# Tag Info

9

The concepts of on-policy vs off-policy and online vs offline are separate, but do interact to make certain combinations more feasible. When looking at this, it is worth also considering the difference between prediction and control in Reinforcement Learning (RL). Online vs Offline These concepts are not specific to RL, many learning systems can be ...

6

This expression: $|\mathcal{A}(s)|$ means $|\quad|$ the size of $\mathcal{A}(s)$ the set of actions in state $s$ or more simply the number of actions allowed in the state. This makes sense in the given formula because $\frac{\epsilon}{|\mathcal{A}(s)|}$ is then the probability of taking each exploratory action in an $\epsilon$-greedy policy. The overall ...

5

I'm using OpenAI's cartpole environment. First of all, is this environment not Markov? The OpenAI Gym CartPole environment is Markov. Whether or not you know the transition probabilities does not affect whether the state has the Markov property. All that matters is that knowing the current state is enough to be determine the next state and reward in ...

4

In this case, $\pi$ has always been an $\epsilon$-greedy policy. In every iteration, this $\pi$ is used to generate ($\epsilon$-greedily) a trajectory from which the new $Q(s, a)$ values are calculated. The last line in the "pseudocode" tells you that the policy $\pi$ will be a new $\epsilon$-greedy policy in the next iteration. Since the policy ...

4

The twist here is that the $a_{t+1}$ in (11) and the $\mu(s_{t+1})$ in (16) are the same and actually the $a_t$ in the on-policy case and the $a_t$ in the off-policy case are different. The key to the understanding is that in on-policy algorithms you have to use actions (and generally speaking trajectories) generated by the policy in the updating steps (to ...

4

I am wondering how to generate datasets when the environment is not as simple as a tic-tac-toe or a maze problem There is no difference in concept, which is why tic-tac-toe and maze problems are used to teach. As you have noted, the main difference between reinforcement learning (RL) and supervised learning is that RL does not use labeled datasets. If you ...

4

This post contains many answers that describe the difference between on-policy vs. off-policy. Your book may be referring to how the current (DQN-based) state-of-the-art (SOTA) algorithms, such as Ape-X, R2D2, Agent57 are technically "off-policy", since they use a (very large!) replay buffer, often filled in a distributed manner. This has a number ...

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There is one thing I don't particularly understand. Why do we need the state-transition probability function when calculating the importance sampling ratio for off-policy prediction? It is not needed for calculation. It must be included in the theory, to compare the correct probability of each trajectory (on-policy vs off-policy). However, the state ...

2

DDPG is an off-policy algorithm simply because of the objective taking expectation with respect to some other distribution that we are not learning about, i.e. the deterministic policy gradient can be expressed as \nabla _{\theta^\mu} J \approx \mathbb{E}_{s_t \sim \rho^\beta} \left[ \nabla _{\theta^\mu} Q(s,a|\theta^Q) | s=s_t, a=\mu(s_t ; \theta ^\mu) \...

2

Expected SARSA can be used either on-policy or off-policy. The policy that you use in the update step determines which it is. If the update step uses a different weighting for action choices than the policy that actually took the action, then you are using Expected SARSA in an off-policy way. Q-learning is a special case of Expected SARSA, where the target ...

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You cannot really do that because you have no way of knowing how good the action really is to make reasonable labels for supervised learning (that's the whole point why we need reinforcement learning). The only way to possibly know that is to make labels based on the return that you got from that action but the return is based on an old trajectory with the ...

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First, some preliminary questions: in this case, what is the optimal policy? It is the policy that maximises return from any given time step $G_t$. You need to be careful with your definition of return with continuing environments. The simple expected sum of future rewards is likely to be positive or negative infinity. There are three basic approaches: ...

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In the DRL nanodegree in Udacity, the instructor says it is possible to combine on- and off-policy learning and suggests the following paper where this has been done: Q-Prop: Sample-Efficient Policy Gradient with An Off-Policy Critic (ICLR 2017). Citing the paper: The core idea is to use the first-order Taylor expansion of the critic as a control ...

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Apparently there is an example of non-convergence for semi-gradient sarsa, according to Rich Sutton (check slide 35). I guess TD(0) is not so different. So, probably your approximator will need to satisfy certain conditions to proof convergence. Maybe this paper will be useful for you. It seems that they show that constraining your network to have relu ...

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It's because, in the actor-critic algorithm, the objective function is an expectation under the $\tau$ of the policy. If we want to use off-policy data, we have to resort to importance sampling relative to the other policy.

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