Q: Vector of vectors of variable lengths

General Usage
, ,
1

Hello,

What is the recommended/best Julia method to create a vector of vectors of variable lengths?

Current method MWE:

# test_vector_of_vectors.jl

begin
    vec_of_vecs = [Float64[] for _ in 1:3]
    for i = 1:3
        push!(vec_of_vecs[1], 0.0)
    end
    for i = 1:5
        push!(vec_of_vecs[2], 1.0)
    end
    for i = 1:7
        push!(vec_of_vecs[3], 2.0)
    end

    @show vec_of_vecs
end

[In the actual code, the variable length vector are workspace arrays, , all initialized to 0.0, to all be passed to a set of functions.]

2

I bit more terse:

vec_of_vecs = [zeros(Float64,s*2+1) for s in 1:3]

If the individual sizes can be calculated from the index. If not I would do it more like:

sizes=[ 7, 3, 5 ]
vec_of_vecs = [zeros(Float64,sizes[s]) for s in 1:3]

Don’t know if this is recommended (by whom) or the best (in what sense). If it is important to have the best performance, because it’s perhaps in a hot loop, my guess is, complete performance depends more on the whole thing you want to do, the context.

3

Or, to set the values too:

vec_of_vecs = [ fill( Float64(s), s*2+1 ) for s in 1:3]
4

Or slightly more succinct:

vec_of_vecs = [zeros(Float64, s) for s in sizes]
3 Likes
5

Could even use broadcasting:

vec_of_vecs = zeros.(s) # default type is Float64
1 Like
6

Should be in the flow of examples:

vec_of_vecs = zeros.(sizes)
1 Like
7

Thanks to all who replied.

The broadcasting method suggested by abraemer and ohell is elegant.

# test_vector_of_vectors.jl

begin
    sizes = Int32[3, 6, 7]
    vec_of_vecs = zeros.(sizes)

    @show vec_of_vecs
end

3-element Vector{Vector{Float64}}:
 [0.0, 0.0, 0.0]
 [0.0, 0.0, 0.0, 0.0, 0.0, 0.0]
 [0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0]
1 Like