# Tag Info

6

The MDP defines the environment (which corresponds to the task that you need to solve), so it defines e.g. the states of the environment, the actions that you can take in those states, the probabilities of transitioning from one state to the other and the probabilities of getting a reward when you take a certain action in a certain state. The policy ...

4

What is reinforcement learning? In reinforcement learning (RL), you typically imagine that there's an agent that interacts, in time steps, with an environment by taking actions. On each time step $t$, the agent takes the action $a_t \in \mathcal{A}$ in the state $s_t \in \mathcal{S}$, receives a reward (or reinforcement) signal $r_t \in \mathbb{R}$ from the ...

4

If we write the pseudo-code for the SARSA algorithm we first initialise our hyper-parameters etc. and then initialise $S_t$, which we use to choose $A_t$ from our policy $\pi(a|s)$. Then for each $t$ in the episode we do the following: Take action $A_t$ and observe $R_{t+1}$, $S_{t+1}$ Choose $A_{t+1}$ using $S_{t+1}$ in our policy $Q(S_t, A_t) = Q(S_t, A_t)... 4 Prediction is the problem of predicting any feature of the environment. In reinforcement learning, the typical feature is the reward or return, but this doesn't have to be always the case. See Multi-timescale nexting in a reinforcement learning robot (2011) by Joseph Modayil et al. Control is the problem of estimating a policy. Clearly, the term control is ... 4 Nbro's answer already addresses the basic definitions, so I won't repeat that. Instead I'll try to elaborate a bit on the other parts of the question. Are there scenarios in RL where the problem cannot be distinctly categorised into the aforementioned problems and is a mixture of the problems? I'm not sure about cases where the "problem" can't be ... 4 Yes, the two update equations are equivalent. As an aside, technically the equation you give is not the Bellman equation, but the update step re-written as an equation - in the Bellman equation instead of$v_{k+1}(s)$or$v_{k}(s)$(showing iterations of approximate value functions), you would have$v_{\pi}(s)$(representing the true value of a state under ... 3 These two definitions are not exactly the same, even though they have a very similar formulation. David Silver's notation is probably an abuse of notation. The first difference between those two definitions is that, in the case of David Silver's slides, the policy is parametrized by$\theta$(i.e. the policy could be represented e.g. by a neural network), ... 3 Supervised learning The supervised learning (SL) problem is formulated as follows. You are given a dataset$\mathcal{D} = \{(x_i, y_i)_{i=1}^N$, which is assumed to be drawn i.i.d. from an unknown joint probability distribution$p(x, y)$, where$x_i$represents the$i$th input and$y_i$is the corresponding label. You choose a loss function$\mathcal{L}: ...

3

Reinforcement learning (RL) control maximises the expected sum of rewards. If you change the reward metric, it will change what counts as optimal. Your reward functions are not the same, so will in some cases change the priority of solutions. As a simple example, consider a choice between trajectories with costs A(0,4,4,4) and B(1,1,1,1). In the original ...

3

I think you are looking at it from the wrong direction, min-max is just a planning algorithm, decision strategy, in the sense that you are describing other algorithms/methods it does not have a category. For example, you have negamax algorithm which is in a sense the same thing the Monte Carlo Search Tree is to Monte Carlo. Min-max category is game theory ...

3

Softmax policy $\pi_\theta(s,a)$ is defined as $\frac{\exp{(\phi(s,a)^T \theta})}{\Sigma \exp{(\phi(s,a) ^T \theta) }}$, where the summation is over the action space. Taking log, this becomes $$\log \pi_\theta(s,a) = log(e^{\phi(s,a) ^T \theta}) - log({\Sigma e^{\phi(s,a) ^T \theta }}) \\ = \phi(s,a) ^T \theta - log({\Sigma e^{\phi(s,a)^T \theta }})$$ ...

3

The rationale behind importance sampling is that $q(x)$ is difficult to sample from but easy to evaluate. Or at least you can easily evaluate some $\tilde{q}$ such that: $$\tilde{q}(z) = Zq(z)$$ where $Z$ (scalar) might be unknown. The geometrical example would be here e.g. sampling uniformly from an area under the curve $q(x)$ (in general it's not easy). ...

3

It is common in Bayesian statistics to only know the posterior up to a constant of proportionality. This means that we can't directly sample from the posterior. However, using importance sample we are able to. Consider our posterior density $\pi$ is only known up to some constant, i.e. $\pi(x) = K \tilde{\pi}(x)$, where $K$ is some constant and we only ...

2

Let's have a look at the introduction of Chapter 2: Multi-armed Bandits in the Reinforcement Learning: An Introduction by Sutton, Barto The most important feature distinguishing reinforcement learning from other types of learning is that it uses training information that evaluates the actions taken rather than instructs by giving correct actions. This ...

2

It should be possible to train an agent using some variant of DQN to beat a random agent around 100% of the time within a few thousand games. It may require one or two more advanced techniques to get the learning time down to a low number of thousands. However, if your agent is winning ~50% of games against a random agent, something has gone wrong, since ...

