How do I implement array indexing?

General Usage
1

I have this array type:

struct Delay{T}
	head_idx::Int
	arr::Vector{T}
	function Delay{T}(length::Int) where T
		if ispow2(length)
			arr = zeros(T,length)
			new(0, arr)
		else
			throw(ArgumentError("Length is not power of 2"))
		end
	end
end

and I implemented the getindex function

function getindex(A::Delay{T},idx) where T
	if idx > 0
		throw(ArgumentError("Only accepts negative indexes"))
	else
		return 1
	end
end

then I define test as

test = Delay{Float32}(8)

When I try to use test[1] it errors with ERROR: MethodError: no method matching getindex(::Delay{Float32}, ::Int64) but if I use getindex function directly it works

julia> getindex(test,-1)
1

What happened?

2

You need to explicitly import the Base.getindex function:


julia> function Base.getindex(A::Delay{T},idx) where T
               if idx > 0
                       throw(ArgumentError("Only accepts negative indexes"))
               else
                       return 1
               end
       end

julia> test[-1]
1

Equivalent to:

import Base.getindex
function getindex(...) ...
1 Like
3

It works! Thanks

Generalizing, what functions should I implement for my array type? In Python, there is this: 3. Data model — Python 3.10.6 documentation which states which methods that I should implement so that my class acts like a list, is there a documentation like this for Julia?

4

The page to read is this one, I think: Interfaces · The Julia Language

Depending on what you need to do with your data you might need only the indexing part, or something else.

2 Likes
5

If you do want it to be an array type, look at the AbstractArray section.

2 Likes