Finding non-intersection of two pytorch tensors

Question

Thanks everyone in advance for your help! What I'm trying to do in PyTorch is something like numpy's setdiff1d. For example given the below two tensors:

t1 = torch.tensor([1, 9, 12, 5, 24]).to('cuda:0')
t2 = torch.tensor([1, 24]).to('cuda:0')

The expected output should be (sorted or unsorted):

torch.tensor([9, 12, 5])

Ideally the operations are done on GPU and no back and forth between GPU and CPU. Much appreciated!

Answer 1

I came across the same problem but the proposed solutions were far too slow when using larger arrays. The following simple solution works on CPU and GPU and is significantly faster than the other proposed solutions:

combined = torch.cat((t1, t2))
uniques, counts = combined.unique(return_counts=True)
difference = uniques[counts == 1]
intersection = uniques[counts > 1]

Answer 2

If you don't want a for loop this can compare all values in one go.

Also you can get the non intersection easily too

t1 = torch.tensor([1, 9, 12, 5, 24])
t2 = torch.tensor([1, 24])

# Create a tensor to compare all values at once
compareview = t2.repeat(t1.shape[0],1).T

# Intersection
print(t1[(compareview == t1).T.sum(1)==1])
# Non Intersection
print(t1[(compareview != t1).T.prod(1)==1])

tensor([ 1, 24])
tensor([ 9, 12,  5])

Answer 3

if you don't want to leave cuda, a workaround could be:

t1 = torch.tensor([1, 9, 12, 5, 24], device = 'cuda')
t2 = torch.tensor([1, 24], device = 'cuda')
indices = torch.ones_like(t1, dtype = torch.uint8, device = 'cuda')
for elem in t2:
    indices = indices & (t1 != elem)  
intersection = t1[indices]