.. _a2c:

.. automodule:: stable_baselines3.a2c


A2C
====

A synchronous, deterministic variant of `Asynchronous Advantage Actor Critic (A3C) <https://arxiv.org/abs/1602.01783>`_.
It uses multiple workers to avoid the use of a replay buffer.


.. warning::

  If you find training unstable or want to match performance of stable-baselines A2C, consider using
  ``RMSpropTFLike`` optimizer from ``stable_baselines3.common.sb2_compat.rmsprop_tf_like``.
  You can change optimizer with ``A2C(policy_kwargs=dict(optimizer_class=RMSpropTFLike, optimizer_kwargs=dict(eps=1e-5)))``.
  Read more `here <https://github.com/DLR-RM/stable-baselines3/pull/110#issuecomment-663255241>`_.


Notes
-----

-  Original paper:  https://arxiv.org/abs/1602.01783
-  OpenAI blog post: https://openai.com/blog/baselines-acktr-a2c/


Can I use?
----------

-  Recurrent policies: ❌
-  Multi processing: ✔️
-  Gym spaces:


============= ====== ===========
Space         Action Observation
============= ====== ===========
Discrete      ✔️      ✔️
Box           ✔️      ✔️
MultiDiscrete ✔️      ✔️
MultiBinary   ✔️      ✔️
Dict          ❌     ✔️
============= ====== ===========


Example
-------

This example is only to demonstrate the use of the library and its functions, and the trained agents may not solve the environments. Optimized hyperparameters can be found in RL Zoo `repository <https://github.com/DLR-RM/rl-baselines3-zoo>`_.

Train a A2C agent on ``CartPole-v1`` using 4 environments.

.. code-block:: python

  from stable_baselines3 import A2C
  from stable_baselines3.common.env_util import make_vec_env

  # Parallel environments
  vec_env = make_vec_env("CartPole-v1", n_envs=4)

  model = A2C("MlpPolicy", vec_env, verbose=1)
  model.learn(total_timesteps=25000)
  model.save("a2c_cartpole")

  del model # remove to demonstrate saving and loading

  model = A2C.load("a2c_cartpole")

  obs = vec_env.reset()
  while True:
      action, _states = model.predict(obs)
      obs, rewards, dones, info = vec_env.step(action)
      vec_env.render("human")


.. note::

  A2C is meant to be run primarily on the CPU, especially when you are not using a CNN. To improve CPU utilization, try turning off the GPU and using ``SubprocVecEnv`` instead of the default ``DummyVecEnv``:

  .. code-block:: python

    from stable_baselines3 import A2C
    from stable_baselines3.common.env_util import make_vec_env
    from stable_baselines3.common.vec_env import SubprocVecEnv

    if __name__=="__main__":
        env = make_vec_env("CartPole-v1", n_envs=8, vec_env_cls=SubprocVecEnv)
        model = A2C("MlpPolicy", env, device="cpu")
        model.learn(total_timesteps=25_000)

  For more information, see :ref:`Vectorized Environments <vec_env>`, `Issue #1245 <https://github.com/DLR-RM/stable-baselines3/issues/1245>`_ or the `Multiprocessing notebook <https://colab.research.google.com/github/Stable-Baselines-Team/rl-colab-notebooks/blob/sb3/multiprocessing_rl.ipynb>`_.

.. note::

  Using gSDE (Generalized State-Dependent Exploration) during inference (see `PR #1767 <https://github.com/DLR-RM/stable-baselines3/pull/1767>`_):

  When using A2C models trained with ``use_sde=True``, the automatic noise resetting that occurs during training (controlled by ``sde_sample_freq``) does not happen when using ``model.predict()`` for inference. This results in deterministic behavior even when ``deterministic=False``.

  For continuous control tasks, it is recommended to use deterministic behavior during inference (``deterministic=True``). If you need stochastic behavior during inference, you must manually reset the noise by calling ``model.policy.reset_noise(env.num_envs)`` at appropriate intervals based on your desired ``sde_sample_freq``.


Results
-------

Atari Games
^^^^^^^^^^^

The complete learning curves are available in the `associated PR #110 <https://github.com/DLR-RM/stable-baselines3/pull/110>`_.


PyBullet Environments
^^^^^^^^^^^^^^^^^^^^^

Results on the PyBullet benchmark (2M steps) using 6 seeds.
The complete learning curves are available in the `associated issue #48 <https://github.com/DLR-RM/stable-baselines3/issues/48>`_.


.. note::

  Hyperparameters from the `gSDE paper <https://arxiv.org/abs/2005.05719>`_ were used (as they are tuned for PyBullet envs).


*Gaussian* means that the unstructured Gaussian noise is used for exploration,
*gSDE* (generalized State-Dependent Exploration) is used otherwise.

+--------------+--------------+--------------+--------------+-------------+
| Environments | A2C          | A2C          | PPO          | PPO         |
+==============+==============+==============+==============+=============+
|              | Gaussian     | gSDE         | Gaussian     | gSDE        |
+--------------+--------------+--------------+--------------+-------------+
| HalfCheetah  | 2003 +/- 54  | 2032 +/- 122 | 1976 +/- 479 | 2826 +/- 45 |
+--------------+--------------+--------------+--------------+-------------+
| Ant          | 2286 +/- 72  | 2443 +/- 89  | 2364 +/- 120 | 2782 +/- 76 |
+--------------+--------------+--------------+--------------+-------------+
| Hopper       | 1627 +/- 158 | 1561 +/- 220 | 1567 +/- 339 | 2512 +/- 21 |
+--------------+--------------+--------------+--------------+-------------+
| Walker2D     | 577 +/- 65   | 839 +/- 56   | 1230 +/- 147 | 2019 +/- 64 |
+--------------+--------------+--------------+--------------+-------------+


How to replicate the results?
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Clone the `rl-zoo repo <https://github.com/DLR-RM/rl-baselines3-zoo>`_:

.. code-block:: bash

  git clone https://github.com/DLR-RM/rl-baselines3-zoo
  cd rl-baselines3-zoo/


Run the benchmark (replace ``$ENV_ID`` by the envs mentioned above):

.. code-block:: bash

  python train.py --algo a2c --env $ENV_ID --eval-episodes 10 --eval-freq 10000


Plot the results (here for PyBullet envs only):

.. code-block:: bash

  python scripts/all_plots.py -a a2c -e HalfCheetah Ant Hopper Walker2D -f logs/ -o logs/a2c_results
  python scripts/plot_from_file.py -i logs/a2c_results.pkl -latex -l A2C


Parameters
----------

.. autoclass:: A2C
  :members:
  :inherited-members:


.. _a2c_policies:

A2C Policies
-------------

.. autoclass:: MlpPolicy
  :members:
  :inherited-members:

.. autoclass:: stable_baselines3.common.policies.ActorCriticPolicy
  :members:
  :noindex:

.. autoclass:: CnnPolicy
  :members:

.. autoclass:: stable_baselines3.common.policies.ActorCriticCnnPolicy
  :members:
  :noindex:

.. autoclass:: MultiInputPolicy
  :members:

.. autoclass:: stable_baselines3.common.policies.MultiInputActorCriticPolicy
  :members:
  :noindex:
