學習目標
- 理解策略評估(Policy Evaluation)和策略提升(Policy Improvement);
- 理解策略迭代(Policy Iteration)算法;
- 理解值迭代(Value Iteration)算法;
- 理解策略迭代和值迭代的不同之處;
- 動態規劃方法的侷限性;
- Python實現格子世界(Gridworld)策略迭代和值迭代。
動態規劃(Dynamic Programming, DP)是一種解決複雜問題的方法,它通過定義問題狀態和狀態之間的關係,將複雜問題拆分成若干較爲簡單的子問題,使得問題能夠以遞推(或者說分治)的方式去解決。所以要能使用動態規劃,這種問題一要能夠分解成許多子問題,二要這些子問題能夠多次被迭代使用。而馬爾科夫決策過程就正好滿足了這兩個條件,MDPs可以看成是各個狀態之間的轉移,而貝爾曼方程則將這個問題分解成了一個個狀態的遞歸求解問題,而值函數就用於存儲這個求解的結果,得到每一個狀態的最優策略,合在一起以後就完成了整個MDPs的求解。但是DP的使用時建立在我們知道MDP環境的模型的基礎上的,所以也稱其爲model based method。
策略評估(Policy Evaluation)
策略評估如其字面意思,就是評價一個策略好不好。計算任意一個策略 的狀態值函數 即可,這也叫做預測(Prediction),上一篇文章已經通過backup圖得到了 的求解公式,如下:
那這個式子怎麼算呢?狀態 的值函數我也不知道啊。這裏我們會使用高斯-賽德爾迭代算法來求解,先人爲給一個初值,再根據下面的式子迭代求解,可以證明,當k趨於無窮時,最後是會收斂到 的。
策略提升(Policy Improvement)
我們已經知道怎麼去評價一個策略好不好,那接下來就要找到那個最好的策略。每到一個狀態,我們可能就會想是不是需要改變一下策略,這樣也許能使回報更大,即選擇一個動作 ,然後再繼續遵循 ,這種方式的值就是動作值函數(還記得在上一篇中提出那個思考嗎,這裏就是一個比較好的回答):
我們用一種貪婪的方式來提升我們策略,即選擇那個能使動作值函數最大的動作:
可以證明,改變了策略 以後,狀態值函數也變大了,即 ,具體證明參見學習資料。
策略迭代(Policy Iteration)
說完了策略評估和策略提升,策略迭代就簡單了,就是反覆使用策略評估和策略提升,最後會收斂到最優策略。
其僞代碼如圖所示
Sutton的書中給了一個Gridworld的例子,如下圖所示,具體規則我就不翻譯了,大致就是說最上角和右下角是終點(終止狀態),每一步的reward都是-1,最終目的是要找到一個最優策略。
我們現在就用這個例子來用Python實現策略迭代。
import numpy as np
from lib.envs.gridworld import GridworldEnv
def policy_eval(policy, env, discount_factor=1.0, theta=0.00001):
"""
Evaluate a policy given an environment and a full description of the environment's dynamics.
Args:
policy: [S, A] shaped matrix representing the policy.
env: OpenAI env. env.P represents the transition probabilities of the environment.
env.P[s][a] is a list of transition tuples (prob, next_state, reward, done).
env.nS is a number of states in the environment.
env.nA is a number of actions in the environment.
theta: We stop evaluation once our value function change is less than theta for all states.
discount_factor: Gamma discount factor.
Returns:
Vector of length env.nS representing the value function.
"""
# Start with a random (all 0) value function
V = np.zeros(env.nS)
while True:
delta = 0
# For each state, perform a "full backup"
for s in range(env.nS):
v = 0
# Look at the possible next actions
for a, action_prob in enumerate(policy[s]):
# For each action, look at the possible next states...
for prob, next_state, reward, done in env.P[s][a]:
# Calculate the expected value
v += action_prob * prob * (reward + discount_factor * V[next_state])
# How much our value function changed (across any states)
delta = max(delta, np.abs(v - V[s]))
V[s] = v
# Stop evaluating once our value function change is below a threshold
if delta < theta:
break
return np.array(V)
def policy_improvement(env, policy_eval_fn=policy_eval, discount_factor=1.0):
"""
Policy Improvement Algorithm. Iteratively evaluates and improves a policy
until an optimal policy is found.
Args:
env: The OpenAI envrionment.
policy_eval_fn: Policy Evaluation function that takes 3 arguments:
policy, env, discount_factor.
discount_factor: gamma discount factor.
Returns:
A tuple (policy, V).
policy is the optimal policy, a matrix of shape [S, A] where each state s
contains a valid probability distribution over actions.
V is the value function for the optimal policy.
"""
def one_step_lookahead(state, V):
"""
Helper function to calculate the value for all action in a given state.
Args:
state: The state to consider (int)
V: The value to use as an estimator, Vector of length env.nS
Returns:
A vector of length env.nA containing the expected value of each action.
