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Problem Set 5: PS4 consensus program.
CPSC 427: Object-Oriented Programming Handout #7

Problem Set 5

1 Assignment Objective
This problem set continues the development begun in Problem Set 4 of a simulator for a
population of simple agents attempting to reach consensus on a choice value. The PS4
assignment handout describes two different such agent algorithms: fickle and follow the
crowd. In PS4, you implemented a simulator for a collection of fickle agents.
In this assignment, you will add a number of new features to the PS4 consensus program.
Just as in real-life, refactoring code to handle new requirements is easy in places and harder
in others.
2 Assignment Goals
• Learn to use polymorphism.
• Experience the effect of refactoring a big class (Simulator) into two related but
smaller classes.
• Learn how to explore a rich parameter space, and gain insight into the behavior of
random processes.
3 Problem
Here is an overview of the new features and required changes:
1. Agent will become a pure abstract class. Recall that that means all functions are
virtual, and all are abstract except for the virtual destructor. The public Agent
functions are the same as before: update() and choice().
2. Fickle is a new class publicly derived from Agent. It is pretty much the same as the
Agent class from PS4. Crowd is another new class publicly derived from Agent. It
implements the follow-the-crowd algorithm described in the PS4 assignment handout.
3. The Simulator of PS4 did two distinct jobs:
(a) It set up the population of agents to be simulated.
(b) It ran the simulation.
In this assignment, you will separate these two tasks. A new class Population will create and manage the agents. The revised Simulator will take a Population reference
as a parameter and run the simulation, as before, until consensus is reached.
4. Population will maintain the aggregated array of Agent that was previously in
Simulation. However, since Agent is now the base class for two different derived
classes, the array elements will be Agent pointers. Each agent will be created using
either new Fickle( val ) or new Crowd( val ), depending on which kind of agent
2 Problem Set 5
is desired. Here val is the initial choice value for that agent. Population will retain
custody of all of these agents, so its destructor must take care to delete them all.
5. The method for constructing agents is different from PS4. Each agent is randomly
assigned to one of the two concrete agent types, Fickle or Crowd. The initial value
v is randomly chosen from the set {0, 1}. Both of these random choices are biased
according to new command line parameters. probFickle is a real number in the
semi-open interval [0, 1) and specifies the probability that an agent is chosen to be of
type Fickle rather than of type Crowd. probOne similarly is a real number in the
range [0, 1) and specifies the probability that an agent’s initial choice is 1 rather than
0.
6. Population has several public functions in addition to constructors, destructor, and
print:
(a) int size() const returns the number of agents.
(b) void sendMessage(int sender, int receiver) simulates a single communication step from sender to receiver.
(c) bool consensusReached() returns true iff consensus has been reached.
(d) int consensusValue() returns the consensus value if consensus has been
reached; otherwise it returns -1.
7. The Simulator constructor now only takes a single parameter of type Population&.
Its run function has signature void run(). To obtain the results of the simulation,
the caller can call two new public functions: numRounds() and consensusValue().
Since the simulator is doing the simulation, it knows how many rounds it has used.
On the other hand, only Population knows the consensus value, so this is a case
where delegation should be used.
8. main.cpp changes considerably. It takes different command line arguments and it
prints different output than PS4.
(a) The new command line arguments are
numAgents probFickle probOne [seed]
where seed is optional as before. numAgents is again the total number of agents.
probFickle and probCrowd are the probabilities discussed in item 5 above.
(b) All output should go to cout. banner() and bye() should be used as usual.
The output from a run has three parts: The initial parameters, the statistics
of the population after the random generation process, and the results of the
simulation. See sample.out for an example of the new output format.
9. The resulting executable file should be called consensus2 to distinguish it from the
PS4 command name.
An important part of this assignment is to test your program on reasonable test cases,
to submit the test case inputs and corresponding outputs, and to report on what you
observe. For example, you should run your code on extreme cases such as 0 agents, 1 agent,
probabilities of 0.0 and 1.0, and so forth.
Try to gain some insight about the extent to which follow-the-crowd agents do better
than fickle agents. For example, what do you observe with a modest size population (say
1000) with different values for probFickle, say, 0.0, 0.01, 0.5, 0.99, 1.0.
Handout #7—October 31, 2018 3
4 Programming Notes
1. random() is now used in two different parts of the code.
(a) Simulator::run() uses uRandom() as before in order to choose first a sender
and then a receiver for a communication step. uRandom() of course is based on
random().
(b) The Population constructor needs random values when constructing each agent.
First it uses randomness to choose an initial choice value for the agent. Then
it uses it to choose the agent type. In both cases, it chooses a double in the
semi-open interval [0, 1) and compares that number with the desired probability
in order to make its decision. For example, to generate the choice value, test if
the random number is less than the desired probability of choosing 1. If it is,
then choose 1, else choose 0.
2. To choose a random real in [0, 1), you can use my code
double Population::
dRandom() {
return random()/(RAND_MAX+1.0); // result is double in [0,1)
}
By default, the type of “1.0” is double, so the coercion rules force the addition and
then the division to both be performed using double arithmetic. If you change “1.0”
to “1”, it won’t compile without warnings, and it won’t work correctly.
3. In order to duplicate my output, you will need to use the random number generator
in exactly the same way as my program does. In particular, you will need to choose
the sender before the receiver, and you will need to choose the initial consensus value
for an agent before choosing the agent type. Of course, you will also need to start
with the same seed.
4. The submission guidelines are the same as in previous assignments. Submit all files
needed to compile your project along with a Makefile. Include a notes.txt file, a
file of sample inputs and a file of the corresponding outputs.
5. Note that the grading rubric for this assignment puts more emphasis on good design,
good style and good choice of test data than the previous assignments.
4 Problem Set 5
5 Grading Rubric
Your assignment will be graded according to the scale given in Figure 1 (see below).
# Pts. Item
1. 4 All relevant standards from previous problem sets are followed regarding submission, identification of authorship on all files, and so forth. A
well-formed Makefile or makefile is submitted that specifies compiler
options -O1 -g -Wall -std=c++17. Running make successfully compiles
and links the project and results in an executable file consensus2. Each
function definition is preceded by a comment that describes clearly what
it does.
2. 3 Sample input and output files are submitted that show good coverage of
the parameter space, e.g., small inputs, large inputs, edge cases for the
probabilities (e.g., 0.0 and 1.0) as well as reasonable intermediate cases.
3. 5 The program shows good style. All functions are clean and concise. Inline
initializations, inline functions, and const are used where appropriate.
Variable names are appropriate to the context. Programs are consistently
indented according to the course indenting style. Each class has a separate
.hpp file and, if needed, a separate .cpp file. However, it is acceptable
to group the three polymorphic agent classes together in the same .hpp
and .cpp files.
4. 2 Everything is private in all classes except for the specified public interface,
any needed special functions (constructors, destructor, move and copy
constructors and assignments), and functions needed for debugging.
5. 6 All of the functionality in section 3 is correctly implemented.
20 Total points.
Figure 1: Grading rubric.

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