Here's a quick rundown on the status quo: * Colon lowering changed in 0.4, as did the AbstractArray's interface. It's *much* easier to make your own arrays in 0.4. I suggest updating to the latest RC.
* There is still a very strong assumption in base Julia that the valid indices in an AbstractArray `A` for dimension `d` are in `1:size(A, d)`. * This means that once a base method has ensured that all the indices it visits are within `1:size(A,d)`, it might wrap its algorithm with `@inbounds`. * This means that if you break that assumption, using a base function could result in the wrong answer and/or memory corruption and/or segfaults. * It also means that the `end` keyword won't lower correctly for your array Now, it is possible to break this assumption and still have things work, but for right now, it's undocumented magic. There's several of us very interested in doing this, so it'll get better. Also new in 0.4 is the eachindex method, which helps here, too. The key is to aggressively check bounds and explicitly error on any indexing method that you know might be ambiguous in its meaning (e.g., does the caller know that your array has funny indexing semantics or not?). Related discussion: https://groups.google.com/forum/#!searchin/julia-users/offsetarray%7Csort:date/julia-users/fNisYpMdZ6o/tahp5AXaAwAJ On Sunday, September 20, 2015 at 9:34:24 PM UTC-4, Greg Plowman wrote: > > Hi, > > I'm trying to define a custom Array type that can be indexed using > arbitrary ranges. > > e.g. A = MyArray(Int, 3:8) would define a 6-element vector with indexes > ranging from 3 to 8, rather than the default 1 to 6. > > I've made some progress, but am now stuck on how to handle colon indexing. > > A[4:6] works by defining appropriate getindex and setindex! > > e.g. setindex!{T,S<:Real}(A::MyArray{T,1}, value, I::AbstractVector{S}) > = ... > > but A[:] = 0 seems to get translated to A[1:6] before dispatch on > setindex!, so I can't hijack the call. > > From subarray.jl, the code below suggests I can specialise on the Colon > type, but this doesn't seem to work for me. Colon appears to be converted > to UnitRange *before* calling setindex! > > sub(A::AbstractArray, I::Union(RangeIndex, Colon)...) = sub(A, ntuple( > length(I), i-> isa(I[i], Colon) ? (1:size(A,i)) : I[i])...) > > > Is there a way around this? > Should I be able to specialise on the colon argument? > > -- Greg >
