Using `append!` instead of `push!` and letting efficient dynamic
reallocation of vector do the resizing:
function vcat_nosplat2a(y)
result = Array(eltype(y[1]), 0)
for a in y
append!(result, a)
end
result
end
(@benchmark shows way less allocations and 2-3x time)
BTW the splat version is just as quick, but perhaps allocation on stack is
problematic (anybody check the limit?)
On Monday, November 23, 2015 at 12:07:18 PM UTC+2, Tomas Lycken wrote:
>
> That doesn’t quite seem to do what you want, Mauro:
>
> julia> arr_of_arr = Vector{Int}[[1],[2,3],[4,5]]
> 3-element Array{Array{Int64,1},1}:
> [1]
> [2,3]
> [4,5]
>
> julia> vcat_nosplat(y) = eltype(y[1])[el[1] for el in y]
> vcat_nosplat (generic function with 1 method)
>
> julia> vcat_nosplat(arr_of_arr)
> 3-element Array{Int64,1}:
> 1
> 2
> 4
>
> It’s quite trivial to achieve the desired result with only a few lines of
> code, though:
>
> julia> function vcat_nosplat2(y)
> result = Array(eltype(y[1]), 0)
> sizehint!(result, sum(map(length, y))) #skip if iterating is more
> expensive than reallcoation
>
> for a in y
> for x in a
> push!(result, x)
> end
> end
>
> result
> end
> vcat_nosplat2 (generic function with 1 method)
>
> julia> vcat_nosplat2(arr_of_arr)
> 5-element Array{Int64,1}:
> 1
> 2
> 3
> 4
> 5
>
> // T
>
> On Sunday, November 22, 2015 at 9:12:55 PM UTC+1, Mauro wrote:
>
> In ODE.jl, I've used
>>
>> vcat_nosplat(y) = eltype(y[1])[el[1] for el in y] # Does vcat(y...)
>> without the splatting
>>
>> I think the eltype might not be needed. There may be better ways though.
>>
>> On Sun, 2015-11-22 at 14:04, Cedric St-Jean <[email protected]> wrote:
>> > I have a big vector of vectors. Is there any way to vcat/hcat them
>> without
>> > splatting?
>> >
>> > arr_of_arr = Vector[[1],[2,3],[4,5]]
>> > vcat(arr_of_arr...)
>> >
>> > I'm asking because splatting big arrays is a performance issue (and
>> IIRC it
>> > blows the stack at some point).
>>
>
>
