Question :
OpenAI’s REINFORCE and actor-critic example for reinforcement learning has the following code:
policy_loss = torch.cat(policy_loss).sum()
loss = torch.stack(policy_losses).sum() + torch.stack(value_losses).sum()
One is using torch.cat, the other uses torch.stack.
As far as my understanding goes, the doc doesn’t give any clear distinction between them.
I would be happy to know the differences between the functions.
Answer #1:
stack
Concatenates sequence of tensors along a new dimension.
cat
Concatenates the given sequence of seq tensors in the given dimension.
So if A and B are of shape (3, 4), torch.cat([A, B], dim=0) will be of shape (6, 4) and torch.stack([A, B], dim=0) will be of shape (2, 3, 4).
Answer #2:
The original answer lacks a good example that is self-contained so here it goes:
# %%
import torch
# stack vs cat
# cat "extends" a list in the given dimension e.g. adds more rows or columns
x = torch.randn(2, 3)
print(f'{x.size()}')
# add more rows (thus increasing the dimensionality of the column space to 2 -> 6)
xnew_from_cat = torch.cat((x, x, x), 0)
print(f'{xnew_from_cat.size()}')
# add more columns (thus increasing the dimensionality of the row space to 3 -> 9)
xnew_from_cat = torch.cat((x, x, x), 1)
print(f'{xnew_from_cat.size()}')
print()
# stack serves the same role as append in lists. i.e. it doesn't change the original
# vector space but instead adds a new index to the new tensor, so you retain the ability
# get the original tensor you added to the list by indexing in the new dimension
xnew_from_stack = torch.stack((x, x, x, x), 0)
print(f'{xnew_from_stack.size()}')
xnew_from_stack = torch.stack((x, x, x, x), 1)
print(f'{xnew_from_stack.size()}')
xnew_from_stack = torch.stack((x, x, x, x), 2)
print(f'{xnew_from_stack.size()}')
# default appends at the from
xnew_from_stack = torch.stack((x, x, x, x))
print(f'{xnew_from_stack.size()}')
print('I like to think of xnew_from_stack as a "tensor list"" that you can pop from the front')
