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How should this problem be framed in the domain of RL for preventing users from exceeding their bank account balance and being overdrawn?

For example, a user has 1000 in an account, and proceeds to withdraw 300, 400, 500, making the user overdrawn by 200 :((300+400+500) - 1000).

Treating this as a supervised learning problem, I could use logistic regression. The input feature is the transaction amounts. The input features for a training instance, 300,400,500 and the output feature occurs if the account is overdrawn or not overdrawn with corresponding values of 1 and 0 respectively. For simplicity, we will assume the number of transactions is consistent and is always 3.

For RL, a state could be represented as a series of transactions, but how should the reward be assigned?

Update:

Here my RL implementation of the problem:

import torch
from collections import defaultdict
gamma = .1
alpha = 0.1
epsilon = 0.1
n_episode = 2000
overdraft_limit = 1000

length_episode = [0] * n_episode
total_reward_episode = [0] * n_episode

episode_states = [[700,100,200,290,500] , [400,100,200,300,500] , [212, 500,100,100,200,500]]

def gen_epsilon_greedy_policy(n_action, epsilon):
    def policy_function(state, Q):
        probs = torch.ones(n_action) * epsilon / n_action
        best_action = torch.argmax(Q[state]).item()
        probs[best_action] += 1.0 - epsilon
        action = torch.multinomial(probs, 1).item()
        return action
    return policy_function

def is_overdrawn(currentTotal):
    return currentTotal >= overdraft_limit

# Actions are overdrawn or not, 0 - means it is not overdrawn, 1 - means that it will be overdrawn
def get_reward(action, currentTotal):
    if action == 0 and is_overdrawn(currentTotal):
        return -1
    elif action == 0 and not is_overdrawn(currentTotal):
        return 1
    if action == 1 and is_overdrawn(currentTotal):
        return 1
    elif action == 1 and not is_overdrawn(currentTotal):
        return -1
    else :
        raise Exception("Action not found") 

def q_learning(gamma, n_episode, alpha,n_action):
    """
    Obtain the optimal policy with off-policy Q-learning method
    @param gamma: discount factor
    @param n_episode: number of episodes
    @return: the optimal Q-function, and the optimal policy
    """
    Q = defaultdict(lambda: torch.zeros(n_action))
    for ee in episode_states : 
        for episode in range(n_episode):
            state = ee[0]
            index = 0
            currentTotal = 0
            while index < len(ee)-1 :
                currentTotal = currentTotal + state
                next_state = ee[index+1] 
                action = epsilon_greedy_policy(state, Q)
#                 print(action)
                reward = get_reward(action, currentTotal)
                td_delta = reward + gamma * torch.max(Q[next_state]) - Q[state][action]
                Q[state][action] += alpha * td_delta

                state = next_state
                index = index + 1

                length_episode[episode] += 1
                total_reward_episode[episode] += reward
                
    policy = {}
    for state, actions in Q.items():
        policy[state] = torch.argmax(actions).item()
    return Q, policy

epsilon_greedy_policy = gen_epsilon_greedy_policy(2, epsilon)

optimal_Q, optimal_policy = q_learning(gamma, n_episode, alpha, 2)

print('The optimal policy:\n', optimal_policy)
print('The optimal Q:\n', optimal_Q)

This code prints:

The optimal policy:
 {700: 0, 100: 0, 200: 1, 290: 1, 500: 0, 400: 0, 300: 1, 212: 0}
The optimal Q:
 defaultdict(<function q_learning.<locals>.<lambda> at 0x7f9371b0a3b0>, {700: tensor([ 1.1110, -0.8890]), 100: tensor([ 1.1111, -0.8889]), 200: tensor([-0.8889,  1.1111]), 290: tensor([-0.9998,  1.0000]), 500: tensor([ 1.1111, -0.8889]), 400: tensor([ 1.1110, -0.8890]), 300: tensor([-1.0000,  1.0000]), 212: tensor([ 1.1111, -0.8888])})

The optimal policy is to inform us if 700 is added to the balance, then the customer will not overdraw (0). If 200 is added to the balance, then the customer will overdraw(1). What avenues can I explore to improve upon this method as this is quite basic, but I'm unsure as to what approach I should take in order to improve the solution.

For example, this solution just looks at the most recent additions to the balance to determine if the customer is overdrawn. Is this a case of adding new features to the training data?

I'm just requesting a critique on this solution so I can improve it. How can I improve the representation of the state?

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  • $\begingroup$ In RL there's an agent that interacts with the environment by selecting actions. In your case, the agent would be pretty much giving yes/no answers, so there would be two actions. How are you input in the sequences? Do you put them in at once or do you want to have a training loop where agent receives them one by one? If you put the whole input at once, then agent just selects whether it thinks the amount is overdrawn and the episode is done. You can give positive award (1, for example) for getting the answer right and negative award for getting it wrong. $\endgroup$ – mark mark Dec 14 '20 at 17:44
  • $\begingroup$ @markmark the sequences are input one by one. $\endgroup$ – blue-sky Dec 14 '20 at 18:44
  • $\begingroup$ @markmark I'm not trying to train the agent to determine if the account is overdrawn which is what I think your suggesting, I'm attempting to train the agent ti to determine if the account will be become overdrawn based on a series of transactions. $\endgroup$ – blue-sky Dec 14 '20 at 18:54
  • 1
    $\begingroup$ You don't need supervised learning, reinforcement learning or any kind of learning. The problem is elementary school algebra?? If not then formulate your question better because right now it doesn't make any sense what you want to accomplish here with learning approaches. $\endgroup$ – Brale Dec 14 '20 at 19:17
  • $\begingroup$ If you're going with RL, I think you need to define action space (what actions can be taken by agent, in this case action is deciding whether overdrawing will happen), observation space (what information is available to agent at every step? A sequence and current accounts balance), what an episode of training/evaluation is like (do you receive multiple sequences for different accounts in every episode?) and the reward(did agent succeed at guessing? give positive reward). $\endgroup$ – mark mark Dec 14 '20 at 19:29

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