Preallocating Vector of Vectors

[rafael.guerra]

@Jeff_Emanuel

There are two different makearr2 's in this thread.

Using the original OP’s:

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

[DNF]

@lmiq

function makearr4(::Val{T},

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

[goerch]

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

[Bardo]

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

[mrVeng]

@lmiq

function makearr4(::Val{T}, Nrow, Ncol) where T
           [ Vector{T}(undef, Ncol) for _ in 1:Nrow ]
       end

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

[mrVeng]

@lmiq

Val(Float64)

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!