# How to turn a list of tensor to tensor?

I have a list and there are many tensor in the list
I want to turn it to just tensor, and I can put it to dataloader

I use for loop and cat the tensor but it is very slow, data size is about 4,800,000

13 Likes

If they’re all the same size, then you could `torch.unsqueeze` them in dimension 0 and then `torch.cat` the results together.

12 Likes
``````a = []
for i in range(100000):
a.append(torch.rand(1, 100, 100)

b = torch.Tensor(100000, 100, 100)
torch.cat(a, out=b)``````
19 Likes

I was always doing something like:

``````a = [torch.FloatTensor().view(1, -1), torch.FloatTensor().view(1, -1)]
torch.stack(a)
``````

Gives me:

``````(0 ,.,.) =
1

(1 ,.,.) =
2
[torch.FloatTensor of size 2x1x1]
``````
16 Likes

Actually

I have two list

list 1
a = [[tensor 40], [tensor 40], [tensor 40], …] (2400000 tensor in list each tensor size is 40)
b = [[tensor 40], [tensor 40], [tensor 40], …] (2400000 tensor in list each tensor size is 40)

I want to concat a and b to c
c is a tensor and size is torch.Size([4800000, 40])

I use this method to solve my problem
a = torch.stack(a)
b = torch.stack(b)
c = torch.cat((a, b))

Thank you for all your help !

11 Likes

Or you can just `torch.stack(a + b)`

14 Likes

It works.
Thank you.

This is how it should be done.

a = torch.stack(a) worked for me

18 Likes

To me it’s sort of unintuitive, why wouldn’t using the tensor class work?

``````torch.tensor([ torch.tensor([i]).repeat(15) for i in range(0,5)])
``````

the list is the same size and it’s really a matrix/tensor already…but somehow only:

``````torch.stack([ torch.tensor([i]).repeat(15) for i in range(0,5)])
tensor([[0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0],
[1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1],
[2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2],
[3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3],
[4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4]])
``````

worked.

Btw, is this this most efficient way to do it in a vectorized way?

2 Likes

check this out but summary use `torch.stack` if you want to respect the original nesting of the lists (by having a tensor with many indices and dimensions) and incrementally create it. There might be better ways but that works for me.

``````
# %%

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')

print()

lst = []
print(f'{x.size()}')
for i in range(10):
x += i  # say we do something with x at iteration i
lst.append(x)
# lstt = torch.stack([x for _ in range(10)])
lstt = torch.stack(lst)
print(lstt.size())

print()

# lst = []
# print(f'{x.size()}')
# for i in range(10):
#     x += i  # say we do something with x at iteration i
#     for j in range(11):
#         x += j
#         lstx
#     lst.append(x)
# # lstt = torch.stack([x for _ in range(10)])
# lstt = torch.stack(lst)
# print(lstt.size())
``````
1 Like

Sorry to bother you after all this time.
I have target['a] which is a list of tensors. when I use

``````torch.unsqueeze(target['a'], 0)
``````

it gives me this error:

TypeError: unsqueeze(): argument ‘input’ (position 1) must be Tensor, not list

what is the correct way to use torch.unsqueeze?