What's the equivalence of theano's inc_subtensor()?

1

To change the values of a subset elements of a tensor, in theano we have inc_subtensor(), what is the equivalence in pytorch?

2

Hi,

You should take a look at the set of functions called index_* they allow you to work with sub-tensors.

3

Yes, I noticed there is torch.index_select() function. However this function returns a new tensor not a view, so if I do

t2 = torch.index_select(t1,  axis, index)
t2 += 1.0

Tensor t1 will stay unchanged. I eventually need t1 to be changed.

4

you can do standard numpy-like indexing:

Try this:

t1 = torch.randn(10, 5)
t2 = t1[:, 3]
t2.fill_(0)
print(t1)
5

@smth What if I need not just an integer index but a list of integers, e.g.
I want indexing like this
t1[:,[1,3,4]] += 1.0
However this is not supported by pytorch now, is there another way or I have to use a for-loop?

6

I think index_add_ is what you are looking for.

7

Thanks, that’s exactly what I’m looking for.

8

And t1[:,[1,3,4]] += 1.0 is implemented, but instead of giving it [1, 3, 4] you need to wrap that in a LongTensor

9

@david-leon index_add_ is documented here with all the other index_* functions

10

@albanD Well, this is awkward :sweat: how I missed it … and thanks a lot!

@apaszke I tried with t1[:,torch.LongTensor([1,3,4])] but no luck, error raised as
TypeError: indexing a tensor with an object of type LongTensor. The only supported types are integers, slices, numpy scalars and torch.LongTensor or torch.ByteTensor as the only argument.
My pytorch version is 0.1.10_2

11

@david-leon yeah sorry for that. It’d need to be indexing along the first dim.

12

Thanks, to be clear for future readers:
t1[torch.LongTensor([1,3,4])] works, but t1[torch.LongTensor([1,3,4]), :] does not.

And one more question which is related: if I want to do indexing like:
t1[[1,3,0], [1,3,4]]
what is the most efficient way to do this in pytorch? In theano we can do it the same as in numpy, however pythorch does not support this yet.

13

I think gather should do it

14

I can’t figure it out with gather, according to its syntax:
torch.gather(input, dim, index, out=None)
gather can only handle one dimensional indexing.

For the time being, I do the indexing via python loop:
a1 = torch.stack([t1[Idx1[i],Idx2[i]] for i in range(3)])
in which
Idx1 = torch.LongTensor([1,3,0]) and Idx2 = torch.LongTensor([1,3,4])
Apparently this is not an efficient nor an elegant way.