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%matplotlib inline

import gym
import itertools
import matplotlib
import numpy as np
import pandas as pd
import sys

if "../" not in sys.path:
sys.path.append("../")

from collections import defaultdict
from lib.envs.windy_gridworld import WindyGridworldEnv
from lib import plotting

matplotlib.style.use('ggplot')

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env = WindyGridworldEnv()

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def make_epsilon_greedy_policy(Q, epsilon, nA):
"""
Creates an epsilon-greedy policy based on a given Q-function and epsilon.

Args:
Q: A dictionary that maps from state -> action-values.
Each value is a numpy array of length nA (see below)
epsilon: The probability to select a random action . float between 0 and 1.
nA: Number of actions in the environment.

Returns:
A function that takes the observation as an argument and returns
the probabilities for each action in the form of a numpy array of length nA.

"""
def policy_fn(observation):
A = np.ones(nA, dtype=float) * epsilon / nA
best_action = np.argmax(Q[observation])
A[best_action] += (1.0 - epsilon)
return A
return policy_fn

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def sarsa(env, num_episodes, discount_factor=1.0, alpha=0.5, epsilon=0.1):
"""
SARSA algorithm: On-policy TD control. Finds the optimal epsilon-greedy policy.

Args:
env: OpenAI environment.
num_episodes: Number of episodes to run for.
discount_factor: Lambda time discount factor.
alpha: TD learning rate.
epsilon: Chance the sample a random action. Float betwen 0 and 1.

Returns:
A tuple (Q, stats).
Q is the optimal action-value function, a dictionary mapping state -> action values.
stats is an EpisodeStats object with two numpy arrays for episode_lengths and episode_rewards.
"""

# The final action-value function.
# A nested dictionary that maps state -> (action -> action-value).
Q = defaultdict(lambda: np.zeros(env.action_space.n))

# Keeps track of useful statistics
stats = plotting.EpisodeStats(
episode_lengths=np.zeros(num_episodes),
episode_rewards=np.zeros(num_episodes))

# The policy we're following
policy = make_epsilon_greedy_policy(Q, epsilon, env.action_space.n)

for i_episode in range(num_episodes):
# Print out which episode we're on, useful for debugging.
if (i_episode + 1) % 100 == 0:
print("\rEpisode {}/{}.".format(i_episode + 1, num_episodes), end="")
sys.stdout.flush()

# Implement this!

return Q, stats

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Q, stats = sarsa(env, 200)

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Episode 200/200.

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In :

plotting.plot_episode_stats(stats)

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``````