Deep deterministic policy gradient
The DQN and its variants have been very successful in solving problems where the state space is continuous and action space is discrete. For example, in Atari games, the input space consists of raw pixels, but actions are discrete—[up, down, left, right, no-op]. How do we solve a problem with continuous action space? For instance, say an RL agent driving a car needs to turn its wheels: this action has a continuous action space.
One way to handle this situation is by discretizing the action space and continuing with a DQN or its variants. However, a better solution would be to use a policy gradient algorithm. In policy gradient methods, the policy 
is approximated directly.
A neural network is used to approximate the policy; in the simplest form, the neural network learns a policy for selecting actions that maximize the rewards by adjusting its weights using the steepest gradient ascent, hence the name: policy gradients.
In this...
