The AdSwitch policy for non-stationary bandits, from [[“Adaptively Tracking the Best Arm with an Unknown Number of Distribution Changes”. Peter Auer, Pratik Gajane and Ronald Ortner]](https://ewrl.files.wordpress.com/2018/09/ewrl_14_2018_paper_28.pdf)

• It uses an additional $$\mathcal{O}(\tau_\max)$$ memory for a game of maximum stationary length $$\tau_\max$$.

Warning

This implementation is still experimental!

class Policies.AdSwitch.Phase

Bases: enum.Enum

Different phases during the AdSwitch algorithm

Checking = 2
Estimation = 1
Exploitation = 3
__module__ = 'Policies.AdSwitch'
Policies.AdSwitch.mymean(x)[source]

Simply numpy.mean() on x if x is non empty, otherwise 0.0.

>>> np.mean([])
/usr/local/lib/python3.6/dist-packages/numpy/core/fromnumeric.py:2957: RuntimeWarning: Mean of empty slice.

Policies.AdSwitch.Constant_C1 = 1.0

Default value for the constant $$C_1$$. Should be $$>0$$ and as large as possible, but not too large.

Policies.AdSwitch.Constant_C2 = 1.0

Default value for the constant $$C_2$$. Should be $$>0$$ and as large as possible, but not too large.

class Policies.AdSwitch.AdSwitch(nbArms, horizon=None, C1=1.0, C2=1.0, *args, **kwargs)[source]

The AdSwitch policy for non-stationary bandits, from [[“Adaptively Tracking the Best Arm with an Unknown Number of Distribution Changes”. Peter Auer, Pratik Gajane and Ronald Ortner]](https://ewrl.files.wordpress.com/2018/09/ewrl_14_2018_paper_28.pdf)

__init__(nbArms, horizon=None, C1=1.0, C2=1.0, *args, **kwargs)[source]

New policy.

horizon = None

Parameter $$T$$ for the AdSwitch algorithm, the horizon of the experiment. TODO try to use DoublingTrickWrapper to remove the dependency in $$T$$ ?

C1 = None

Parameter $$C_1$$ for the AdSwitch algorithm.

C2 = None

Parameter $$C_2$$ for the AdSwitch algorithm.

phase = None

Current phase, exploration or exploitation.

current_exploration_arm = None

Currently explored arm. It cycles uniformly, in step 2.

current_exploitation_arm = None

Currently exploited arm. It is $$\overline{a_k}$$ in the algorithm.

batch_number = None

Number of batch

last_restart_time = None

Time step of the last restart (beginning of phase of Estimation)

length_of_current_phase = None

Length of the current tests phase, computed as $$s_i$$, with compute_di_pi_si().

step_of_current_phase = None

Timer inside the current phase.

current_best_arm = None

Current best arm, when finishing step 3. Denote $$\overline{a_k}$$ in the algorithm.

current_worst_arm = None

Current worst arm, when finishing step 3. Denote $$\underline{a_k}$$ in the algorithm.

current_estimated_gap = None

Gap between the current best and worst arms, ie largest gap, when finishing step 3. Denote $$\widehat{\Delta_k}$$ in the algorithm.

last_used_di_pi_si = None

Memory of the currently used $$(d_i, p_i, s_i)$$.

all_rewards = None

Memory of all the rewards. A dictionary per arm, mapping time to rewards. Growing size until restart of that arm!

__str__()[source]

-> str

startGame()[source]

Start the game (fill pulls and rewards with 0).

getReward(arm, reward)[source]

Get a reward from an arm.

read_range_of_rewards(arm, start, end)[source]

Read the all_rewards attribute to extract all the rewards for that arm, obtained between time start (included) and end (not included).

statistical_test(t, t0)[source]

Test if at time $$t$$ there is a $$\sigma$$, $$t_0 \leq \sigma < t$$, and a pair of arms $$a,b$$, satisfying this test:

$| \hat{\mu_a}[\sigma,t] - \hat{\mu_b}[\sigma,t] | > \sqrt{\frac{C_1 \log T}{t - \sigma}}.$

where $$\hat{\mu_a}[t_1,t_2]$$ is the empirical mean for arm $$a$$ for samples obtained from times $$t \in [t_1,t_2)$$.

• Return True, sigma if the test was satisfied, and the smallest $$\sigma$$ that was satisfying the test, or False, None otherwise.
find_Ik()[source]

Follow the algorithm and, with a gap estimate $$\widehat{\Delta_k}$$, find $$I_k = \max\{ i : d_i \geq \widehat{\Delta_k} \}$$, where $$d_i := 2^{-i}$$. There is no need to do an exhaustive search:

$I_k := \lfloor - \log_2(\widehat{\Delta_k}) \rfloor.$
__module__ = 'Policies.AdSwitch'
compute_di_pi_si()[source]

Compute the values of $$d_i$$, $$p_{k,i}$$, $$s_i$$ according to the AdSwitch algorithm.

choice()[source]

Choose an arm following the different phase of growing lengths according to the AdSwitch algorithm.