# Preallocating Vector of Vectors

if you know the total size ahead of time, you can use:

basically you store the data contiguously in-memory, and when you set/get index, you just “view”, this should save a lot of memory in the limit of having a lot of inner vectors

You want something more flexible than a multidimensional array provides. With individual vectors you can grow the vectors separately for example. And as (almost) always, you have to pay for flexibility with performance.

Why is that an issue?

Thank you! Unfortunately, this causes a pointer issue:

``````c = makearr3(Float64, Nrow, Ncol) #Vector{Vector{Float64}} with 1000 elements
c[1][1] #NaN
c[2][1] #NaN
c[1][1] = 123. #123.0
c[2][1] #123.0, not NaN

``````
1 Like

``````function makearr2(T, Nrow, Ncol)
[ Vector{T}(undef, Ncol) for _ in 1:Nrow ]
end
``````

? That doesn’t have the pitfall of using the exact same (`===`) vector for all indices.

I have to say, I’m also curious - why are views not ok?

This is the same function as the the second function in my post, right? Apologies if I oversee something.

In the actual code, I define functions on the inner array - usually there are already `@views` involved, and I also want to be sure that the type of the inner array matches the original type for safey reasons.

I will have a look at `ArraysOfArrays.jl`, and otherwise I just take the additional time as flexibility costs.

It is. The reason it doesn’t get much more concise is because you want to express that you have a container of containers, with each container being

1. distinct
2. able to grow/shrink freely without impacting other vectors

A multidimensional matrix doesn’t have those restrictions, so it’s layout/construction can be simpler. Down in the weeds this means that a matrix can just be a single big chunk of memory, because each row/column has the same size as any other, whereas a vector of vectors has disjoint blocks of memory for each vector, to allow each object to live independently.

4 Likes

Thanks! I think I should copy-paste your comment to the relevant struct field so I never forget why I chose that specific Array format :D.

Just to add that, in these cases, use `push!` to increase the arrays (do not reallocate the whole stuff). `push!` can be quite efficient, because the vectors are saved with some extra space and pushing new data only needs to do reallocations eventually.

``````julia> x = [[1,2],[3,4]];

julia> @allocated push!(x[1],3)
48

julia> @allocated push!(x[1],3)
0

julia> @allocated push!(x[1],3)
80

julia> @allocated push!(x[1],3)
0

julia> @allocated push!(x[1],3)
0

julia> @allocated push!(x[1],3)
0

julia> @allocated push!(x[1],3)
144

julia> x
2-element Vector{Vector{Int64}}:
[1, 2, 3, 3, 3, 3, 3, 3, 3]
[3, 4]

``````

Thanks
If not mistaken, writing the loop instead of the comprehension seems to be faster & allocate less (Julia 1.7):

``````function makearr3(T, Nrow, Ncol)
w = Vector{Vector{T}}(undef, Ncol)
for i in 1:Ncol
w[i] =  Vector{T}(undef, Nrow)
end
return w
end
``````

I don’t see that:

``````julia> function makearr3(T, Nrow, Ncol)
w = Vector{Vector{T}}(undef, Ncol)
for i in 1:Ncol
w[i] =  Vector{T}(undef, Nrow)
end
return w
end
makearr3 (generic function with 1 method)

julia> @btime makearr3(Float64, 500, 1000);
395.621 μs (1491 allocations: 3.98 MiB)

julia> function makearr2(T, Nrow, Ncol)
[ Vector{T}(undef, Ncol) for _ in 1:Nrow ]
end
makearr2 (generic function with 1 method)

julia> @btime makearr2(Float64, 500, 1000);
191.369 μs (1000 allocations: 3.89 MiB)

``````

What does improve the performance is to allow the function to specialize for the type of element being created:

``````julia> function makearr4(::Val{T}, Nrow, Ncol) where T
[ Vector{T}(undef, Ncol) for _ in 1:Nrow ]
end
makearr4 (generic function with 1 method)

julia> @btime makearr4(Val(Float64), 500, 1000);
81.485 μs (501 allocations: 3.88 MiB)

``````

Odd.
Here running Julia 1.7 on Win10 laptop:

``````using BenchmarkTools
@btime makearr1(Float64, \$Nrow, \$Ncol) #   3.9 μs (2 allocations: 3.81 MiB)
@btime makearr2(Float64, \$Nrow, \$Ncol) # 181.3 μs (1003 allocations: 3.89 MiB)
@btime makearr3(Float64, \$Nrow, \$Ncol) #  59.8 μs (501 allocations: 3.88 MiB)
``````

`makearr2` and `makearr3` are not equivalent. One creates Nrow vectors of length Ncol. The other is switched.

There are two different `makearr2`'s in this thread.

2 Likes

Interesting. Here, with 1.7, the loop version is improved (considering the correction in `makearray3` pointed above):

``````
julia> @btime makearr2(Float64, 500, 1000);
195.416 μs (1000 allocations: 3.89 MiB)

julia> @btime makearr3(Float64, 500, 1000);
83.967 μs (501 allocations: 3.88 MiB)

julia> @btime makearr4(Val(Float64), 500, 1000);
85.357 μs (501 allocations: 3.88 MiB)

``````

In Julia 1.6 I have:

``````julia> @btime makearr3(Float64, 500, 1000);
189.542 μs (1001 allocations: 3.89 MiB)

``````

Thus, the gain associated to the specialization in 1.6 is given in 1.7 for some reason.

1 Like

Using the original OP’s:

``````function makearr2(T, Nrow, Ncol)
return [Vector{T}(undef, Nrow) for _ in 1:Ncol]
end
``````

I think the most idiomatic would be `::Type{T}`, though it probably doesn’t make any performance difference.

1 Like

I have the suspicion this is variation of our recent discussion How to do custom allocators. ArrayOfArrays.jl probably uses some custom allocators, too.

If your vectors all have the same size, you could try zeros(m,n) and then go with
Zero-Cost Abstractions in Julia: Indexing Vectors by Name with LabelledArrays - Stochastic Lifestyle

1 Like

This is a really good tip with `push!`, and I would not have expected that if I hadnt seen that. Thank you!

1 Like

I ended up with

``````function VectorOfVector(::Type{T}, Nrow::Integer, Ncol::Integer) where {T}
return [Vector{T}(undef, Nrow) for _ in Base.OneTo(Ncol)]
end
``````

which is basically the same as your version. Thank you!

2 Likes