Reinforcement Learning & Apprenticeship Learning

1. Reinforcement Learning & Apprenticeship Learning Chenyi Chen

2. Markov Decision Process (MDP) • What’s MDP? • A sequential decision problem • Fully observable, stochastic environment • Markovian transition model: the nth state is only determined by (n-1)th state and (n-1)thaction • Each state has a reward, and the reward is additive

3. Markov Decision Process (MDP) • State s: a representation of current environment;

4. Markov Decision Process (MDP) • Example: Tom and Jerry, control Jerry (Jerry’s perspective) • State: the position of Tom and Jerry, 25*25=625 in total; One of the states One of the states

5. Markov Decision Process (MDP) • State s: a representation of current environment; • Action a: the action can be taken by the agent in state s;

6. Markov Decision Process (MDP) • Example: Tom and Jerry, control Jerry (Jerry’s perspective) • State: the position of Tom and Jerry, 25*25=625 in total; • Action: both can move to the neighboring 8 squares or stay; One of the states One of the states

7. Markov Decision Process (MDP) • State s: a representation of current environment; • Action a: the action can be taken by the agent in state s; • Reward R(s): the reward of current state s (+,-,0); • Value (aka utility) of state s: different from reward, related with future optimal actions;

8. An Straightforward Example • 100 bucks if you came to class • Reward of “come to class” is 100 • You can use the money to: • Eat food (you only have 50 bucks left) • Stock market (you earn 1000 bucks, including the invested 100 bucks) • The value (utility) of “come to class” is 1000

9. Markov Decision Process (MDP) • Example: Tom and Jerry, control Jerry (Jerry’s perspective) • State: the position of Tom and Jerry, 25*25=625 in total; • Action: both can move to the neighboring 8 squares or stay; • Reward: 1) Jerry and cheese at the same square, +5; 2) Tom and Jerry at the same square, -20; 3) otherwise 0; One of the states One of the states

10. Markov Decision Process (MDP) • State s: a representation of current environment; • Action a: the action can be taken by the agent in state s; • Reward R(s): the reward of current state s (+,-,0); • Value (aka utility) of state s: different from reward , related with future optimal actions; • Transition probability P(s’|s,a):given the agent is in state s and taking action a, the probability of reaching state s’ in the next step;

11. Markov Decision Process (MDP) • Example: Tom and Jerry, control Jerry (Jerry’s perspective) • State: the position of Tom and Jerry, 25*25=625 in total; • Action: both can move to the neighboring 8 squares or stay; • Reward: 1) Jerry and cheese at the same square, +5; 2) Tom and Jerry at the same square, -20; 3) otherwise 0; • Transition probability: about Tom’s moving pattern. One of the states One of the states

12. Markov Decision Process (MDP) • Example: Tom and Jerry, control Jerry (Jerry’s perspective) …

13. Markov Decision Process (MDP) • State s: a representation of current environment; • Action a: the action can be taken by the agent in state s; • Reward R(s): the reward of current state s (+,-,0); • Value (aka utility) of state s: different from reward , related with future optimal actions; • Transition probability P(s’|s,a):given the agent is in state s and taking action a, the probability of reaching state s’ in the next step; • Policy π(s)->a: a table of state-action pairs, given state s, output action a that should be taken.

14. Bellman Equation • The expected utility of state s obtained by executing π starting in s is given by ( is a discount factor): , where • The optimal policy is given by: • Denote as , the optimal policy chooses the action that maximizes the expected utility of the subsequent state:

15. Bellman Equation • Bellman Equation: • The utility of a state is the immediate reward for that state plus the expected discounted utility of the next state, assuming that the agent choose the optimal action • with , is the unique solution to Bellman equation

16. Value Iteration initialize , as a discount factor repeat for each state sinSdo ifthen until return Bellman Equation:

17. Value Iteration • Naïve example: R(s)=3, R(s’)=5,=0.9 Initially U(s)=0, U(s’)=0 (1) U(s)=3+0.9*0=3, U(s’)=5+0.9*3=7.7 (2) U(s)=3+0.9*7.7=9.93, U(s’)=5+0.9*9.93=13.937 (3) U(s)=3+0.9*13.937=15.5433, U(s’)=5+0.9*15.5433= 18.989 … (29) U(s)=39.3738, U(s’)=40.4364 (30 ) U(s)=39.3928, U(s’)= 40.4535 • Solve the equation: U(s)=3+0.9*U(s’) U(s’)=5+0.9*U(s) the true value is: U(s)= 39.4737, U(s’)= 40.5263 Value iteration

18. Reinforcement Learning • Similar to MDPs • But we assume the environment model (transition probability P(s’|s,a) ) is unknown

19. Reinforcement Learning • How to solve it? • Solution #1: Use Monte Carlo method to sample the transition probability, then implement Value Iteration limitation: too slow for problems with many possible states because it ignores frequencies of states

20. Monte Carlo Method • A broad class of computational algorithms that rely on repeated random sampling to obtain numerical results; • Typically one runs simulations many times in order to obtain the distribution of an unknown probabilistic entity. From Wikipedia

21. Monte Carlo Example :the element is the probability P(s’|s,a) initialize tablewith all elements repeat at current state s, random choose a valid action a simulate for one step, get a new state s’ until sampled enough times return

