a
sensible range and, if not, give the user a meaningful message to try again; it
keeps on looping until the user inputs the data in the correct format. This type
of loop will be a conditional loop as the programmer does not know in advance
how long it might take the user to get the format or range correct.
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SUBJECT CONTENT
An example of this algorithm using pseudocode is shown below:
1.
2.
3.
4.
5.
6.
Ask for user to input value.
Store the input value.
Loop while input value in not within acceptable range.
Ask user to re-enter the value in correct range.
Store the input value.
End loop.
An example of this in Visual Basic might be as follows:
Test_score = InputBox(“Please enter a test score out of 100”)
While (Test_score < 0) OR (Test_score > 100)
Test_score = InputBox(“ERROR….Please re-enter a score
between 0 and 100”)
Wend
Exercises
1.
A section of program code is shown below. What type of statement is it?
let minimum_age be 21
A
B
C
D
Conditional loop
Fixed loop
Conditional statement
Assignment
2.
A program is required to calculate how many days there are for a given
number of weeks. Use pseudocode or a programming language of your
choice to show how this could be implemented.
3.
Below is an algorithm to work out the speed of a missile. Look at the
sequence of steps and explain where there is a problem. Rewrite the
steps to produce correct results.
1.
2.
3.
4.
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Take in distance travelled by missile.
Display the speed of the missile.
Take in the time taken to travel this distance.
Calculate the speed of the missile.
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4.
A computing professor has asked his students to design and write a
program to satisfy the following specification.
Create a program to take in the marks of 20 students in a recent class
test out of 100. It should calculate and display the average mark to the
nearest whole number.
One student produces the design below.
1
2
3
4
5
6
Set total to 0.
Loop 20 times.
Get valid mark.
Add mark to total.
End loop.
Calculate and display average mark.
What type of loop has the student chosen in her design?
5.
The example in the question above requires further refinement to ensure
that only a valid mark is accepted. Using the same design notation, write
the refinement for step 3 or alternatively write the code in a language
with which you are familiar. Assume that the user is to be prompted to
enter the mark.
6.
A section of code in a program is to be repeated until a particular item of
data is entered by the user. Which type of loop is required to implement
this?
A
B
C
D
Fixed loop
Conditional loop
Nested loop
Infinite loop
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SUBJECT CONTENT
Errors in programs
When a programmer is inputting code and running it to see if it works, the
code won’t always work. This may be because of errors in the code the
programmer has typed, called bugs.
The types of errors programmers make come into three categories:
1.
Syntax errors
These are errors made when a programmer types instructions into the
editor, for example misspelling a reserved word (INTERGER instead of
INTEGER in Visual Basic) or variable name (variable is declared as
HIGH_SCORE, but later on it is mistyped as THIGH_SORE).
2.
Run-time errors
Run-time errors don’t come to light until the program is executed or run.
A run-time error happens when a program gets data but doesn’t know
what to do with it. A very simple example of this might be if a program
asks for the user to enter a test score which the program will then
proceed to assign a grade to, such as an A, B, C etc, dependent on what
percentage the learner gets in the test. In this example, if the user puts
in text as the test score, eg seventy instead of 70, the programming
code may then falter because it was expecting a number and not a string
and therefore can’t execute the rest of the code properly.
3.
Logic errors
Logic errors occur when the program has the wrong commands or the
commands are not in the correct order or sequence. Essentially the
commands the programmer gives are not clear or specific enough and
therefore the computer gives unexpected results. An example of this in
real life might be if staff asked learners to complete a homework task. If
staff do not give very specific instructions the homework might not come
back in time, might not be the learner’s own work or might not be of the
requisite quality. Logic errors can be hard to spot because the program
might run but the end result might be incorrect and without thorough
testing it may go unnoticed. An example of this might be a satellite
navigation program that when Perth in Scotland is typed in as a location,
directs the user to Perth in Australia. The program gives a result but the
result in incorrect.
