Add files for prisoner's dilemma

prisoner/iteratedpd.m

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+%% Iterates the Prisoner Dilemma between two players
+% 
+% Returns four arguments: the two lists of strategies (0=defect, 1=coop)
+% and the two scores. 
+
+function [X,Y,Xscore,Yscore]=iteratedpd(r,p1strat,p2strat,payoff)
+
+% Init
+mdefect=payoff(1,1);
+mcoop=payoff(2,2);
+suckerlose=payoff(1,2);
+suckerwin=payoff(2,1);
+
+X=[];
+Y=[];
+Xscore=0;
+Yscore=0;
+
+for i=1:r
+
+    % Determine the next move based on the past history
+    newX=play(X,Y,p1strat);
+    newY=play(Y,X,p2strat);
+
+    % Add the last move 
+    X=[X,newX];
+    Y=[Y,newY];
+
+    % Update the score
+    if (newX==0) %P1 defect
+        if(newY==0)
+            Xscore=Xscore+mdefect;
+            Yscore=Yscore+mdefect;
+        else
+            Xscore=Xscore+suckerwin;
+            Yscore=Yscore+suckerlose;
+        end
+    else %P1 cooperate
+        if(newY==0)
+            Xscore=Xscore+suckerlose;
+            Yscore=Yscore+suckerwin;
+        else
+            Xscore=Xscore+mcoop;
+            Yscore=Yscore+mcoop;
+        end
+    end
+end
+end
+
+
+
+

prisoner/play.m

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+%% Play a round of Prisoner Dilemma
+% Evaluate previous history and execute a strategy. 
+%
+% Returns 1 to cooperate or 0 to defect.
+% 
+% Strategies implemented:
+%
+% * 1 = always defect
+% * 2 = always cooperate
+% * 3 = Tit-for-tat
+% * 4 = GRIM (defect forever if opponent ever defects)
+% * Any other input implements RANDOM- 50% chance of either.
+
+function x=play(player,opp,strategy) 
+
+if (strategy == 1) %always defect
+    x=0;
+
+elseif (strategy == 2) %always cooperate
+    x=1;
+
+elseif(strategy==3) %titfortat
+    if(isempty(opp))
+        x=1; % Be nice first
+    else
+        x=opp(length(opp)); % Repeat last opponent's move
+    end
+
+elseif(strategy==4) %GRIM
+    if(isempty(opp)) % Be nice first
+        x=1;
+    else
+        if(sum(opp)<length(opp)) % After one defect, always defect
+            x=0;
+        else
+            x=1;
+        end
+    end
+
+else % random
+    s=rand(1,1);
+
+    if (strategy==5)
+        threshold = 0.5;
+    else
+        threshold = 0.9;
+    end
+
+    if(s<threshold)
+        x=0;
+    else
+        x=1;
+    end
+end

prisoner/tournament.m

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+%% Runs an IPD (Iterated Prisoner Dilemma) tournament
+% Players with different strategies iteratedtly face each others and decide
+% to cooperate or defect.
+
+% Strategies are based on the previous history of the game
+% The right number of random players can enrich the dynamics of the game
+
+fprintf('\nStart the tournament...\n');
+R=200; % number of rounds
+
+%% PLAYERS & STRATEGIES
+% Set up 4 players and assign them different strategies
+% 1 always defect
+% 2 always cooperate
+% 3 Tit-for-Tat
+% 4 GRIM
+% 5 Random 0.5 defect
+% 6 Random 0.9 defect
+Q=[3,4,5,6]
+n=length(Q);
+
+%% MATRIX PAYOFF
+% Set up the payoff matrix according to Axelrod (1984) The Evolution of
+% Cooperation.
+% Notice: 3+3 > 5+0
+
+bothDefect = 1;
+bothCooperate = 3;
+temptationToDefect = 5;
+suckerPayoff = 0;
+
+payoffMatrix = [bothDefect,suckerPayoff;
+                temptationToDefect,bothCooperate]
+
+%% START THE TOURNAMENT
+%
+Z=zeros(n,n); % tournament table
+
+for i=1:n
+    for j=1:n
+        [hP1,hP2,scoreP1,scoreP2]=iteratedpd(R,Q(i),Q(j),payoffMatrix);
+        Z(i,j)=scoreP1; 
+    end
+end
+
+
+%% PRINT RESULTS
+%
+display(Z);
+scores = zeros(n,1);
+for i=1:n
+    scores(i)=sum(Z(i,1:n));
+end
+display(scores);
+
+imagesc(Z);
+
+% Who wins?
+% Why?
+