2

Let's first clarify a couple of details: The policy $\pi$ we're talking about is an $\epsilon$-soft policy (defined to mean that $\pi(a \vert s) \geq \frac{\epsilon}{\vert \mathcal{A}(s) \vert}$ for all states and all actions). We're not trying to prove equality of $v_{\pi}$ and $v_*$, but of $v_{\pi}$ and $\tilde{v}_*$, where $\tilde{v}_*$ denotes the ...

2

In Supervised learning, the goal is to learn a mapping from points in a feature space to labels. So that for any new input data point, we are able to predict its label. whereas in Unsupervised learning data set is composed only of points in a feature space, i.e. there are no labels & here the goal is to learn some inner structure or organization in the ...

2

When doing gradient descent update for this single example, should the target output to set for the network be equivalent to $Q(s_1,a_0), Q(s_1,a_1), r_2 + \gamma max_aQ(s',a',\theta) , Q(s_1,a_3),...$ ? Other than what looks like a couple of small typos, then yes. This is an implementation issue for DQN, where you have decided to create a function that ...

2

Isn't the environment constantly changing in this game? The current state of the agent and the environment is constantly changing as you play, but not necessarily the transition probabilities. For simplicity, you may assume that the transition probabilities do not change (e.g. if the dealer and the deck are the same every time you play). How would the ...

2

There is this paper Representation and Reinforcement Learning for Personalized Glycemic Control in Septic Patients, presented in the Machine Learning for Health Workshop in NIPS 2017. Here is a quote from the paper where the authors describe the clustering approach: After we generated the state representation, we used the k-means clustering algorithm ...

2

Let's say your old policy is $\pi_b$ and your current one is $\pi_a$. If you collected trajectory by using policy $\pi_b$ you would get return $G$ whose expected value is \begin{align} E_{\pi_b}[G_t|S_t = s] &= E_{\pi_b}[R_{t+1} + G_{t+1}]\\ &= \sum_a \pi_b(a|s) \sum_{s', r} p(s', r|s, a) [r + E_{\pi_b}[G_{t+1}|S_{t+1} = s']]\\ \end{align} You can ...

2

Q-values represent expected return after taking action $a$ in state $s$, so they do tell you how good it is to take an action in the specific state. Better actions will have larger Q-values. Q-values can be used to compares actions but they are not very meaningful in representing performance of the agent since you have nothing to compare them with. You don't ...

2

Here is the commit I fixed few minor errors, but the major one was when I saw what the line histories = [deque(maxlen=self.reward_steps)] * len(self.env.envs) was doing. It was just repeating the same queue. In [2]: histories = [deque(maxlen=5)] * 4 In [3]: histories ...

2

By definition of $V_{n+1}$, we have: $V_{n+1} = \frac{\sum_{k=1}^{n} W_{k} G_{k}}{\sum_{k=1}^{n} W_{k}} \; \tag{1}$ Then, taking the $n^{th}$ term out of the sum in the numerator, we have: $V_{n+1} = \frac{W_{n}G_{n} \; + \; \sum_{k=1}^{n-1} W_{k} G_{k}}{\sum_{k=1}^{n} W_{k}} \; \tag{2}$ Then, from the definition of $V_n$, $V_{n} = \frac{\sum_{k=1}^{n-1} ... 2 First, some preliminary questions: in this case, what is the optimal policy? It is the policy that maximises return from any given time step$G_t$. You need to be careful with your definition of return with continuing environments. The simple expected sum of future rewards is likely to be positive or negative infinity. There are three basic approaches: ... 2 Can deep reinforcement learning algorithms be deterministic in their reproducibility in results? Yes, but only if you control all places in the code where stochastic methods are used (typically by seeding the affected RNGs): Neural network weight initialisation Action choice for$\epsilon$-greedy or other behaviour policy (does not apply in your case, ... 2 There are different TD algorithms, e.g. Q-learning and SARSA, whose convergence properties have been studied separately (in many cases). In some convergence proofs, e.g. in the paper Convergence of Q-learning: A Simple Proof (by Francisco S. Melo), the required conditions for Q-learning to converge (in probability) are the Robbins-Monro conditions$\sum_{...

2

Recall that our goal is to be able to accurately estimate the true value of each state by computing a sample average over returns starting from that state: $$v_{q}(s) \doteq \mathbb{E}_{q}\left[G_{t} | S_{t}=s\right] \approx \frac{1}{n} \sum_{i=1}^{n} Return_i$$ where $Return_i$ is the return obtained from the $i^{th}$ trajectory. The problem is that the \$\...

2

I think there is an intersection. There are problems that are in reinforcement learning and in learning in multi-agent systems. There are problems in reinforcement learning, but not exactly in multi-agent systems. And there is learning in multi-agent systems that is not through reinforcement learning. For sort you can say: multi-agent reinforcement learning. ...

2

Aside from the points raised in nbro's answer, I'd like to point out that for a single MDP (a single instance of a "problem"), it may be sensible to study it from perspectives that include no policy at all, or multiple different policies. For instance, if I have an MDP, I may be interested in studying it by looking at various inherent properties of the ...

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