What is the preferred way to evoke similar on MArrays?

[MilesCranmer]

I am 99% sure someone has posted about this before but I couldn't find it. Please link if you know.

I wrote generic code for AbstractArrays in my library, and use similar to construct arrays of the same container and eltype. All of the arrays I deal with are mutable.

However, it seems that there is weird behavior for MArray:

X = MMatrix{5,100}(rand(5, 100));

We can use similar just fine if we want the same array shape:

julia> typeof(similar(X))
MMatrix{5, 100, Float64, 500}

however, if I want a similar array to a slice of X, it gives me a regular Vector:

julia> typeof(similar(X, 100))
Vector{Float64}

If I try to use @view instead:

julia> typeof(similar(@view X[1, :]))
SizedVector{100, Float64, Vector{Float64}}

rather than an MArray. (Creating a copy with X[1, :] is too slow)

Why is this?

Thanks,
Miles

[MilesCranmer]

Okay it looks like I am able to get away with:

similar(X, axes(X, 2))

is this the right way to do it?

[mateuszbaran]

Yes, using axes is the preferred way to get a similar array of a different shape.

[MilesCranmer]

Thanks!