# Tag Info

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

The Markov assumption is used when deriving the Bellman equation for state values: $$v(s) = \sum_a \pi(a|s)\sum_{r,s'} p(r,s'|s,a)(r + \gamma v(s'))$$ One requirement for this equation to hold is that $p(r,s'|s,a)$ is consistent. The current state $s$ is a key argument of that function. There is no adjustment for history of previous states, actions or ...

4

Designing reward functions Designing a reward function is sometimes straightforward, if you have knowledge of the problem. For example, consider the game of chess. You know that you have three outcomes: win (good), loss (bad), or draw (neutral). So, you could reward the agent with $+1$ if it wins the game, $-1$ if it loses, and $0$ if it draws (or for any ...

3

You are referring to catastrophic forgetting which could be an issue in any neural net. More specifically for DQN refer to this article.

3

Suppose you learned your action-value function perfectly. Recall that the action-value function measures the expected return after taking a given action in a given state. Now, the goal when solving an MDP is to find a policy that maximizes expected returns. Suppose you're in state $s$. According to your action-value function, let's say actions $a$ maximizes ...

3

In the policy gradient theorem, we don't need to write $r$ as a function of $a$ because the only time we explicitly 'see' $r$ is when we are taking the expectation with respect to the policy. For the first couple lines of the PG theorem we have \begin{align} \nabla v_\pi(s) &= \nabla \left[ \sum_a \pi(a|s) q_\pi (s,a) \right] \;, \\ &= \sum_a \left[ \...

3

If your objective is for the agent to attain some goal (say, reaching a target), then a valid reward function is to assign a reward of 1 when the goal is attained and 0 otherwise. The problem with this reward function is that it's too sparse, meaning the agent has little guidance on how to modify their behavior to become better at attaining said goal, ...

3

The true answers are 1 and 3. 1 because the required conditions for tabular Q-learning to converge is that each state action pair will be visited infinitely often, and Q-learning learns directly about the greedy policy, $\pi(a|s) := \arg \max_a Q_\pi(s,a)$, and because Q-learning converges to the optimal Q-value function we know that the policy will be ...

3

You would still be picking a single action. Your action space is now $\mathcal{A} = \mathcal{O} \times \mathcal{I}$ where I've chosen $\mathcal{O}$ to be the set of possible orders from your problem and $\mathcal{I}$ to be the set of possible items. Provided both of these sets are finite, then you should still be able to approach this problem with DQN. ...

3

I don't think people generally do use neural nets for grid world. As long as the state and action spaces are small enough, you should be able to store Q values in a table like you suggested. Neural nets come in handy when the state space is very large (or even continuous), so you can't afford to store a table of Q values. Also, neural nets have the ability ...

3

To answer your question, the specifics of some of the OpenAI Gym environments can be found on their wiki: The episode ends when you reach 0.5 position, or if 200 iterations are reached. There is a deeper question in what you asked, though: My initial understanding was that an episode should end when the Car reaches the flagpost. The environment certainly ...

2

The episode ends when either the car reaches the goal, or a maximum number of timesteps has passed. By default the episode will terminate after 200 steps. You can customize this with the _max_episode_steps attribute of the environment.

2

Why is the expected return in Reinforcement Learning (RL) computed as a sum of cumulative rewards? That is the definition of return. In fact when applying a discount factor this should formally be called discounted return, and not simply "return". Usually the same symbol is used for both ($R$ in your case, $G$ in e.g. Sutton & Barto). There ...

2

Convergence analysis is about proving that your policy and/or value function converge to some desired value, which is usually the fixed-point of an operator or an extremum. So it essentially proves that theoretically the algorithm achieves the desired function. Without convergence, we have no guarantees that the value function will be accurate or the policy ...

2

The unrolling step is due to the fact you end up with an equation that you can keep expanding indefinitely. Note that we start with calculating $\nabla v_\pi(s)$ and arrive at $$\nabla v_\pi(s) = \sum_a\left[ \nabla \pi(a|s) q_\pi(s,a) + \pi(a|s) \sum_{s'}p(s'|s,a) \nabla v_\pi (s') \right]\;,$$ which contains a term for $\nabla v_\pi(s')$. This is a ...

2

Am I correct in this understanding that with the increasing complexity of problems, tabular RL methods are getting obsolete? Individual problems don't get any more complex, but the scope of solvable environments increases due to research and discovery of better or more apt methods. Using deep RL methods with large neural nets can be a lot less efficient for ...

