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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 asNone /... / integer / boolean / slice / tensorare available in the C++ API, making translation from Python indexing code to C++very simple. The main difference is that, instead of using the[]-operatorsimilar 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 asNone /Ellipsis /Slicelive in thetorch::indexing namespace, and it’s recommended to putusingnamespacetorch::indexingbefore any indexing code for convenient use of those index types.

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

Getter#

Python

C++ (assumingusingnamespacetorch::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++ (assumingusingnamespacetorch::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++ (assumingusingnamespacetorch::indexing)

None

None

Ellipsis

Ellipsis

...

"..."

123

123

True

true

False

false

: or::

Slice() orSlice(None,None) orSlice(None,None,None)

1: or1::

Slice(1,None) orSlice(1,None,None)

:3 or:3:

Slice(None,3) orSlice(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})