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Hints and pseudo code for Exercise 6.6.3 (Axelrod, 1980)

  1. Implement the following five strategies:

    • always cooperate
    • always defect
    • random: cooperate with probability 12 , and defect otherwise
    • tit for tat: cooperate on the first turn, then do whatever the other player did in the previous turn
    • tit for two tat: start by cooperating, and defect only if the other player has defected twice in a row

    Each strategy should be a function, accepting as input a list storing the previous turns of the game, and returning 1 for cooperate and 0 for defect.

    Hints: note that some strategies are deterministic (e.g., always cooperate, tit for tat), while some are random; moreover, some use information on what the other player played last, and as such we need to pass the moves of the other player to the function.

    Pseudocode:

    def name_of_the_strategy(previous_moves_other_player):
        if given other player's last move we should cooperate:
           return 1
        else:
           return 0
  2. Write a function that accepts the names of two strategies and plays them against each other in a game of iterated prisoner's dilemma for a given number of turns. Who wins between random and always_defect? And between random and tit_for_tat?

    Hints: this is how you can match the name of the function directly:

    Pseudocode:

    def play_strategies(strategy_1, strategy_2, nsteps = 200):
        pl1 = globals()[strategy_1]
        pl2 = globals()[strategy_2]
        # We create two vectors to store the moves of the players
        steps_pl1 = []
        steps_pl2 = []
        # and two variables for keeping the scores. 
        # (because we said these are numbers of years in prison, we 
        # use negative payoffs, with less negative being better)
        points_pl1 = 0
        points_pl2 = 0
        # Iterate over the number of steps
        for i in range(nsteps):
        	# decide strategy for player 1 and 2
            # calculate payoff
    	if both cooperate: -1 point each
            # if pl2 loses: -3 to pl2, 0 to pl1
            # if pl1 lose: -3 to pl1, 0 to pl2
            # if both defect: -2 points each
            # add the moves to the history
        # return the final scores
        return((points_pl1, points_pl2))
  3. Implement a round-robin tournament in which each strategy is played against every other (including against itself) for 10 rounds of 1000 turns each. Who is the winner?

    Hints: for this, you will need to cycle through each pair of strategies. Create a list of strategy names, and then create two nested for loops: the first goes through all strategies, the second only from the current onward.

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