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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})

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