Prof. Dr. Lars-Erik CedermanETH - Center for Comparative and International Studies (CIS)

Seilergraben 49, Room G.2, lcederman@ethz.chNils Weidmann, CIS Room E.3 weidmann@icr.gess.ethz.ch

http://www.icr.ethz.ch/teaching/compmodels

Lecture, December 7, 2004

Introduction to Computational Modeling of Social Systems

RePast Tutorial I

2Today’s agenda

• The RePast toolkits and its features• The Iterated Prisoner‘s Dilemma• How to build a model (1)• SimpleIPD

3What is RePast?

• Recursive Porous Agent Simulation Toolkithttp://repast.sourceforge.net

• Repast is an open-source software framework for creating agent-based simulations using Java

• Initially developed by the Social Science Research Computing at the University of Chicago

• Will be further developed by the RePast Organization for Architecture and Development (ROAD) and Argonne National Laboratory

4Why RePast?

• Alternatives: Swarm, Ascape, NetLogo...• “RePast is at the moment the most suitable

simulation framework for the applied modeling of social interventions based on theories and data” (2004):http://jasss.soc.surrey.ac.uk/7/1/6.html

• Modeled on Swarm but easier to use and better documented

• Important criteria:– abstraction, ease of use and user-friendliness– flexibility and extensibility– performance and scalability– support for modeling, simulation & experimentation– Interoperability (GIS, statistical packages, …)

5

What does RePast offer?

• Skeletons of agents and their environment

• Graphical user interface• Scheduling of simulations• Parameters management• Behavior display• Charting• Data collection• Batch and parallel runs• Utilities and sample models

6Tutorial Sequence

December 7 SimpleIPD: strategy space

December 14 EvolIPD: RWRDecember 21 GraphIPD: charts

and GUIGridIPD: 2DK

January 11 ExperIPD: batch runs and parameter sweeps

7

Iterated Prisoner’s Dilemma

• Cohen, Riolo, and Axelrod. 1999. “The Emergence of Social Organization in the Prisoner's Dilemma” (SFI Working Paper 99-01-002)http://www.santafe.edu/research/publications/wpabstract/199901002

• In The Evolution of Cooperation, Robert Axelrod (1984) created a computer tournament of IPD– cooperation sometimes emerges– Tit For Tat a particularly effective strategy

8

Prisoner’s Dilemma Game

Column: C D

C 3,3 0,5Row:

D 5,0 1,1

9

One-Step Memory Strategies

C

DC

D

p

q

Memory:C

D

Strategy = (i, p, q) i = prob. of cooperating at t = 0

p = prob. of cooperating if opponent cooperated

q = prob. of cooperating if opponent defected

t-1 t

10The Four Strategies

Name i p q

ALLC 1 1 1

TFT 1 1 0

ATFT 0 0 1

ALJD 0 0 0

11TFT meets ALLD

t0 1 2 3 4

Row(TFT)

Column(ALLD)

i=1

i=0

p=1; q=0

0

5

1 1

1 1 1

1

= 8

= 3+ + +

+ + +

D

C

D

C

D

D

D

D

D

D

p=0; q=0

CumulatedPayoff

12

Payoffs for 4x4 strategies

Own strategy

Other‘s strategyALLC TFT ATFT ALLD

pay/move

sum

pay/move

sum

pay/move

sum

pay/move

sum

ALLC 333312

333312

00000

00000

TFT 333312

333312

01539

01113

ATFT 555520

51039

13138

10001

ALLD 555520

51118

155516

11114

13

How to build a model (1)

• Extending the SimpleModel class with our own model class:

import uchicago.src.sim.engine.SimpleModel;

public class MyModel extends SimpleModel {...

}

14

Setting up the simulation

import uchicago.src.sim.engine.SimpleModel;

public class MyModel extends SimpleModel {public static final int TFT = 1;public static final int ALLD = 3;

private int a1Strategy = TFT;private int a2Strategy = ALLD;

...

public void setup() {super.setup();a1Strategy = TFT;a2Strategy = ALLD;

}

public void buildModel() {super.buildModel();Agent a1 = new Agent(a1Strategy);Agent a2 = new Agent(a2Strategy);agentList.add(a1);agentList.add(a2);

}}

Called when the

simulation is first started and when

setup button pressed

Executed every time the simulation is

run

SimInit.loadModel()

15Running the simulation

import uchicago.src.sim.engine.SimpleModel;

public class MyModel extends SimpleModel {...

public void setup() {...}

public void buildModel() {...}

public void preStep() {...}

public void step() {super.step();

for (Iterator it = agentList.iterator(); it.hasNext();) {

Agent agent = (Agent) iterator.next(); agent.play(); }

}

public void postStep() {...}}

Executed at every time

step

16Model parameters

import uchicago.src.sim.engine.SimpleModel;

public class MyModel extends SimpleModel {public static final int TFT = 1;public static final int ALLD = 3;

private int a1Strategy = TFT;private int a2Strategy = ALLD;

public MyModel() {params = new String[] {“A1Strategy”};name = “Example Model”;}

...

public void setA1Strategy(int strategy) {this.a1Strategy = strategy;}

public int getA1Strategy() {return a1Strategy;}}

Introspection

17SimpleModel methods

• setStoppingTime(long time)set the time at which the current simulation run will stop

• setRngSeed(long seed)set the seed for the random number generator (default: 1)

• getNextIntFromTo(int from, int to)returns the next random double between from and to (exclusive)

• getNextDoubleFromTo(double from,double to)returns the next random double between from and to (exclusive)

• atPause() will be executed whenever your simulation is paused

• atEnd() will be executed whenever your simulation ends

18SimpleModel fields

• boolean isGui true if the simulation is running in GUI mode, false if running in batch mode

• long startAtthe time step at which to start executing the preStep(), step(), postStep() actions (default: 1)

• Schedule schedulecan be used to add your own actions to the schedule...

• For the full list, consult the RePast API:http://repast.sourceforge.net/docs/api/