3 files changed, 27 insertions, 7 deletions

diff --git a/SelectTable.lua b/SelectTable.lua
index 64cd105..61918f7 100644
--- a/SelectTable.lua
+++ b/SelectTable.lua
@@ -7,7 +7,13 @@ function SelectTable:__init(index)
 end
 
 function SelectTable:updateOutput(input)
-   self.output = input[self.index]
+   assert(math.abs(self.index) <= #input, "arg 1 table idx out of range")
+   if self.index < 0 then
+      self.output = input[#input + self.index + 1]
+   else
+      self.output = input[self.index]
+   end
+   
    return self.output
 end
 
@@ -31,7 +37,11 @@ local function zeroTableCopy(t1, t2)
 end
 
 function SelectTable:updateGradInput(input, gradOutput)
-   self.gradInput[self.index] = gradOutput
+   if self.index < 0 then
+      self.gradInput[#input + self.index + 1] = gradOutput
+   else
+      self.gradInput[self.index] = gradOutput
+   end
    zeroTableCopy(self.gradInput, input)
    return self.gradInput
 end
diff --git a/doc/table.md b/doc/table.md
index 57c222d..c2aeb83 100755
--- a/doc/table.md
+++ b/doc/table.md
@@ -636,7 +636,7 @@ Forwarding a batch of 2 examples gives us something like this:
 
 `module` = `SelectTable(index)`
 
-Creates a module that takes a `table` as input and outputs the element at index `index`.
+Creates a module that takes a `table` as input and outputs the element at index `index` (positive or negative). 
 This can be either a `table` or a [`Tensor`](https://github.com/torch/torch7/blob/master/doc/tensor.md#tensor).
 
 The gradients of the non-`index` elements are zeroed `Tensor`s of the same size. This is true regardless of the
@@ -645,14 +645,12 @@ dept of the encapsulated `Tensor` as the function used internally to do so is re
 Example 1:
 ```lua
 > input = {torch.randn(2, 3), torch.randn(2, 1)}
-                                                                      [0.0002s]
 > =nn.SelectTable(1):forward(input)
 -0.3060  0.1398  0.2707
  0.0576  1.5455  0.0610
 [torch.DoubleTensor of dimension 2x3]
 
-                                                                      [0.0002s]
-> =nn.SelectTable(2):forward(input)
+> =nn.SelectTable(-1):forward(input)
  2.3080
 -0.2955
 [torch.DoubleTensor of dimension 2x1]
diff --git a/test.lua b/test.lua
index 82aefc8..bfece0e 100644
--- a/test.lua
+++ b/test.lua
@@ -2964,8 +2964,20 @@ function nntest.SelectTable()
       equal(gradInput[idx], gradOutputs[idx], "gradInput[idx] dimension " .. idx)
       equal(gradInput[nonIdx[idx]], zeros[nonIdx[idx]], "gradInput[nonIdx] dimension " .. idx)
    end
-   module:float()
+   
+   -- test negative index
+   local idx = -2
+   module = nn.SelectTable(idx)
+   local output = module:forward(input)
+   equal(output, input[#input+idx+1], "output dimension " .. idx)
+   local gradInput = module:backward(input, gradOutputs[#input+idx+1])
+   equal(gradInput[#input+idx+1], gradOutputs[#input+idx+1], "gradInput[idx] dimension " .. idx)
+   equal(gradInput[nonIdx[#input+idx+1]], zeros[nonIdx[#input+idx+1]], "gradInput[nonIdx] dimension " .. idx)
+   
+   -- test typecast
    local idx = #input
+   module = nn.SelectTable(idx)
+   module:float()
    local output = module:forward(input)
    equal(output, input[idx], "type output")
    local gradInput = module:backward(input, gradOutputs[idx])