2

In reinforcement learning (RL), an immediate reward value must be returned after each action, alomng with the next state. This value can be zero though, which will have no direct impact on optimality or setting goals. Unless you are modifying the reward scheme to try and make an environment easier to learn (sometimes called reward shaping), then you should ...

2

Your policy gradient algorithms appear to be working as intended. All standard MDPs have one or more deterministic optimal solutions, and those are the policies that solvers will converge to. Making any of these policies more random will often reduce their effectiveness, making them sub-optimal. So once consistently good actions are discovered, the learning ...

2

Lets assume $\sup_{s,a} r(s,a)<b$. Then for continuing problems the upper bound can be obtained by \begin{align} \sum_{t=0}^{\infty} \gamma^{t}r(s_t,a_t) &\le \sum_{t=0}^{\infty} \gamma^{t} \sup_{s,a}r(s,a) \nonumber \\ &=\sum_{t=0}^{\infty} \gamma^{t} b = \frac{b}{1-\gamma}. \end{align} We can use the same bound for episodic tasks with ...

2

My answer to:Is there an upper limit to the maximum cumulative reward in a deep reinforcement learning problem? Yes but depending on the environment, if dealing with the theoretical environment, where there are infinite number of time steps. Calculating the upper bound In reinforcement learning (deep RL inclusive), we want to maximize the discounted ...

2

In any reinforcement learning problem, not just Deep RL, then there is an upper bound for the cumulative reward, provided that the problem is episodic and not continuing. If the problem is episodic and the rewards are designed such that the problem has a natural ending, i.e. the episode will end regardless of how well the agent does in the environment, then ...

2

Reinforcement Learning: An Introduction by Richard Sutton and Andrew Barto is undoubtedly one of the best books, to begin with. Despite its age, the book is still the canonical introduction to reinforcement learning. It does require some patience, but I think it's very approachable and rigorous at the same time!

2

why is it not possible to suggest a policy solely on the basis of state-values; why do we need state-action values? A policy function takes state as an argument and returns an action $a = \pi(s)$, or it may return a probability distribution over actions $\mathbf{Pr}\{A_t=a|S_t=s \} =\pi(a|s)$. In order to do this rationally, an agent needs to use the ...

2

If you have multiple types of rewards (say, R1 and R2), then it is no longer clear what would be the optimal way to act: it can happen that one way of acting would maximize R1 and another way would maximize R2. Therefore, optimal policies, value functions, etc., would all be undefined. Of course, you could say that you want to maximize, for example, R1+R2, ...

1

The difference between Vanilla Policy Gradient (VPG) with a baseline as Value function and Advantage Actor Critic (A2C) is very similar to the difference between Monte Carlo Control and SARSA: The value estimates used in updates for VPG are based on full sampled returns, calculated at the end of episodes. The value estimates used in updates for A2C are ...

1

The policy doesn't change over time. That is, the values will change, otherwise we would not be learning anything, but our rules for action selection don't. I.e. we always take action according to the distribution postulated to our current estimate of the policy $\pi_\theta(a|s)$, we don't suddenly start taking $\max_a \pi_\theta(a|s)$, which would be a true ...

1

I'm using my own implementation of A2C (Advantage Actor Critic) in an industrial application based on Markov Process (present state alone provides sufficient knowledge to make an optimal decision). It's simple and versatile, its performance proven in many different applications. The results so far have been promising. One of my colleagues had issues with ...

1

This is not quite the loss that is stated in the paper. For standard policy gradient methods the objective is to maximise $v_{\pi_\theta}(s_0)$ -- note that this is analogous to minimising $-v_{\pi_\theta}(s_0)$. This is for a stochastic policy. In DDPG the policy is now assumed to be deterministic. In general, we can write v_\pi(s) = \mathbb{E}_{a\sim\pi}[...

1

I don't recommend changing the rules of the environment. What you could do: Perform a method called bucketing i.e. take a value from a continuous state space see which discrete bucket it should go into and then let your agent use the bucket number as the observation. e.g. Say I do have a continuous state space with one variable in range $[-\infty,\infty]$ ...

1

The term REINFORCE actually corresponds to a method of estimating gradients, it is not particular to reinforcement learning. The paper you linked doesn't appear to deal with RL at all, so the issue they're describing is not one that you should expect to find in a policy gradient application. If you're using REINFORCE to estimate policy gradients in RL (this ...

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