Section 2:
Programming Concepts
2.7: Simulation
AQA Computing A2 © Nelson Thornes 2009
Section 2:
Programming Concepts
Key points about computer simulation
Computer simulation involves the construction of models
using computer programs, which can represent real or
imaginary situations.
Computer simulations allow users to study or try things that
would be difficult or impossible to do in real life.
Queues often form an integral part of computer simulation.
Computer simulation models need to deal with the behaviour
of a system over a period of time.
Simulations also often include some element of randomness,
often through the use of a pseudo random number generator.
AQA Computing A2 © Nelson Thornes 2009
Section 2:
Programming Concepts
Simulation examples
There are many examples of computer simulations, for example:
The Bank of England may use simulations to see what
might happen to the economy if interest rates were
reduced or increased.
A factory may use a simulation to see the increase in
output and profit if more machines were installed.
A supermarket may use a simulation to find out the
optimum number of checkouts to open at different times
of day, in order to reduce queuing and increase
customer satisfaction.
AQA Computing A2 © Nelson Thornes 2009
Section 2:
Programming Concepts
Entities and Attributes
Entities are the components that make up a system.
Attributes are the characteristics of an entity.
For example, in a supermarket, some entities might be
Customer, Queue, Checkout Operator.
An attribute of the customer might be waiting.
An attribute of the queue might be its length.
An attribute of the checkout operator might be busy.
AQA Computing A2 © Nelson Thornes 2009
Section 2:
Programming Concepts
States, events and activities
A system also has states, events and activities.
State – What is happening at the moment in the system.
For example, in a queuing system, the states are:
 Nobody in the queue, server waiting.
 Nobody in the queue, a customer being served.
Event – An event is what causes a change of state to take place.
For example,
 a customer arrives.
Activity – is defined as a process which takes time


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The serving of a customer.
The inter-arrival time between customers arriving.
Section 2:
Programming Concepts
Simulation example 1: Taxi company
A taxi cab company has a problem
They only have one mechanic, Bob. Bob is very experienced but he cannot
repair the taxis quickly enough on his own and taxis are out of action for
too long queuing up waiting to be repaired. This means the company is
losing money.
One of the taxi drivers, Olly, used to be a mechanic. Should they use him to
repair taxis as a backup? If so, the taxis may be back on the road more
quickly.
Olly is a quicker mechanic, but less experienced and thorough than Bob. The
more time Olly is working as a mechanic, the less time he will be driving
his taxi and earning taxi fares for the company.
What should the company do to ensure they make the maximum profit?
A computer simulation could help to model this situation.
AQA Computing A2 © Nelson Thornes 2009
Section 2:
Programming Concepts
Taxi scenario: Entities and Attributes
What would be the entities in this system?
Taxi
Mechanic
Queue
What attributes are there of the taxi?
How long waiting for repair
Whether on or off the road
Etc.
Can you think of attributes of
the other entities?
AQA Computing A2 © Nelson Thornes 2009
Section 2:
Programming Concepts
Taxi scenario:
States, events and activities
Possible states
The mechanics are both busy; taxis are waiting.
Olly is driving his cab; no taxis are waiting for repair.
Possible events
A taxi breaks down.
Bob starts a repair.
Bob finishes a repair.
Possible activities
Bob is repairing a taxi.
Olly is repairing a taxi.
Can you think of others?
AQA Computing A2 © Nelson Thornes 2009
Section 2:
Programming Concepts
Simulation example 2: Traffic flow
A town is becoming very congested with traffic most of the day,
particularly at rush-hour.
A range of alternatives have been suggested including more
traffic lights, building a bypass and putting in speed restrictors.
A computer simulation is needed to investigate all of these
alternatives to see how much traffic congestion would reduce
with each alternative.
For example, a simulation might consider the effect of
introducing traffic lights and enforcing a speed limit.
What would be the events and variables
in this simulation?
AQA Computing A2 © Nelson Thornes 2009
Section 2:
Programming Concepts
Traffic flow scenario:
Entities and Attributes
What would be the entities in this system?
A car
The road
The traffic lights
What attributes are there of the traffic lights?
Colour
Time between changes
Can you think of attributes of
the other entities?
AQA Computing A2 © Nelson Thornes 2009
Section 2:
Programming Concepts
Traffic flow scenario:
State, events and activities
Possible states –
There are no cars waiting at the traffic lights;
the lights are red.
All traffic is stationary.
Possible events –
The traffic lights change to red.
The traffic lights change to green.
Possible activities –
A car is moving along the road.
A car is waiting at the traffic lights.
Can you think of others?
AQA Computing A2 © Nelson Thornes 2009
Section 2:
Programming Concepts
Timing of events
Many simulations involve changing the timing of events –
these are called discrete event simulations.
Events may occur at random time intervals.
To do this a (pseudo) random number generator can be
used. A number is generated between 0 and 1 using a
mathematical formula. It can then be multiplied by a number
to generate larger numbers.
Probability may be used to give realistic timing of events in
the simulation. These may be based on averages such as
how many customers arrive in a given period of time.
AQA Computing A2 © Nelson Thornes 2009
Section 2:
Programming Concepts
Summary
Computer simulations can be used for a wide range of
different situations, where a “what if” question needs to
be asked.
Computer simulations involve creating a model of a reallife, or imaginary situation.
Creating a model of a system involves determining the
entities, attributes, states, events, attributes and activities
within the system.
AQA Computing A2 © Nelson Thornes 2009