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From: Daniele Futtorovic <da.futt.news@laposte-dot-net.invalid>
Newsgroups: comp.lang.java.programmer
Subject: Re: Basic prisoner's dilemma?
Date: Mon, 18 Apr 2011 19:22:53 +0200
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On 18/04/2011 04:51, theglazeb allegedly wrote:
> Hi there!
>
> I was wondering if you were able to help with me a basic prisoner's
> dilemma strategy? How would I implement a basic tit for tat strategy?
> And perhaps try to avoid the "death spiral" from occurring when two
> players use the tit for tat strategy?
>
> See below
>
> Thanks!
>
> /*
> * File: PDStrategy.java
> * ---------------------
> * This class encapsulates the strategy for an Iterated Prisoner's
> * Dilemma game. To enter the contest, all you have to do is change
> * the definition of chooseAction so that it takes account of the
> * history and does something more clever than cooperating (or
> * defecting) all the time.
> */
>
> import java.util.*;
>
> class PDStrategy {
>
> /*
> * Method: chooseAction
> * --------------------
> * This method determines the player's action for the current
> * game, based on two ArrayLists containing the history of play:
> * one for your algorithm and one for the opponent. Each of these
> * ArrayLists contains strings, which are guaranteed to be one of
> * the uppercase strings "COOPERATE" or "DEFECT". Your method
> * should look at the history and return one of those two strings
> * representing the action you take on the current round.
> */
>
> public String chooseAction(ArrayList<String>  myHistory,
> ArrayList<String>  oppHistory) {
> return "COOPERATE"; //currently, it is only cooperating
> }
>
> }

Basic tit-for-tat is as follows:
- if it's the first time you encounter that opponent (this implies you
keep a 'memory' of previous encounters), cooperate.
- otherwise, replicate that opponent's last move.

In code (assuming oppHistory contains only the history with "this"):

  public String chooseAction(
   ArrayList<String> myHistory, ArrayList<String> oppHistory)
  {
   return oppHistory.isEmpty() ? "COOPERATE" : oppHistory.get(
oppHistory.size());
  }
(or oppHistory.get(0), depending on whether it's used as a FIFO or a LIFO).


You can add various tricks, but don't expect to get far with them,
because tit-for-tat is the evolutionary stable strategy.

If you're interested in the topic apart from the programming aspect, I
*very* strongly suggest you read Dawkins' "Selfish Gene" and "Blind
Watchmaker". Even if you're not, by the way -- those are IMHO must-reads.

-- 
DF.
An escaped convict once said to me:
"Alcatraz is the place to be"