Scheme of work – Cambridge IGCSE® Computer Studies (0420)
Unit 4: Algorithm design and testing
Recommended prior knowledge
Students need to have studied Unit 3 before starting this unit.
Context
Students need to study this unit before Unit 5 and before or alongside Unit 12. For both Units 4 and 5, together with Unit 6, Computer Studies Support Booklet –
Part 3 (http://teachers.cie.org.uk/docs/dynamic/31798.pdf) provides notes on section 3 of the syllabus and practice problems (with answers in Computer Studies
Support Booklet – Answers (http://teachers.cie.org.uk/docs/dynamic/31801.pdf).
Outline
Defining the scope of separate modules, designing algorithms that relate clearly to the requirements of the system and identifying hardware needs arising from the
required output. The use of structure diagrams for top-down design, program flowcharts and libraries of subroutines. Using dry runs and trace tables to work out the
purpose of an algorithm, suggesting and using suitable test data and identifying and correcting errors in algorithms. Study of these topics can be illustrated by case
studies of existing solutions to problems and reinforced through practical work.
Syllabus
ref
Learning
objectives
3.1
Introduction
v1 2Y04
Suggested teaching activities
Learning resources
Students could be introduced to the need for algorithms in developing
software solutions as follows:
1. Ask for a volunteer. Write a small set of random numbers on a board
and ask the volunteer to sort them into ascending numerical order.
Explain that the volunteer is using an unseen strategy or algorithm.
2. Ask the volunteer to look away. Write a new small set of random
numbers on the board and ask the volunteer to give you instructions
for sorting them, asking necessary questions such as “What is the first
number?”
Once a volunteer has succeeded in this task, explain that the
volunteer has revealed their algorithm.
3. Ask the successful volunteer to list the instructions for sorting a new
small set of random numbers before you write the new set on the
board. You can try to pick numbers that will not work with the
volunteer’s instructions. Execute the list of instructions without any
Cambridge IGCSE Computer Studies (0420)
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Syllabus
ref
3.1.1
Learning
objectives
Make an overall
plan
Suggested teaching activities
further intervention from the volunteer. If the instructions fail to
achieve the sort, allow the volunteer or other students to amend the
instructions and start the procedure from the beginning.
Once the instructions are successful, explain that this is an algorithm.
Students should perform practical exercises, using appropriate tools from
3.1.2 below, to:
• use top-down design to define the scope of separate modules within
each solution
• design algorithms to meet the requirements of modules
• explain algorithms and how they relate to a solution
• explain how hardware needs arise from the input and output required
for a solution.
Learning resources
LWS coursebook pp. 240–2
http://raptor.martincarlisle.com/Introduction%20to%20Algo
rithmic%20Thinking.doc
An introduction to algorithmic thinking
It may be useful to point out that:
• a computer must store input data prior to processing and (usually) the
results of processing after processing in memory locations, or
“variables”, which we label with identifiers
• even within structure diagrams and program flowcharts, we can use
pseudocode as shorthand for the process of assignment
• x ← 3 means the value 3 is written as the new value stored in the
memory location labelled x
• x ← y means the value stored in the memory location labelled y is
copied to the memory location labelled x; in other words, the value
stored in the memory location labelled y is read without altering that
value and written as the new value stored in the memory location
labelled x.
Some possible examples to demonstrate solution design are:
• finding the average of a set of input numbers
• finding largest and smallest numbers in a set of input numbers
• calculating the frequency distribution of ranges of numbers in a set of
input numbers (e.g. when a series of temperatures T are input, how
many are in each of the ranges -20 ≤ T < 0, 0 ≤ T < 20 and
20 ≤ T < 40?)
These could be followed by case studies such as an automatic
supermarket stock control system for calculating stock levels and
automatically re-ordering items.
v1 2Y04
Cambridge IGCSE Computer Studies (0420)
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Syllabus
ref
Learning
objectives
3.1.2
Algorithm tools
3.1.3
v1 2Y04
Interpret and test
algorithms
Suggested teaching activities
Learning resources
Students need to become familiar with:
• top-down design using structure diagrams
• program flowcharts to represent algorithms
• system flowcharts to represent whole or partial solutions, including
hardware
• the fact that algorithms can often be re-used as part of the solution of
a different problem and that, similarly, libraries of procedure and
subroutine program code can also be re-used.
LWS coursebook pp. 242–7
Students need to be able to:
• work out the purpose of an algorithm using dry runs
• choose and use suitable test data for testing:
• input validation or other conditional processing (three types:
normal, boundary/extreme and abnormal/erroneous)
• correctness of a calculation
• use trace tables to find the value of variables at each stage in dry run
• identify errors in given algorithms and suggest ways of removing these
errors.
Cambridge IGCSE Computer Studies (0420)
www.smartdraw.com/resources/tutorials/#/resources/tutori
als/Entity-Structure-Diagrams
A tutorial on how to draw structure diagrams using
SmartDraw, of which a free, trial version is available
http://raptor.martincarlisle.com/
RAPTOR, free program flowchart interpreter software that
allows students to draw a flowchart and check its
functioning by executing it
LWS coursebook pp. 247–51
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