Tensor Indexing API

Indexing a tensor in the PyTorch C++ API works very similar to the Python API. All index types such as None / ... / integer / boolean / slice / tensor are available in the C++ API, making translation from Python indexing code to C++ very simple. The main difference is that, instead of using the []-operator similar to the Python API syntax, in the C++ API the indexing methods are:

• torch::Tensor::index (link)

• torch::Tensor::index_put_ (link)

It’s also important to note that index types such as None / Ellipsis / Slice live in the torch::indexing namespace, and it’s recommended to put using namespace torch::indexing before any indexing code for convenient use of those index types.

Here are some examples of translating Python indexing code to C++:

Getter

Python	C++ (assuming using namespace torch::indexing)
tensor[None]	tensor.index({None})
tensor[Ellipsis, ...]	tensor.index({Ellipsis, "..."})
tensor[1, 2]	tensor.index({1, 2})
tensor[True, False]	tensor.index({true, false})
tensor[1::2]	tensor.index({Slice(1, None, 2)})
tensor[torch.tensor([1, 2])]	tensor.index({torch::tensor({1, 2})})
tensor[..., 0, True, 1::2, torch.tensor([1, 2])]	tensor.index({"...", 0, true, Slice(1, None, 2), torch::tensor({1, 2})})

Setter

Python	C++ (assuming using namespace torch::indexing)
tensor[None] = 1	tensor.index_put_({None}, 1)
tensor[Ellipsis, ...] = 1	tensor.index_put_({Ellipsis, "..."}, 1)
tensor[1, 2] = 1	tensor.index_put_({1, 2}, 1)
tensor[True, False] = 1	tensor.index_put_({true, false}, 1)
tensor[1::2] = 1	tensor.index_put_({Slice(1, None, 2)}, 1)
tensor[torch.tensor([1, 2])] = 1	tensor.index_put_({torch::tensor({1, 2})}, 1)
tensor[..., 0, True, 1::2, torch.tensor([1, 2])] = 1	tensor.index_put_({"...", 0, true, Slice(1, None, 2), torch::tensor({1, 2})}, 1)

Translating between Python/C++ index types

The one-to-one translation between Python and C++ index types is as follows:

Python	C++ (assuming using namespace torch::indexing)
None	None
Ellipsis	Ellipsis
...	"..."
123	123
True	true
False	false
: or ::	Slice() or Slice(None, None) or Slice(None, None, None)
1: or 1::	Slice(1, None) or Slice(1, None, None)
:3 or :3:	Slice(None, 3) or Slice(None, 3, None)
::2	Slice(None, None, 2)
1:3	Slice(1, 3)
1::2	Slice(1, None, 2)
:3:2	Slice(None, 3, 2)
1:3:2	Slice(1, 3, 2)
torch.tensor([1, 2])	torch::tensor({1, 2})