# How do I implement array indexing?
**URL:** https://discourse.julialang.org/t/how-do-i-implement-array-indexing/51506
**Category:** General Usage
**Created:** [December 9, 2020, 9:04am UTC](https://discourse.julialang.org/t/how-do-i-implement-array-indexing/51506 "2020-12-09T09:04:36Z")
**Posts on this page:** 5
**Page:** 1
### Author:  [@minetest2048](https://discourse.julialang.org/u/minetest2048)
#### Post date: [December 9, 2020, 9:04am UTC](https://discourse.julialang.org/t/how-do-i-implement-array-indexing/51506/1 "2020-12-09T09:04:36Z")
I have this array type:
```julia
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
```julia
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
```julia
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
julia> getindex(test,-1)
1
```
What happened?
---
### Author:  [@lmiq](https://discourse.julialang.org/u/lmiq)
#### Post date: [December 9, 2020, 9:15am UTC](https://discourse.julialang.org/t/how-do-i-implement-array-indexing/51506/2 "2020-12-09T09:15:52Z")
You need to explicitly import the `Base.getindex` function:
```julia
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:
```julia
import Base.getindex
function getindex(...) ...
```
---
### Author:  [@minetest2048](https://discourse.julialang.org/u/minetest2048)
#### Post date: [December 9, 2020, 9:41am UTC](https://discourse.julialang.org/t/how-do-i-implement-array-indexing/51506/3 "2020-12-09T09:41:46Z")
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](https://docs.python.org/3/reference/datamodel.html#emulating-container-types) which states which methods that I should implement so that my class acts like a list, is there a documentation like this for Julia?
---
### Author:  [@lmiq](https://discourse.julialang.org/u/lmiq)
#### Post date: [December 9, 2020, 10:07am UTC](https://discourse.julialang.org/t/how-do-i-implement-array-indexing/51506/4 "2020-12-09T10:07:54Z")
The page to read is this one, I think: [Interfaces · The Julia Language](https://docs.julialang.org/en/v1/manual/interfaces/#Indexing)
Depending on what you need to do with your data you might need only the indexing part, or something else.
---
### Author:  [@Elrod](https://discourse.julialang.org/u/Elrod)
#### Post date: [December 9, 2020, 10:13am UTC](https://discourse.julialang.org/t/how-do-i-implement-array-indexing/51506/5 "2020-12-09T10:13:18Z")
If you do want it to be an array type, look at the [AbstractArray](https://docs.julialang.org/en/v1/manual/interfaces/#man-interface-array) section.