How to Build a Repast Model - 1

In writing our simulation, as mentioned above, we want to describe how to setup our model, and then describe what happens every time step. SimpleModel provides methods that we override in our own model to do the setup specific to our model. These two methods are setup() and buildModel(). We use them as follows:

import uchicago.src.sim.engine.SimpleModel;

public class MyModel extends SimpleModel {


  public static

  final int TIT_FOR_TAT = 0; public static
  final int ALWAYS_DEFECT = 1; private int

  p1Strategy = TIT_FOR_TAT; private
  int p2Strategy = ALWAYS_DEFECT;

  ...

  public void setup() {
    super.setup();
    p1Strategy = TIT_FOR_TAT;
    p2Strategy = ALWAYS_DEFECT;
  }

  public void buildModel() {
    Player p1 = new Player(p1Strategy);
    Player p2 = new Player(p2Strategy);
    p1.setOtherPlayer(p2);
    p2.setOtherPlayer(p1);
    agentList.add(p1);
    agentList.add(p2);
  }
}

Where setup() "tears downs" the simulation, we use buildModel() to create the objects that our simulation uses. So, it is here where your agents should be created, and added to the master list of agents, agentList. agentList is an ArrayList provided by SimpleModel for this purpose. The assumption here is that our Player class is constructed with a reference to the initial strategy and a reference to the other Player to play.

The order of execution is that setup() is called first and then buildModel(). However, as mentioned above, setup() is called when the simulation is first started, and whenever the setup button is pressed. buildModel() is not called until the simulation is run, that is, it is not called until either the run, step or initialize buttons are pressed. (See below for more on what code is executed by what buttons.) This provides an opportunity for the user to change any parameters via the gui.

Now that setup is finished, we need to define what occurs each timestep of the simulation. SimpleModel provides three methods for this. They are preStep(), step() and postStep(). Each tick they are executed in that order, first preStep(), then step(), and lastly postStep(). The intention here is to separate the core behavior in step() from any necessary pre- or post- processing. In our example, we have no such need for any pre- or post- processing, so we only use the step() method.

import uchicago.src.sim.engine.SimpleModel;

public class

MyModel extends SimpleModel { public static

  final int TIT_FOR_TAT = 0; public static
  final int ALWAYS_DEFECT = 1; private int

  p1Strategy = TIT_FOR_TAT; private
  int p2Strategy = ALWAYS_DEFECT;

  ...

  public void setup() {
    super.setup();
    p1Strategy = TIT_FOR_TAT;
    p2Strategy = ALWAYS_DEFECT;
  }

  public void buildModel() {
    Player p1 = new Player(p1Strategy);
    Player p2 = new Player(p2Strategy);
    p1.setOtherPlayer(p2);
    p2.setOtherPlayer(p1);
    agentList.add(p1);
    agentList.add(p2);
  }

  public void step() {
    int size = agentList.size();
    for (int i = 0; i < size; i++) {
      Player p = (Player)agentList.get(i);
      p.play();
    }
  }
}

A different game might require a pre- or postStep() method. For example, assume something like a cooperation game with many players where after each player has played its neighbors and received a payoff, each player then polls its neighborhood for the best strategy, and sets its own strategy accordingly. In this case, the actual play would be defined as above in the step method, and the neighborhood polling and strategy setting would occur in a postStep() method.

There is an alternative to using step() in your model. You can use the auto step mechanism. If you choose do this, SimpleModel will iterate through all your agents and call the step method defined in your agents. In order for this to work, your agents must implement the Stepable interface. This interface has a single method, step(). Note that step() here is a method in your agent class not your model class. This autostep takes the place of your model's step method described above. To use the auto step mechanism, you set the autoStep instance variable to true in the constructor of your model. You can also set the shuffle instance variable to true or false depending on whether you wish to shuffle the agentList before auto-stepping through it. preStep() and postStep() work as above, being called before and after the autoStep, respectively. Regardeless of whether we use preStep(), postStep(), both, or auto step, this is all that is required to create a simulation with SimpleModel: fill in setup(), buildModel() and step() (or auto step), as described above.

import uchicago.src.sim.engine.SimpleModel;

public class MyModel extends SimpleModel {

  ...

  public MyModel() {
    params = new String[] {"P1Strategy"};
  }

  ...

}

Once you've created your parameter accessor methods and placed your parameter name in the param array, you will see the parameter displayed in the parameter pane when the simulation is run. The value of this parameter is whatever is returned by its get accessor method. The parameter is set by entering a new value in the parameters text box. The change is registed either by pressing enter, or when the text box loses focus. This newly entered value then becomes the argument to the parameter's set accessor method. Its important to note here that its the accessor methods that are important, the actual variable where the parameter is stored (e.g. p1Strategy) is never seen by the parameter mechanism.