How Pytorch Tensor get the index of specific value

Question

In python list, we can use list.index(somevalue). How can pytorch do this?
For example:

    a=[1,2,3]     print(a.index(2)) 

Then, 1 will be output. How can a pytorch tensor do this without converting it to a python list?

Answer 1

I think there is no direct translation from list.index() to a pytorch function. However, you can achieve similar results using tensor==number and then the nonzero() function. For example:

t = torch.Tensor([1, 2, 3]) print ((t == 2).nonzero(as_tuple=True)[0]) 

This piece of code returns

1

[torch.LongTensor of size 1x1]

Answer 2

For multidimensional tensors you can do:

(tensor == target_value).nonzero(as_tuple=True) 

The resulting tensor will be of shape number_of_matches x tensor_dimension. For example, say tensor is a 3 x 4 tensor (that means the dimension is 2), the result will be a 2D-tensor with the indexes for the matches in the rows.

tensor = torch.Tensor([[1, 2, 2, 7], [3, 1, 2, 4], [3, 1, 9, 4]]) (tensor == 2).nonzero(as_tuple=False) >>> tensor([[0, 1],         [0, 2],         [1, 2]])