Q-Learning and SARSA

In this post I’m going to be covering two active reinforcement learning methods: q-learning and SARSA. Both methods do not depend on an MDP. Meaning, we are not guaranteed the presence of P. Instead, we learn which actions can be taken in a state during playthroughs. This is represented by a q-table, which is a table of states and actions that yield a q-value, which represents expected utility when taking an action in a given state. We can say that the expected utility of a state is the best action associated with that state. See the equation below.

In active reinforcement learning, we are updating the q-table after the agent makes a decision on the next move to make. To get the next move, you can use an epsilon greedy approach like in the code below or you can use the q-values in the q-table as weights for a probability-based selection. Note that a valid strategy is a greedy q-learning agent where only the max_next action selection strategy is used. This will method will converge quicker, but at the cost of likely finding a sub-optimal policy.

def max_next(self, s):
    valid_choices = [new_s for new_s in self.Q[s]]
    best_s = None
    best_u = -inf

    for new_s in valid_choices:
        next_u = self.Q[s][new_s]
        if next_u > best_u:
            best_u = next_u
            best_s = new_s

    return best_s, best_u
    
def get_next(self, s, eps=0.05):
    if random() < eps:
        best_s = choice([new_s for new_s in self.Q[s]])
        best_q = self.Q[s][best_s]
    else:
        best_s, best_q = self.max_next(s)

    return best_s, best_q

With an action selected, we can update the q-table. We do this with the following equation:

As you can see, there is a look ahead here. Given the current state and action to be taken, we say that its q-value is dependent on s' which is the result of taking action a in state s, where we find the optimal action to take in s’ based on the q-table. This equation is almost exactly the same as what we saw in temporal difference learning.

def train(self, max_iterations):
    GAMMA = 0.9

    N = {}
    for s in self.S:
        N[s] = 1

    for _ in trange(max_iterations):
        s = self.START
        while s not in self.E:
            N[s] += 1
            new_s, _ = self.get_next(s)

            ALPHA = (60.0/(59.0 + N[s]))
            self.Q[s][new_s] += ALPHA*(self.R[s] + GAMMA*self.u(new_s) - self.Q[s][new_s])
            s = new_s

        self.Q[s][0] = self.R[s]

That is q-learning in a nutshell, and now I’m going to turn my attention to SARSA which received its name for s,a,r,s’,a. In SARSA, we use a very similar seeming equation.

The difference is that in q-learning we don’t use the best action in the environment but we do use it in calculating the q-value. In SARSA, we don’t use the max value but we do take whatever action is selected. This is why q-learning is considered an off-policy algorithm and SARSA is on-policy: the calculation of the q-values is based on the policy. In a sense, SARSA is learning what will actually happen whereas q-learning will eventually learn to behave well regardless of the policy, which is why SARSA is the better choice when we care about performance while the agent is learning.

def train(self, max_iterations):
    GAMMA = 0.9

    N = {}
    for s in self.S:
        N[s] = 1

    for _ in trange(max_iterations):
        self.reset()
        s = self.START
        s_1, _ = self.get_next(s)
        while s not in self.E:
            N[s] += 1
            s_2, _ = self.get_next(s_1)

            ALPHA = (60.0/(59.0 + N[s]))
            self.Q[s][s_1] += ALPHA*(self.R[s] + GAMMA*self.Q[s_1][s_2] - self.Q[s][s_1])

            s = s_1
            s_1 = s_2

        self.Q[s][0] = self.R[s]

Running both of these algorithms on GridWorld with a 20x20 grid, they were able to find a solution after about 600 full playthroughs the majority of the time. Q-learning was slower by about a tenth of a second on successful runs on these successful runs. This makes sense because once SARSA requires less computation since it doesn’t compute U(s,a).

I hope you enjoyed this post. You can find the code on GitHub.