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Testing
Testing is the process of checking that the program works as planned. When
testing a program, the programmer should run the program a number of times
with different kinds of user input to see how the program responds and
whether the output from the program is as expected. When picking data to test
the program with, a programmer should be methodical and pick data that will
test the program as thoroughly as possible. Test data should cover the
following categories:
 Normal – the programmer tests the program with the average sort of data
the program would be expected to deal with.
 Extreme – the programmer tests the program with data right on the limit or
boundary of what the program would be expected to deal with.
 Exceptional – the programmer tests the program with data outside of what
the program has been designed to process. How does the program handle
a user typing in ‘fifteen’ instead of 15 when asked for their age?
Example test data
A program is designed to take in learner test scores, add them together and
produce a class average. The test is out of 30. Below is an example of the test
data that could be used to check that the program works.
 Normal test data: 10, 15, 20.
 Extreme test data: 0 and 30.
 Exceptional test data: –1,100, fifteen.
Maintenance
After a working program has been produced, a programmer’s work isn’t done,
Sometimes it is necessary to go back to a program:
 to fix a problem spotted later by users (perfective maintenance)
 to update the program to work with a new piece of hardware or software
(adaptive maintenance)
 to add new features to the software (perfective maintenance).
Programmers can make the job of going back and changing software easier
by making their programs more readable. Improving the readability of a
program can be achieved using the following techniques:
 Add lines of comments into the code to document when the program was
written and by whom, what the purpose of the program is and explaining
what certain parts of the program do. These little ‘sticky notes' in
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SUBJECT CONTENT
the program are ignored by the computer but can really help a programmer
looking back at a program written some time ago or at code that has been
written by someone else. Their proper name is internal commentary.
 Variables should be given meaningful names, for example if a variable
stores a high score for a game, it could be called something that gives the
reader a clue as to what it holds such as ‘high_score’ rather than something
meaningless like ‘variable1’ or ‘x’ or ‘spam_fritter’.
 When writing a story or essay it is important to punctuate the text with full
stops, commas and paragraphs. These help the reader scan the text and
take in the information in manageable chunks. Similarly, programmers
should punctuate their code with white space and indentation (like
tabulation in word processing), making the code easier to understand when
it comes to fixing errors or making changes.

Internal
comment
White space
Meaningful
variable
name
Meaningful
Screenshot of a maintainable program in Visual Basic
variable name
Indentation
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Exercises
1.
A program has been written to process scores out of 20 in a recent
science test. Choose the most suitable set of test data below to fully test
this code.
A
B
C
D
12, 3, 16, 9, 10, 15
-1, 20, 0, 21, 15, 2
0, 4, 16, 10, 19, 20
1, 2, 3, 4, 5, 6
2.
A program is being written to take in the age of customers to allow a
company to target them with suitable advertising and promotions. A
program like this should be tested with several sets of test data to make
sure that the age-inputting part of the program is working. List and label
any six data items that would thoroughly test that this part of the design
works properly.
3.
Swap your test data from question 2 with your neighbour and evaluate
their test data. Did they miss anything out? Did they include a type of
test data that you didn’t? Give your neighbour some verbal feedback
after looking at their work and together come up with an agreed set of
six items of labelled test data. Be ready to give an example to the rest of
the class.
4.
Name and describe two kinds of programming error that can be
made.
5.
Name and describe three steps a programmer can take to make code
more readable.
6.
Why is it important that a programmer makes their code readable?
7.
In pairs, debate which is more important in making code more readable:
meaningful variable names or internal commentary. Each person should
take one of these measures and spend 1 minute putting a case forward
to the other partner. When both people have made their argument, try to
agree on which is the most important. If you cannot, why not? Be ready
to justify your findings to the rest of the class.
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SUBJECT CONTENT
Glossary
Below is a glossary of common computing terms that you may come across
during your studies.
