# Slicing and merging of CartesianIndices

Hello,

Let’s assume I have a two-dimensional array and the list of Cartesian indices which allow me to extract some subset of the array elements:

``````A = zeros((6, 9))

ci = CartesianIndices((2:2:6, 3:3:9))

A[ci]
``````

Now, having `ci`, how I can extract the Cartesian indices which correspond separately to the first and second dimensions of the array? If I will define them manually, they will be

``````ci1 = CartesianIndices((2:2:6,))
ci2 = CartesianIndices((3:3:9,))

A[ci1, :]
A[:, ci2]
``````

And the other way around, having `ci1` and `ci2`, how I can merge them into the original `ci`?
The last question is related to my earlier post https://discourse.julialang.org/t/combine-cartesianindices-for-effective-cuda-kernels/

Thank you.

You can do use `ci.indices` though I think that’s an undocumented internal:

``````ci1 = CartesianIndices(ci.indices)
ci2 = CartesianIndices(ci.indices)
``````

You can get the same information with `first(ci)`, `step(ci)`, `last(ci)` but that’s less convenient.

To recover the full list:

``````CartesianIndices((ci1.indices, ci2.indices))
``````

Another idea using arrays of `CartesianIndex` rather than `CartesianIndices`:

``````julia> i1 = getindex.(ci[:, 1], 1)
3-element Vector{Int64}:
2
4
6

julia> i2 = getindex.(ci[1, :], 2)
3-element Vector{Int64}:
3
6
9

julia> CartesianIndex.(i1, i2')
3×3 Matrix{CartesianIndex{2}}:
CartesianIndex(2, 3)  CartesianIndex(2, 6)  CartesianIndex(2, 9)
CartesianIndex(4, 3)  CartesianIndex(4, 6)  CartesianIndex(4, 9)
CartesianIndex(6, 3)  CartesianIndex(6, 6)  CartesianIndex(6, 9)
``````
1 Like

Thank you.
`indices` is exactly what I need.