Include noise
=============

A common variation on iterated prisoner’s dilemma tournaments is to add
stochasticity in the choice of actions, simply called noise. This noise is
introduced by flipping plays between C and D with some probability that is
applied to all plays after they are delivered by the player [Bendor1993]_.

The presence of this persistent background noise causes some strategies to
behave substantially differently. For example, :code:`TitForTat` can fall into
defection loops with itself when there is noise. While :code:`TitForTat` would
usually cooperate well with itself::

    C C C C C ...
    C C C C C ...

Noise can cause a C to flip to a D (or vice versa), disrupting the cooperative
chain::

    C C C D C D C D D D ...
    C C C C D C D D D D ...

To create a noisy tournament you simply need to add the `noise` argument::

    >>> import axelrod as axl
    >>> players = [axl.Cooperator(), axl.Defector(),
    ...            axl.TitForTat(), axl.Grudger()]
    >>> noise = 0.1
    >>> tournament = axl.Tournament(players, noise=noise)
    >>> results = tournament.play()
    >>> plot = axl.Plot(results)
    >>> p = plot.boxplot()
    >>> p.show()

.. image:: _static/noisy_tournaments/demo_strategies_noisy_boxplot.svg
   :width: 50%
   :align: center

Here is how the distribution of wins now looks::

    >>> p = plot.winplot()
    >>> p.show()

.. image:: _static/noisy_tournaments/demo_strategies_noisy_winplot.svg
   :width: 50%
   :align: center