强化学习------Sarsa算法

简介

SARSA（State-Action-Reward-State-Action）是一个学习马尔可夫决策过程策略的算法，通常应用于机器学习和强化学习学习领域中。它由Rummery 和 Niranjan在技术论文“Modified Connectionist Q-Learning（MCQL）” 中介绍了这个算法，并且由Rich Sutton在注脚处提到了SARSA这个别名。
State-Action-Reward-State-Action这个名称清楚地反应了其学习更新函数依赖的5个值，分别是当前状态S1，当前状态选中的动作A1，获得的奖励Reward，S1状态下执行A1后取得的状态S2及S2状态下将会执行的动作A2。我们取这5个值的首字母串起来可以得出一个词SARSA。

算法的核心思想可以简化为：

用伪代码可以表示为：

算法实战

我们使用openAI的gym中的CliffWalking-v0作为环境

#!/usr/bin/env python 
# -*- coding:utf-8 -*-
import numpy as np
import gym
import time
import gridworld

#Sarsa算法
class Sarsa():

    def __init__(self,num_states,num_actions,e_greed=0.1,lr=0.9,gamma=0.8):
        #建立Q表格
        self.Q = np.zeros((num_states,num_actions))
        self.e_greed = e_greed   #探索概率
        self.num_states = num_states
        self.num_actions = num_actions
        self.lr = lr   #学习率
        self.gamma = gamma #折扣因子


    def predict(self,state):
        """
        通过当前状态预测下一个动作
        :param state:
        :return:
        """
        #获取当前状态的所有动作的切片
        Q_list = self.Q[state,:]
        #随机选取其中最大值中的某一个（防止存在多个最大值时，总是选最前面的问题）
        action = np.random.choice(np.flatnonzero(Q_list == Q_list.max()))
        return  action

    def action(self,state):
        """
        选取动作
        :param state:
        :return:
        """
        #探索,随机选择一个动作
        if np.random.uniform(0,1) < self.e_greed:
            action = np.random.choice(self.num_actions)
        else:   #直接选取最大Q值的动作
            action = self.predict(state)
        return action

    def learn(self,state,action,reward,next_state,next_action,done):
        cur_Q = self.Q[state,action]
        # 当游戏结束时，不存在next_action和next_state
        if done:
            target_Q = reward
        else:
            target_Q = reward + self.gamma*self.Q[next_state,next_action]

        self.Q[state,action] += self.lr*(target_Q - cur_Q)

#训练
def train_episode(env,agent,is_render):
    total_reward = 0
    #初始化环境
    state,_ = env.reset()
    action = agent.action(state)
    while True:
        #执行动作返回结果
        next_state,reward,done,_,_ = env.step(action)
        #根据状态获取动作
        next_action = agent.action(next_state)
        #更新参数
        agent.learn(state,action,reward,next_state,next_action,done)
        #循环执行
        action = next_action
        state = next_state
        total_reward += reward
        if is_render:
            env.render()
        if done:
            break

    return  total_reward
#测试
def test_episode(env,agent,is_render=False):
    total_reward = 0
    # 初始化环境
    state,_ = env.reset()
    while True:
        action = agent.predict(state)
        next_state, reward, done, _,_ = env.step(action)

        state = next_state
        total_reward += reward
        env.render()
        time.sleep(0.5)
        if done:
            break

    return total_reward
#训练
def train(env,episodes=500,lr=0.1,gamma=0.9,e_greed=0.1):
    agent = Sarsa(
        num_states = env.observation_space.n,
        num_actions = env.action_space.n,
        lr = lr,
        gamma = gamma,
        e_greed = e_greed
    )
    is_render = False
    #先训练episodes次
    for e in range(episodes):
        ep_reward = train_episode(env,agent,is_render)
        print('Episode %s : reward= %.1f'%(e,ep_reward))
        #每执行50轮就显示一次
        if e%50 == 0:
            is_render = True
        else:
            is_render = False
    #训练结束后，我i们测试模型
    test_reward = test_episode(env,agent)
    print('test_reward= %.1f' % (test_reward))


if __name__ == '__main__':
    env = gym.make("CliffWalking-v0")
    env = gridworld.CliffWalkingWapper(env)
    train(env)

运行效果

另附工具类

#   Copyright (c) 2020 PaddlePaddle Authors. All Rights Reserved.
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
#     http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.

# -*- coding: utf-8 -*-

import gym
import turtle
import numpy as np

# turtle tutorial : https://docs.python.org/3.3/library/turtle.html

def GridWorld(gridmap=None, is_slippery=False):
    if gridmap is None:
        gridmap = ['SFFF', 'FHFH', 'FFFH', 'HFFG']
    env = gym.make("FrozenLake-v0", desc=gridmap, is_slippery=False)
    env = FrozenLakeWapper(env)
    return env


class FrozenLakeWapper(gym.Wrapper):
    def __init__(self, env):
        gym.Wrapper.__init__(self, env)
        self.max_y = env.desc.shape[0]
        self.max_x = env.desc.shape[1]
        self.t = None
        self.unit = 50

    def draw_box(self, x, y, fillcolor='', line_color='gray'):
        self.t.up()
        self.t.goto(x * self.unit, y * self.unit)
        self.t.color(line_color)
        self.t.fillcolor(fillcolor)
        self.t.setheading(90)
        self.t.down()
        self.t.begin_fill()
        for _ in range(4):
            self.t.forward(self.unit)
            self.t.right(90)
        self.t.end_fill()

