Specialized Base.findfirst for ranges

pnalysz · September 6, 2019, 8:34am

As far as I see at the moment julia uses the same findfirst method for all AbstractArray which simply iterates through all the items. Would it not make sense to have specialized method for ranges that would find the item’s index based on start/step/stop?

Thanks,
Piotr

Tamas_Papp · September 6, 2019, 8:58am

There is a method for StepRange but not for UnitRange:

github.com

JuliaLang/julia/blob/36c59c144122dbf09fc0e1f0f1e9defbbd2f254d/base/array.jl#L1760


    for (i, a) in pairs(A)
        testf(a) && return i
    end
    return nothing
end


# Needed for bootstrap, and allows defining only an optimized findnext method
findfirst(testf::Function, A::Union{AbstractArray, AbstractString}) =
    findnext(testf, A, first(keys(A)))


function findfirst(p::Union{Fix2{typeof(isequal),T},Fix2{typeof(==),T}}, r::StepRange{T,S}) where {T,S}
    first(r) <= p.x <= last(r) || return nothing
    d = convert(S, p.x - first(r))
    iszero(d % step(r)) || return nothing
    return d ÷ step(r) + 1
end


"""
    findprev(A, i)


Find the previous index before or including `i` of a `true` element of `A`,

so yes, it could make sense. If someone is interested, just make a PR.

pnalysz · September 6, 2019, 9:10am

Thank you, I overlooked it. Actually StepRange is enough for me.