# How to create a VecEnv on GPUs¶

ElegantRL supports massively parallel simulation through GPU-accelerated VecEnv.

Here, we talk about how to create a VecEnv on GPUs from scratch and go through a simple chasing example, a deterministic environment with continuous actions and continuous state space. The goal is to move an agent to chase a randomly moving robot. The reward depends on the distance between agent and robot. The environment terminates when the agent catches the robot or the max step is reached.

To keep the example simple, we only use two packages, PyTorch and Numpy.

```import torch
import numpy as np
```

Now, we start to create the environment, which usually includes initialization function, reset function, and step function.

For initialization function, we specify the number of environments `env_num`, the GPU id `device_id`, and the dimension of the chasing space `dim`. In the chasing environment, we keep track of positions and velocities of the agent and the robot.

```class ChasingVecEnv:
def __init__(self, dim=2, env_num=4096, device_id=0):
self.dim = dim
self.init_distance = 8.0

# reset
self.p0s = None  # position
self.v0s = None  # velocity
self.p1s = None
self.v1s = None

self.distances = None
self.current_steps = None

'''env info'''
self.env_name = 'ChasingVecEnv'
self.state_dim = self.dim * 4
self.action_dim = self.dim
self.max_step = 2 ** 10
self.if_discrete = False
self.target_return = 6.3

self.env_num = env_num
self.device = torch.device(f"cuda:{device_id}")
```

The second step is to implement a reset function. The reset function is called at the beginning of each episode and sets initial state to current state. To utilize GPUs, we use data structures for multi-dimensional tensors provided by the torch package.

```def reset(self):
self.p0s = torch.zeros((self.env_num, self.dim), dtype=torch.float32, device=self.device)
self.v0s = torch.zeros((self.env_num, self.dim), dtype=torch.float32, device=self.device)
self.p1s = torch.zeros((self.env_num, self.dim), dtype=torch.float32, device=self.device)
self.v1s = torch.zeros((self.env_num, self.dim), dtype=torch.float32, device=self.device)

self.current_steps = np.zeros(self.env_num, dtype=np.int)

for env_i in range(self.env_num):
self.reset_env_i(env_i)

self.distances = ((self.p0s - self.p1s) ** 2).sum(dim=1) ** 0.5

return self.get_state()
```

The last function is the step function, that includes a transition function and a reward function, and signals the terminal state. To compute the transition function, we utilize mathematical operations from the torch package over the data (tensors). These operations allow us to compute transitions and rewards of thousands of environments in parallel.

Note

Unlike computing the transition function and reward function in parallel, we check the terminal state in a sequential way. Since sub-environments may terminate at different time steps, when a sub-environment is at terminal state, we have to reset it manually.

```def step(self, action1s):
'''transition function'''
action0s = torch.rand(size=(self.env_num, self.dim), dtype=torch.float32, device=self.device)
action0s_l2 = (action0s ** 2).sum(dim=1, keepdim=True) ** 0.5
action0s = action0s / action0s_l2.clamp_min(1.0)

self.v0s *= 0.50
self.v0s += action0s
self.p0s += self.v0s * 0.01

action1s_l2 = (action1s ** 2).sum(dim=1, keepdim=True) ** 0.5
action1s = action1s / action1s_l2.clamp_min(1.0)

self.v1s *= 0.75
self.v1s += action1s
self.p1s += self.v1s * 0.01

'''reward function'''
distances = ((self.p0s - self.p1s) ** 2).sum(dim=1) ** 0.5
rewards = self.distances - distances - action1s_l2.squeeze(1) * 0.02
self.distances = distances

'''check terminal state'''
self.steps += 1  # array
dones = torch.zeros(self.env_num, dtype=torch.float32, device=self.device)
for env_i in range(self.env_num):
done = 0
if distances[env_i] < 1:
done = 1
rewards[env_i] += self.init_distance
elif self.steps[env_i] == self.max_step:
done = 1

if done:
self.reset_env_i(env_i)
dones[env_i] = done

'''next_state'''
next_states = self.get_state()
return next_states, rewards, dones, None
```

For more information about the chasing environment, we provide a Colab version to play with, and its code can be found here.