[Torch5-devel] apply() and torch5-devel mailing list

[Ronan C. <ro...@co...>]

Hey

---- disclaimer

i created a torch5-devel list. so if you want to discuss something on 
the development of torch, write here. new added features should also be 
advertised here. i will not send any mail personally anymore, so i 
strongly encourage you to subscribe to the list if you are interested by 
this kind of news.

https://lists.sourceforge.net/lists/listinfo/torch5-devel

---- end of disclaimer

   I added in lab package the lab.apply() function.

Suppose you have a 1000x10000 random tensor and you want to apply a tanh 
on it. You can:

x = lab.rand(1000,10000)

-- superfast, takes 0.7s
z = lab.tanh(x)

-- superslow, takes 44.7s
z = torch.Tensor():resizeAs(x)
for i=1,x:size(1) do
    for j=1,x:size(2) do
       z[i][j] = math.tanh(x[i][j])
    end
end

-- superslow, takes 15.5s
-- we can do it here, because we know the tensor is contiguous in memory
z = torch.Tensor():resizeAs(x)
local xs = x:storage()
local zs = z:storage()
for i=1,xs:size() do
    zs[i] = math.tanh(xs[i])
end

-- supercool, takes 1.4s
z = lab.apply(x, math.tanh)


So this lab.apply() function is extremely efficient (considering it does 
lua calls underneath) and should be used instead of loops whenever you 
can. (well, except of course if an optimized C function already exists 
like for tanh())

Ronan.