REINFORCE Correction

[IbrahimSobh]

Hello,

In deep-reinforcement-learning/reinforce/REINFORCE.ipynb

R is implemented as a single value in the following code:

discounts = [gamma**i for i in range(len(rewards)+1)]
R = sum([a*b for a,b in zip(discounts, rewards)])

However it should be implemented as discounted values as follow:

        discounted_rewards = []
        for t in range(len(rewards)):
          Gt = 0 
          pwr = 0
          for r in rewards[t:]:
              Gt = Gt + gamma**pwr * r
              pwr = pwr + 1
          discounted_rewards.append(Gt)
        
        policy_loss = []
        for log_prob, Gt in zip(saved_log_probs, discounted_rewards):
          policy_loss.append(-log_prob * Gt)

This important correction is compatible with the REINFORCE algorithm and leads to a faster and more stable training as shown in the figure.

[lukysummer]

I agree. Expected return G_t (sum of FUTURE awards) should be multiplied with each log p(At|St), which decreases as t increases, not the cumulative reward/episode R which is same for all t.