Getindex for AbstractArray with Function

New to Julia
1

There is a AbstractArray:

m=[[1,2,3] [2,3,4]]

"""
3×2 Matrix{Int64}:
 1  2
 2  3
 3  4

"""

How can I dispath on Function to Base.getindex to have this custom method?

m[>=(2),:]

"""
1×2 Matrix{Int64}:

 2  3
 3  4

"""


m[:,>=(1)]

m[[1,2], >=(1)]

etc.....
2

Hi! It seems to me that you want to do something like

m[2:end, :]

is that correct?

1 Like
3

yes, this can get the same result.

But above is just a simple example, I have many more compcated situation, which have the similar style to that.

4

Maybe you could engineer something with findall instead of overloading getindex?

5

findall is good, but not elegent. With methods mentioned above, I can:

Base.getindex(x::M,i::F) where T=x[i.(@view(x[:,1])),:]

Base.getindex(x::V,i::F)=x[i.(x)]

Base.getindex(x::M,i::Va{Pair})=begin
    mask=trues(size(x,1))
    for (k,v) in i
        mask = mask .&& (@view(x[:,k]) .|> v)
    end
    x[mask,:]

end

m[>=(2)]

"""
2×2 Matrix{Int64}:
 2  3
 3  4

"""

m[1=>>=(2),2=>==(3)]

"""
1×2 Matrix{Int64}:
 2  3
"""
6

It’s hard to get a grip on what you are trying to do. For example,

julia> m[2:end, :]
2×2 Matrix{Int64}:
 2  3
 3  4

julia> m[m[:, 1].>=2, :]
2×2 Matrix{Int64}:
 2  3
 3  4

neither of which are the same. Can you explain what x[>=(2), :] is supposed to do? Why does it return [2 3]?

1 Like
7

Sorry for that, I made a mistake, It is supposed to

8

So you are looking for calculations on the indices, not on the values when selecting rows?

Maybe something like

m[foo.(axes(m, 1)), :]

for a function foo that returns a Bool. If you are working on the values, not the indices, then

m[foo.(m[:, 1]), :]

maybe with a @view in front.

I would not recommend overloading getindex for Array and Function, since that would be type piracy. Instead, you could make your own function, maybe

selectrows(f::Function, m::AbstractMatrix) = m[f.(axes(m, 1)), :]

and then, selectrows(>=(2), m), etc.

2 Likes