    def move_player(self, x, y):
        self.t.up()
        self.t.setheading(90)
        self.t.fillcolor('red')
        self.t.goto((x + 0.5) * self.unit, (y + 0.5) * self.unit)

    def render(self):
        if self.t == None:
            self.t = turtle.Turtle()
            self.wn = turtle.Screen()
            self.wn.setup(self.unit * self.max_x + 100,
                          self.unit * self.max_y + 100)
            self.wn.setworldcoordinates(0, 0, self.unit * self.max_x,
                                        self.unit * self.max_y)
            self.t.shape('circle')
            self.t.width(2)
            self.t.speed(0)
            self.t.color('gray')
            for i in range(self.desc.shape[0]):
                for j in range(self.desc.shape[1]):
                    x = j
                    y = self.max_y - 1 - i
                    if self.desc[i][j] == b'S':  # Start
                        self.draw_box(x, y, 'white')
                    elif self.desc[i][j] == b'F':  # Frozen ice
                        self.draw_box(x, y, 'white')
                    elif self.desc[i][j] == b'G':  # Goal
                        self.draw_box(x, y, 'yellow')
                    elif self.desc[i][j] == b'H':  # Hole
                        self.draw_box(x, y, 'black')
                    else:
                        self.draw_box(x, y, 'white')
            self.t.shape('turtle')

        x_pos = self.s % self.max_x
        y_pos = self.max_y - 1 - int(self.s / self.max_x)
        self.move_player(x_pos, y_pos)


class CliffWalkingWapper(gym.Wrapper):
    def __init__(self, env):
        gym.Wrapper.__init__(self, env)
        self.t = None
        self.unit = 50
        self.max_x = 12
        self.max_y = 4

    def draw_x_line(self, y, x0, x1, color='gray'):
        assert x1 > x0
        self.t.color(color)
        self.t.setheading(0)
        self.t.up()
        self.t.goto(x0, y)
        self.t.down()
        self.t.forward(x1 - x0)

    def draw_y_line(self, x, y0, y1, color='gray'):
        assert y1 > y0
        self.t.color(color)
        self.t.setheading(90)
        self.t.up()
        self.t.goto(x, y0)
        self.t.down()
        self.t.forward(y1 - y0)

    def draw_box(self, x, y, fillcolor='', line_color='gray'):
        self.t.up()
        self.t.goto(x * self.unit, y * self.unit)
        self.t.color(line_color)
        self.t.fillcolor(fillcolor)
        self.t.setheading(90)
        self.t.down()
        self.t.begin_fill()
        for i in range(4):
            self.t.forward(self.unit)
            self.t.right(90)
        self.t.end_fill()

    def move_player(self, x, y):
        self.t.up()
        self.t.setheading(90)
        self.t.fillcolor('red')
        self.t.goto((x + 0.5) * self.unit, (y + 0.5) * self.unit)

    def render(self):
        if self.t == None:
            self.t = turtle.Turtle()
            self.wn = turtle.Screen()
            self.wn.setup(self.unit * self.max_x + 100,
                          self.unit * self.max_y + 100)
            self.wn.setworldcoordinates(0, 0, self.unit * self.max_x,
                                        self.unit * self.max_y)
            self.t.shape('circle')
            self.t.width(2)
            self.t.speed(0)
            self.t.color('gray')
            for _ in range(2):
                self.t.forward(self.max_x * self.unit)
                self.t.left(90)
                self.t.forward(self.max_y * self.unit)
                self.t.left(90)
            for i in range(1, self.max_y):
                self.draw_x_line(
                    y=i * self.unit, x0=0, x1=self.max_x * self.unit)
            for i in range(1, self.max_x):
                self.draw_y_line(
                    x=i * self.unit, y0=0, y1=self.max_y * self.unit)

            for i in range(1, self.max_x - 1):
                self.draw_box(i, 0, 'black')
            self.draw_box(self.max_x - 1, 0, 'yellow')
            self.t.shape('turtle')

        x_pos = self.s % self.max_x
        y_pos = self.max_y - 1 - int(self.s / self.max_x)
        self.move_player(x_pos, y_pos)


if __name__ == '__main__':
    # 环境1：FrozenLake, 可以配置冰面是否是滑的
    # 0 left, 1 down, 2 right, 3 up
    env = gym.make("FrozenLake-v0", is_slippery=False)
    env = FrozenLakeWapper(env)

    # 环境2：CliffWalking, 悬崖环境
    # env = gym.make("CliffWalking-v0")  # 0 up, 1 right, 2 down, 3 left
    # env = CliffWalkingWapper(env)

    # 环境3：自定义格子世界，可以配置地图, S为出发点Start, F为平地Floor, H为洞Hole, G为出口目标Goal
    # gridmap = [
    #         'SFFF',
    #         'FHFF',
    #         'FFFF',
    #         'HFGF' ]
    # env = GridWorld(gridmap)

    env.reset()
    for step in range(10):
        action = np.random.randint(0, 4)
        obs, reward, done, info = env.step(action)
        print('step {}: action {}, obs {}, reward {}, done {}, info {}'.format(\
                step, action, obs, reward, done, info))
        env.render()  # 渲染一帧图像