I came across these two lines (back-to-back) of code in a torch project:
im4[{1,{},{}}] = im3[{3,{},{}}]
im4[{3,{},{}}] = im3[{1,{},{}}]
What do these two lines do? I assumed they did some sort of swapping.
This is covered in indexing in the Torch Tensor Documentation
Indexing using the empty table {} is shorthand for all indices in that dimension. Below is a demo which uses {} to copy an entire row from one matrix to another:
> a = torch.Tensor(3, 3):fill(0)
0 0 0
0 0 0
0 0 0
> b = torch.Tensor(3, 3)
> for i=1,3 do for j=1,3 do b[i][j] = (i - 1) * 3 + j end end
> b
1 2 3
4 5 6
7 8 9
> a[{1, {}}] = b[{3, {}}]
> a
7 8 9
0 0 0
0 0 0
This assignment is equivalent to: a[1] = b[3].
Your example is similar:
im4[{1,{},{}}] = im3[{3,{},{}}]
im4[{3,{},{}}] = im3[{1,{},{}}]
which is more clearly stated as:
im4[1] = im3[3]
im4[3] = im3[1]
The first line assigns the values from im3's third row (a 2D sub-matrix) to im4's first row and the second line assigns the first row of im3 to the third row of im4.
Note that this is not a swap, as im3 is never written and im4 is never read from.