Algorithms
Analysis
Application software
Applications package
Arithmetic/logic unit(ALU)
Array
ASCII
Back up
Backing storage
Backing storage medium
Binary
Binary language
Biometrics
Bit
Byte
Central processing unit
(CPU)
Coding the program
Compiler
Computer system
Conditional loop
50
Set of instructions used or sequence of steps
taken to solve a problem.
The analysis stage involves reading and
understanding a problem.
Comprises all the programs you apply to a
task, such as writing a report or sending an
email.
Software that performs a particular task such
as word processing or desktop publishing.
Component of the processor of a computer
that does the calculations and makes the
decisions.
Set of data items of the same type grouped
together using a single identifier.
A 7-bit code allowing 128 different characters
to be represented.
A second copy of a program or file made in
case the original is lost or damaged.
Permanently holds data on media such as disk
or memory card.
The physical item which holds data such as
USB pen drive.
Counting using two digits, 1 and 0.
Machine language represented using a series
of 0s and 1s.
Security method using physical characteristics
to identify a user such as fingerprints, retinal
scan or voiceprint.
A binary digit, either one (1) or zero (0).
A collection of eight bits.
The hardware component that processes data.
Writing programming language instructions.
Translates a high-level language into machine
code in one go.
A combination of all the components required
to process and store data using a computer.
A condition must be met for the loop to be
carried out.
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Conditional statement
Control unit
Debugging
Documentation
Embedded system
Encryption
Evaluation
Exceptional test data
Extreme test data
Field
File management
File server
Fixed loop
Flowchart
Graphical user interface
(GUI)
Hardware
High-level programming
language
HTML
Human computer
interface (HCI)
Icons
Implementation
Simple decisions, for example IF condition is
true THEN carry out instructions.
Controls parts of the processor and ensures
instructions are in the correct order.
The process of finding and correcting program
errors.
All the supporting paperwork for a program.
Minicomputer inside another piece of
equipment such as a car or a mobile phone.
Formatting a message into a secret code so
that it can’t be understood by anyone who is
not authorised to view it.
Evaluation involves reviewing a solution
against a checklist of requirements,
highlighting any strengths, weaknesses and
improvements needed.
Test data that should be rejected by the
program.
Test data that is on the limit of acceptability.
A single item of data stored in a record.
A function of an operating system dealing with
how files are held, controlling the process of
saving or loading files from disk.
Central disk storage for user's programs on
network.
A loop that is carried out a set number of times
A pictorial representation of the logical steps it
takes to solve a problem.
Allows users to interact with a program in a
graphical environment.
The equipment or physical devices that are
associated with a computer.
Supports English-like syntax.
Hypertext mark-up language – the language
used to write web pages.
The means by which a computer and user
communicate.
Symbols or pictures on a screen.
Changing a program design into instructions
that the computer can understand.
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Import
Information
Input
Input device
Integer
Interface
Internal commentary
Interpreter
Logic error
Loop
Low-level machine
language
Machine code
Macro
Mailbox
Maintenance
Meaningful variable
names
Menu
Non-volatile
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To bring in data from one program file to
another.
Data that has been processed.
Describes the entry of data items into
computer memory using hardware devices
such as a keyboard or mouse.
Allows data to be entered, such as a scanner
or a mouse.
A whole number without a decimal point.
Part of a computer system that allows different
devices to communicate with the processor by
compensating for any differences in their
operation.
Text inside a program that explains what a
program does.
Changes a high-level language into machine
code one line of code at a time, every time the
program is executed.
An error in a program that does not necessarily
stop the program from working but can
produce an unexpected or incorrect result.
Instructions might be performed in the wrong
order, too many times or not at all.
To repeat a set of instructions a number of
times, either fixed or conditional.
Made up of 1s and 0s that the computer
understands. Also called machine language.
Computer language made up of binary
numbers (0 and 1 only).
A set of commands activated by a single
keystroke.
