Evolutionary Reinforcement Learning Systems Presented by Alp Sardağ

Goal Two main branches of reinforcement learning: –Search the space of functions that asses values to utility of states. –Search the space of functions that asses values to behaviors (Q-learning). Describe the evolutionary algorithm approach to reinforcement learning and compare and contrast with more common TD methods

Sequential Decision Task This is an example of sequential decision task, the optimal sequence For an agent that starts from a1 is R,D,R,D,D,R,R,D

Learning from reinforcements The EA and TD methods have similarities: –Solve diffucult sequential decision tasks. –These methods are normally model-free, whereas dynamic programming approaches learn from a complete mathematical model of the underlying system.

Supervised vs. Unsupervised Learning In supervised learning the agent receives the correct decisions paired with specific sensory input. In reinforcement learning, the agent does not learn from examples of correct behavior but uses system payoffs as aguide to form effective decision policies. –A decision making may not receive feedback after every decision. –They are applicable in problems where significant domain knowledge is either unavailable or costly to obtain.

TD Reinforcement Learning Q-learning update scheme: Q(x,a)  Q(x,a)+  (R(i)+maxQ(y,b)-Q(x,a)) After a long-run: Q-values will converge to the optimal values. A reinforcement learning system can thus use the Q values to evaluate each action that is possible from a given state.

Evolutionary Algorithm Evolutionary algorithms are global search techniques. They are built on Darwin’s theory of evolution by natural selection. Numerous potential solutions are encoded in structures, called chromosomes. During each iteration, the EA evaluates solutions adn generates offspring based on the fitness of each solution in the task. Substructures, or genes, of the solutions are then modified through genetic operators such as mutation or recombination. The idea: structures that led to good solutions in previous evaluations can be mutated or combined to form even better solutions.

Basic Steps in Evolutionary Algorithm Procedure EA begin t=0; initialize P(t); evaluate structures in P(t) while termination condition not satisfied do begin t=t+1; select P(t) form P(t-1); alter structures in P(t); evaluate structures in P(t); end end.

Similarities EA and RL EA method can learn “online” through the direct interaction with the underlying system They are adaptive to changes. Online learning is often time-consuming and dangerous. Teherfore researchers in EA and TD learning train in simulation then apply the learned control policy to real system.

Differences between TD and EA approach This can be summarized along three dimensions: –Policy representation –Credit assignment –Memory

Policy Representation What they represent: –TD methods form q(x,a)  v. –EA methods uses direct mapping from state to recommended action (e.G. Q(x)  a). Conclusion: TD try to solve a more diffucult problem than reinforcement posed. In addition to choosing best action, it tells us why.

The size of hypothesis space The hypothesis space for direct policy representation (e.g. Q(x)  a) –The total number of functions: c n where n: number of states & c: number of actions The hypothesis space for a value function representation (e.g. Q(x,a)  v) –The total number of functions: w cn where w: the number of possible values NOTE: the size of hypothesis space does not reflect the difficulty of the problem northe efficiency of the methods that search the space.

Credit Assignment TD reinforcement learning: Credit is chained backward. In this manner, payoffs are distributed across sequence of actions. A single reward value becomes associated with each individual state and decision pair. EA reinforcement learning: rewards are only associated with sequences of decisions. Thus, which individual decisions are most responsible for a good or poor decision policy is irrelevant to the EA.

Issue in Credit Assignment In TD, the update is focused on a single state and action. In EA, after a recombination the evaluation is on the entire sequence of actions.

Memory TD reinforcement learning: maintain statistics concerning every state-action pairs. This method sustain information about both good and bad state-action pairs. EA reinforcement learning: maintain information only about good states-action pairs. Thus, memory losses occur.

Example State loss can also occur in EA reinforcement learning. Unlike TD the information content decreases in EA, population actually decreases during learning.

Issues in RL Exploration vs. Exploitation Perceptual Aliasing Generalization Unstable Environments

Exploration vs. Exploitation for TD Too much exploration could result in lower average rewards and too little could prevent the learning system from discovering new optimal states. –Solution 1: –Solution 2: U(i)  (1-  )U(i)+  (R(i) + max a F(  j U(j),N(a,i))) where F(u,n) = { R + if n<N e uotherwise

Exploration vs. Exploitation for EA Too much exploration could result in lower average rewards and too little could prevent the learning system from discovering new optimal states. –Solution comes from the nature of EA. Early in an evolutionary search, selection pressure is low because most policies within the population have lower fitness. As the evolution proceeds, highly fit policy evolve, increasing the selective pressure within in the population.

Perceptual Aliasing or Hidden State In real world situations, the agent often will not have access to complete information on the state of its world. –TD methods are vulnerable to hidden states. Ambiguous state information misleads the TD method. –Since EA methods associate values with entire decision sequences, credits are based on net results.

Example In TD reinforcement learning:

Generalization The generalization of policy desicions from one area of the state space to another. Since the number of possible states of the world grows exponentially with the size of the task. Solution is to apply action decisions from observed states to unobserved states. (ANN, rule bases)

Comparison of EA and TD Generalization of TD: In some large-scale problems, approximating the value function works well, while in some simple toy problems it fails. In discrete table one update affects one state, whereas in generalized case more than one state and create noisiness. (who cares toy problems?) Generalization of EA: Since it make less frequent update and base them on more global informatio, it is more diffucult for single observation to affect the global decision problem.

Unstable Environments The agent must adapt its current decision policy in response to changes that occur in its world.(e.g. Faulty sensors, new obstructions, etc.) –Because TD makes constant updates to decision polic, it should respond to changes as soon as they occur. –Since EA do not update any policy until an individual or a population of individuals have been completely evaluated over several actions, the response to changes delayed.

Evolutionary Reinforcement Learning Implementations Learning Classifier Systems SAMUEL GENITOR SANE

Learning Classifier Systems Mesaages trigger Classifiers which are symbolic if-then rules that map sensory input to action Classifiers are in a competition, resolved by a bidding algorithm. Classifiers messages may trigger new classifiers. Predate TD learning. EA selects, mutates, and recombines classifiers that received the most credit by bidding algorithm. The result was dissaponting.

SAMUEL A system that learns to solve sequential decision problems. SAMUEL searches the space of decision policies for stes of condition- action rules. Each individual is a rule set that specifies its behavior. Each gene is a rule that maps states to actions Three major components: –Problem specific module: task environment simulation –Performance module: interacts with the world, obtain payoffs –Learning module: uses GA to evolve behaviors.

GENITOR GENITOR uses neural networks to represent the decision policy, meaning generalization the state space. GENITOR relies solely on its EA to adjust the weights in NN. NN (individual) is represented in the population as a sequence of connection weights. The croosover is realized on the basis that the offspring performance. Genetic operators are applied aynchronously.

SANE SANE (symbiotic, Adaptive Neuro-Evolution) was designed as a fast, efficient method fro building NN in domains where it is not possible to generate trainning data. NN forms a direct mapping from sensors to actions and provides effective generalization over the state space. Individuals are complete NNs. Two separate population: –Population of neurons : population of building blocks of NN. –Population of network blueprints : population of combination of building blocks of NN.

SANE

Conclusion EA and TD learn through interactions with the actual system. EA and TD do not require precise mathematical model of the domain. The main differences: –Policy representation –Credit assignment –Memory