Torch / Lua, how to select a subset of an array or tensor?

Question

I'm working on Torch/Lua and have an array dataset of 10 elements.

dataset = {11,12,13,14,15,16,17,18,19,20}

If I write dataset[1], I can read the structure of the 1st element of the array.

th> dataset[1]
11  

I need to select just 3 elements among all the 10, but I don't know which command to use. If I were working on Matlab, I would write: dataset[1:3], but here does not work.

Do you have any suggestions?

Answer 1

In Torch

th> x = torch.Tensor{1, 2, 3, 4, 5, 6, 7, 8, 9, 10}

To select a range, like the first three, use the index operator:

th> x[{{1,3}}]
1
2
3

Where 1 is the 'start' index, and 3 is the 'end' index.

See Extracting Sub-tensors for more alternatives using Tensor.sub and Tensor.narrow

---

In Lua 5.2 or less

Lua tables, such as your dataset variable, do not have a method for selecting sub-ranges.

function subrange(t, first, last)
  local sub = {}
  for i=first,last do
    sub[#sub + 1] = t[i]
  end
  return sub
end

dataset = {11,12,13,14,15,16,17,18,19,20}

sub = subrange(dataset, 1, 3)
print(unpack(sub))

which prints

11    12   13

---

In Lua 5.3

In Lua 5.3 you can use table.move.

function subrange(t, first, last)
     return table.move(t, first, last, 1, {})
end