SVector(1, 2, 3)[1:2] isa Array

[tkf]

It seems that indexing with UnitRange and StepRange turns a SArray to an Array.

julia> SVector(1, 2, 3)[1:2]
2-element Array{Int64,1}:
 1
 2

julia> SVector(1, 2, 3)[1:2:2]
1-element Array{Int64,1}:
 1

Tried it with current master (b454bc9) and Julia 0.6.2 and 0.7 (3a47cd4b2e).

[andyferris]

That’s intentional, because we can’t statically determine the size of the output matrix it is fastest to treat it as a dynamically allocates array.

You can index with another static array, if that helps.

[tkf]

Thanks. But how about dynamically calculate the length and then use the same code as in inds::StaticVector? Is it still slower than going to Array?

import Base: getindex
import StaticArrays: _getindex
using Base: @propagate_inbounds, @_propagate_inbounds_meta
using StaticArrays

const Indexer = Union{StaticVector{<:Any, Int},
                      OrdinalRange{<:Int, <:Int}}

@propagate_inbounds function getindex(a::StaticArray, inds::OrdinalRange{<:Int, <:Int})
    _getindex(a, Length(length(inds)), inds)
end

# getindex for inds::StaticVector is the same as in the current master:
@propagate_inbounds function getindex(a::StaticArray, inds::StaticVector{<:Any, Int})
    _getindex(a, Length(inds), inds)
end

@generated function _getindex(a::StaticArray, ::Length{L}, inds::Indexer) where {L}
    exprs = [:(a[inds[$i]]) for i = 1:L]
    return quote
        @_propagate_inbounds_meta
        similar_type(a, Size(L))(tuple($(exprs...)))
    end
end


@assert SVector(1, 2, 3)[1:2] == SVector(1, 2)
@assert SVector(1, 2, 3)[end:-1:1] == SVector(3, 2, 1)

[tkf]

OK. I run a quick benchmark and going to the standard array is a lot faster, even though it takes some time to do GC. But I guess actually it makes sense since each inds[$i] call is done separately without utilizing any kind of continuity.

But I suppose range-based indexing to SArray will come once you define static ranges, as mentioned here?:
#223 (comment)

Anyway, I'm closing it since it was just my luck of understanding of StaticArrays.