# Clipped Proximal Policy Optimization - Reinforcement Learning Coach

# Clipped Proximal Policy Optimization

Actions space: Discrete|Continuous

References: Proximal Policy Optimization Algorithms

## Network Structure

## Algorithm Description

### Choosing an action - Continuous action

Same as in PPO.

### Training the network

Very similar to PPO, with several small (but very simplifying) changes:

1. Train both the value and policy networks, simultaneously, by defining a single loss function, which is the sum of each of the networks loss functions. Then, back propagate gradients only once from this unified loss function.

2. The unified network's optimizer is set to Adam (instead of L-BFGS for the value network as in PPO).

3. Value targets are now also calculated based on the GAE advantages. In this method, the
   values are predicted from the critic network, and then added to the GAE based advantages, in order to get a
   value for each action. Now, since our critic network is predicting a
   value for each state, setting the
   calculated action-values as a target, will on average serve as a
   state-value target.

4. Instead of adapting the penalizing KL divergence coefficient used in PPO, the likelihood ratio
   is clipped, to achieve a similar effect. This is done by defining the policy's loss function to be the minimum between the standard surrogate loss and an epsilon clipped surrogate loss:

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