"""
A = np.zeros(env.nA)
for a in range(env.nA):
for prob, next_state, reward, done in env.P[state][a]:
A[a] += prob * (reward + discount_factor * V[next_state])
return A
# Start with a random policy
policy = np.ones([env.nS, env.nA]) / env.nA
while True:
# Evaluate the current policy
V = policy_eval_fn(policy, env, discount_factor)
# Will be set to false if we make any changes to the policy
policy_stable = True
# For each state...
for s in range(env.nS):
# The best action we would take under the currect policy
chosen_a = np.argmax(policy[s])
# Find the best action by one-step lookahead
# Ties are resolved arbitarily
action_values = one_step_lookahead(s, V)
best_a = np.argmax(action_values)
# Greedily update the policy
if chosen_a != best_a:
policy_stable = False
policy[s] = np.eye(env.nA)[best_a]
# If the policy is stable we've found an optimal policy. Return it
if policy_stable:
return policy, V
env = GridworldEnv()
random_policy = np.ones([env.nS, env.nA]) / env.nA
v = policy_eval(random_policy, env)
policy, v = policy_improvement(env)
print("Policy Probability Distribution:")
print(policy)
print("")
print("Reshaped Grid Policy (0=up, 1=right, 2=down, 3=left):")
print(np.reshape(np.argmax(policy, axis=1), env.shape))
print("")
print("Value Function:")
print(v)
print("")
print("Reshaped Grid Value Function:")
print(v.reshape(env.shape))
print("")
得到如下結果:
可以看出,這和書上得到的最優策略時一致的。
值迭代(Value Iteration)
策略迭代有一個缺點,就是每一步都要進行策略評估,當狀態空間很大的時候是非常耗費時間的。值迭代是直接將貝爾曼最優化方程拿來迭代計算的,這一點是不同於策略迭代的,我們直接對比兩者的僞代碼。
所以值迭代會直接收斂到最優值,從而我們就可以得到最優策略,因爲它就是一個貪婪的選擇。再反過去看一下策略迭代的過程,策略評估過程是應用貝爾曼方程來計算當前最優策略下的值函數,接着進行策略提升,即在每個狀態都選擇一個最優動作來最大化值函數,以改進策略。但是想一下,在策略評估過程我們一定要等到它收斂到準確的值函數嗎?答案是不一定,我們可以設定一個誤差,中斷這個過程,用一個近似的值函數用以策略提升(格子世界的例子中就可以看出,在迭代到第三步以後,其實最優策略就已經確定了),而我們提出這個方法的時候並不是這麼做的,而是等到策略評價過程收斂,這是一個極端的選擇,相當於在迭代貝爾曼最優化方程!所以,換句話說,值迭代其實可以看成是策略迭代一個極端情況。
一般來說,策略迭代的收斂速度更快一些,在狀態空間較小時,最好選用策略迭代方法。當狀態空間較大時,值迭代的計算量更小一些。
同樣,還是以格子世界爲例,用Python實現一遍值迭代算法:
import numpy as np
from lib.envs.gridworld import GridworldEnv
def value_iteration(env, theta=0.0001, discount_factor=1.0):
"""
Value Iteration Algorithm.
Args:
env: OpenAI env. env.P represents the transition probabilities of the environment.
env.P[s][a] is a list of transition tuples (prob, next_state, reward, done).
env.nS is a number of states in the environment.
env.nA is a number of actions in the environment.
theta: We stop evaluation once our value function change is less than theta for all states.
discount_factor: Gamma discount factor.
Returns:
A tuple (policy, V) of the optimal policy and the optimal value function.
"""
def one_step_lookahead(state, V):
"""
Helper function to calculate the value for all action in a given state.
Args:
state: The state to consider (int)
V: The value to use as an estimator, Vector of length env.nS
Returns:
A vector of length env.nA containing the expected value of each action.
"""
A = np.zeros(env.nA)
for a in range(env.nA):
for prob, next_state, reward, done in env.P[state][a]:
A[a] += prob * (reward + discount_factor * V[next_state])
return A
V = np.zeros(env.nS)
while True:
# Stopping condition
delta = 0
# Update each state...
for s in range(env.nS):
# Do a one-step lookahead to find the best action
A = one_step_lookahead(s, V)
best_action_value = np.max(A)
# Calculate delta across all states seen so far
delta = max(delta, np.abs(best_action_value - V[s]))
# Update the value function
V[s] = best_action_value
# Check if we can stop
if delta < theta:
break
# Create a deterministic policy using the optimal value function
policy = np.zeros([env.nS, env.nA])
for s in range(env.nS):
# One step lookahead to find the best action for this state
A = one_step_lookahead(s, V)
best_action = np.argmax(A)
# Always take the best action
policy[s, best_action] = 1.0
return policy, V
env = GridworldEnv()
policy, v = value_iteration(env)
print("Policy Probability Distribution:")
print(policy)
print("")
print("Reshaped Grid Policy (0=up, 1=right, 2=down, 3=left):")
print(np.reshape(np.argmax(policy, axis=1), env.shape))
print("")
print("Value Function:")
print(v)
print("")
print("Reshaped Grid Value Function:")
print(v.reshape(env.shape))
print("")
輸出結果與策略迭代一致。
參考
[1] Reinforcement Learning: An Introduction- Chapter 4: Dynamic Programming
[2] David Silver’s RL Course Lecture 3 - Planning by Dynamic Programming(video, slides)
[3] Quora: https://www.quora.com/How-is-policy-iteration-different-from-value-iteration by Sergio Valcarcel Macua
[4] 策略迭代與值迭代的區別
[5] github開源代碼
《Reinforcement Learning: An Introduction 第二版》PDF書籍與David Silver課程,歡迎關注我的公衆號“野風同學”,回覆“RL”即可獲取。
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