Extracting the top-k value-indices from a 1-D Tensor

Question

Given a 1-D tensor in Torch (torch.Tensor), containing values which can be compared (say floating point), how can we extract the indices of the top-k values in that tensor?

Apart from the brute-force method, I am looking for some API call, that Torch/lua provides, which can perform this task efficiently.

Answer 1

As of pull request #496 Torch now includes a built-in API named torch.topk. Example:

> t = torch.Tensor{9, 1, 8, 2, 7, 3, 6, 4, 5}

-- obtain the 3 smallest elements
> res = t:topk(3)
> print(res)
 1
 2
 3
[torch.DoubleTensor of size 3]

-- you can also get the indices in addition
> res, ind = t:topk(3)
> print(ind)
 2
 4
 6
[torch.LongTensor of size 3]

-- alternatively you can obtain the k largest elements as follow
-- (see the API documentation for more details)
> res = t:topk(3, true)
> print(res)
 9
 8
 7
[torch.DoubleTensor of size 3]

At the time of writing the CPU implementation follows a sort and narrow approach (there are plans to improve it in the future). That being said an optimized GPU implementation for cutorch is currently being reviewed.