# End keyword in creating CartesianIndex

**URL:** https://discourse.julialang.org/t/end-keyword-in-creating-cartesianindex/30461
**Category:** Internals & Design
**Tags:** question, array
**Created:** [October 29, 2019, 8:57pm UTC](https://discourse.julialang.org/t/end-keyword-in-creating-cartesianindex/30461 "2019-10-29T20:57:08Z")
**Posts on this page:** 1
**Showing post:** 2

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### Author: ![mbauman](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/mbauman/32/31082_2.png) [@mbauman](https://discourse.julialang.org/u/mbauman)
#### Post date: [October 29, 2019, 9:16pm UTC](https://discourse.julialang.org/t/end-keyword-in-creating-cartesianindex/30461/2 "2019-10-29T21:16:17Z")

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The basic technique Keno was referencing is something that’s generally quite feasible, and I think there is a package _somewhere_ that does it (but I cannot find it at the moment).

```nohighlight
julia> struct End end

julia> Base.to_indices(A, inds, I::Tuple{End, Vararg}) = (inds[1][end], to_indices(A, Base.tail(inds), Base.tail(I))...)

```

With just those two definitions, we can now use `End()` in many places that didn’t use to be possible:

```nohighlight
julia> A = rand(3,5)
3×5 Array{Float64,2}:
 0.0496429 0.1541 0.178673 0.987561 0.100913
 0.273997 0.984579 0.255768 0.862814 0.200435
 0.317385 0.445612 0.106676 0.00631464 0.934804

julia> A[End(), 2]
0.4456115553735922

julia> view(A, End(), 2:3)
2-element view(::Array{Float64,2}, 3, 2:3) with eltype Float64:
 0.4456115553735922
 0.10667647668776192

```

Now, the trouble you’ll rapidly run into is that you cannot do computation on this `End()` thing.

```nohighlight
julia> A[End() - 1, :]
ERROR: MethodError: no method matching -(::End, ::Int64)

```

So you can manually add in “deferred” computations, essentially allowing it to build up a thunk that it can evaluate once it knows the size of the array. This is the huge advantage to the `end` keyword — it is just syntax for the appropriate `lastindex` function call, which returns a _real_ integer on which you can do real computation.

* * *

Now this finally brings us to your actual question, which is can you use this thing with a `CartesianIndex`? The answer there is a flat no. `CartesianIndex` is hardcoded to only handle `Int`s. But the game isn’t over — you can of course construct your own type that will work _just like_ CartesianIndex but allow for more things to be stored in it. It’s just a bit more complicated, but that [`to_indices`](https://docs.julialang.org/en/v1/base/arrays/#Base.to_indices) is the only API that you need to extend.

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