Where electronic mail is stored while waiting to
be read/downloaded.
Consists of all the improvements and
corrections made to a program after it is in
production.
Names given to variables, functions,
procedures, etc. Should have meaning and
indicate the purpose of the item named.
List on screen from which choices may be
made by the user.
Describes storage whose contents are retained
when power is lost.
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Object code
Object-oriented
programming
Online help
Online tutorial
Operating system
Optical character
recognition (OCR)
Optical storage
Output
Password
Pixel
Portability
Processing data items
Processor
Program
Programming
Programming language
Pseudocode
Random access
Translated machine language.
A programming model that focuses on objects
or ‘things’, and describes their features (or
attributes) and their behaviours.
Help available in the form of information
screens built into the program being used.
Step-by-step lessons on how to use a
computer program.
Programs that control the operation of a
computer system.
Characters on paper can be digitised into an
editable document for use on the computer.
A form of non-magnetic storage read by a laser
beam being reflected off the pits and bumps on
the disk surface.
Describes the operation of retrieving
information from memory and sending it to a
device, such as a monitor or printer, so people
can view, interpret and use the results.
A secret code used to gain access to private
information on a computer system or to log on
to a network.
Short for picture element; small dots used to
make up a picture on a screen.
Term used to describe whether a program can
work on a number of different computer
systems.
May involve organising items, checking them
for accuracy or performing calculations with
them.
The part of a computer that carries out the
process.
A list of instructions that tell the computer what
to do
The act of developing and writing programs.
Includes languages such as Visual Basic, C#,
C++ and Java. Used to write programs.
Design notation using a numbered list of steps
using English-like phrases representing the
logical steps it takes to solve a problem.
Files are accessed directly.
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Random access memory
(RAM)
Read-only memory (ROM)
Readability
Record
Registers
Rich text format
Scripting languages
Scrolling
Selection
Sequential access
Software
Spreadsheet
Standard file formats
Structure diagram
Structured listing
Technical guide
Template
Text editor
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Computer chip that stores data temporarily;
data are lost when the computer is powered
down.
Computer chip that stores data permanently:
data are not lost when the computer Is
powered down.
How easy it is for another programmer to
understand and adapt a program.
Data structure consisting of multiple fields of
information.
Fast-access memory locations inside a
processor.
Stores information about text, including
formatting.
Examples are Python, Perl and PHP. Used to
write programs that are typed directly from a
keyboard and stored as text rather than as
binary executable files.
Moving the display on the screen using cursor
keys or a mouse.
Involves making a decision using a construct
such as an IF...statement.
Files are accessed one after another.
The computer instructions that tell the
hardware what to do.
A program used for numeric data, calculations
and graphs divided into rows and columns.
The individual cells in a spreadsheet can
contain text, numbers and formulae.
A way of storing data for use by a number of
different application packages.
Represents the logical structure of a program
design using a tree of linked boxes.
A print-out or screen display of the lines of
code in a computer program.
Contains the installation instructions and
technical requirements of a piece of software.
Ready-made structure/layout for a document.
Allows source program code to be entered and
changed.
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Translator
User guide
Username
Validation
Viruses
Converts a high-level language program into
low level machine code so the processor can
use it.
Explains the functions and features of a piece
of software.
The means of identifying a user on a computer
system or network.
A check carried out by the computer on any
data to see if it is unreasonable or incomplete,
for example to see if a value is within a range
(range check).
Harmful piece of software that attaches itself to
a file, reproduces and spreads.
Qualifications
The requirements for a qualification in Computing Science at National 4 and 5
are laid out in the SQA documentation, which can be accessed from
http://www.sqa.org.uk/sqa/48477.html.
To gain a course award, a learner must pass all units as well as the Added
Value Unit at National 4 or the course assessment at National 5. The course
assessment will contain the Added Value Unit and will consist of a question
paper and an assignment. The assignment will be assessed under controlled
conditions and marked externally.
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