22. Reinforcement Learning • How to solve it? • Solution #1: Use Monte Carlo method to sample the transition probability, then implement Value Iteration limitation: too slow for problems with many possible states because it ignores frequencies of states • Solution #2: Q-learning the major algorithm for reinforcement learning

23. Q-learning Bellman Equation: • Q-value is defined by: • The relationship between utility and Q-value is: • The optimal policy is given by: • Q-learning algorithm is used to learn this Q-value table

24. Q-learning Q: a table of Q-values indexed by state and action, initially zero s, a,R(s): state, action, and reward. Initial state is given by the environment, and initial action is randomly picked up γ: discount factor α: learning rate f(.): greedy function, at the beginning, Q-table is bad, so we make some random choice While not coverage run one step to obtain s’ from s and a through the environment (e.g. the game engine) ) return Q-value is defined by:

25. Playing Atari with Deep Reinforcement Learning • The Atari 2600 is a video game console released in September 1977 by Atari, Inc.  • Atari emulator: Arcade Learning Environment (ALE)

26. What did they do? • Train a deep learning convolutional neural network • Input is current state (raw image sequence) • Output is all the legal action and corresponding Q(s,a) value • Let the CNN play Atari games

27. What’s Special? • Input is raw image! • Output is the action! • Game independent, same convolutional neural network for all games • Outperform human expert players in some games

28. Problem Definition • State: • Action: possible actions in the game • Reward: score won in the Atari games (output of the emulator) • Learn the optimal policy through training

29. A Variant of Q-learning In the paper: Q-value is defined by:

30. Deep Learning Approach Approach the Q-value with a convolutional neural network Q(s,a;θ) VS Straightforward structure The structure used in the paper

31. How to Train the Convolutional Neural Network? Loss function: Where: Q-value is defined as: Do gradient descent:

32. Some Details • The distribution of action a(ε-greedy policy): choose a “best” action with probability 1-ε, and selects a random action with probability ε, ε annealed linearly from 1 to 0.1 • Input image preprocessing function φ(st) • Build a huge database to store historical samples

33. During Training… Add new data sample to database Do mini-batch gradient descent on parameter θ for one step with probability 1-ε or random action with probability ε Play the game for one step

34. CNN Training Pipeline

35. After Training… Play the game

36. Results Screen shots from five Atari 2600 games: (Left-to-right) Beam Rider, Breakout, Pong, Seaquest, Space Invaders Comparison of average total reward for various learning methods by running an ε-greedy policy with ε = 0.05 for a fixed number of steps

37. Results • The leftmost plot shows the predicted value function for a 30 frame segment of the game Seaquest. The three screenshots correspond to the frames labeled by A, B, and C respectively

38. Apprenticeship Learning via Inverse Reinforcement Learning • Teach the computer to do something by demonstration, rather than by telling it the rules or reward • Reinforcement Learning: tell computer the reward, let it learn by itself using the reward • Apprenticeship Learning: demonstrate to the computer, let it mimic the performance

39. Why Apprenticeship Learning? • For standard MDPs, a reward for each state needs to be specified • Specify a reward some time is not easy, what’s the reward for driving? • When teaching people to do something (e.g. driving), usually we prefer to demonstrate rather than tell them the reward function

40. How Does It Work? • Reward is unknown, but we assume it’s a linear function of features, is a function mapping state s to features, so:

41. Example of Feature • State st of the red car is defined as: st==1 left lane, st==2 middle lane, st==3 right lane • Feature φ(st) is defind as: [1 0 0] left lane, [0 1 0] middle lane, [0 0 1] right lane • w is defined as: w=[0.1 0.5 0.3] R(left lane)=0.1, R(middle lane)=0.5, R(right lane)=0.3 • So in this case staying in the middle lane is preferred

42. How Does It Work? • Reward is unknown, but we assume it’s a linear function of features, is a function mapping state s to features, so: • The value (utility) of policy π is: The expected utility obtained by executing π starting in s is given by: , where

43. How Does It Work? • Define feature expectation as: • Then: • Assume the expert’s demonstration defines the optimal policy: • We need to sample the expert’s feature expectation by (sample m times):

44. What Does Feature Expectation Look Like? • State st of the red car is defined as: st==1 left lane, st==2 middle lane, st==3 right lane • Feature φ(st) is defind as: [1 0 0] left lane, [0 1 0] middle lane, [0 0 1] right lane • During sampling, assume γ=0.9 Step 1, red car in middle lane μ=0.9^0*[0 1 0]=[0 1 0] Step 2, red car still in middle lane μ= [0 1 0]+0.9^1*[0 1 0]=[0 1.9 0] Step 3, red car move to left lane μ= [0 1.9 0]+0.9^2*[1 0 0]=[0.81 1.9 0] …

45. How Does It Work? • We want to mimic the expert’s performance by minimize the difference between and • If we have , and assuming Then

46. Pipeline

47. Supporting Vector Machine (SVM) • The 2nd step of the pipeline is a SVM problem Which can be rewritten as:

48. Pipeline

49. Their Testing System

50. Demo Videos http://ai.stanford.edu/~pabbeel/irl/