# How to get the Cartesian indices of a row/column in a \`Matrix\`?

**URL:** <https://discourse.julialang.org/t/how-to-get-the-cartesian-indices-of-a-row-column-in-a-matrix/91940>\
**Category:** General Usage\
**Tags:** indexing, array, cartesianindices\
**Created:** [December 21, 2022, 9:42am UTC](https://discourse.julialang.org/t/how-to-get-the-cartesian-indices-of-a-row-column-in-a-matrix/91940 "2022-12-21T09:42:27Z")\
**Posts on this page:** 1\
**Showing post:** 2

<div class="post-metadata">

**Author:** ![singularitti](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/singularitti/32/17678_2.png) [@singularitti](https://discourse.julialang.org/u/singularitti)\
**Post date:** [December 21, 2022, 9:59am UTC](https://discourse.julialang.org/t/how-to-get-the-cartesian-indices-of-a-row-column-in-a-matrix/91940/2 "2022-12-21T09:59:33Z")

</div>

I guess `axes` does work (as suggested in [Eachindex for individual axes of multidimensional arrays?](https://discourse.julialang.org/t/eachindex-for-individual-axes-of-multidimensional-arrays/81264)), even for `OffsetArrays`:

```julia
 using OffsetArrays: OffsetMatrix, Origin

julia> a = OffsetMatrix(rand(4, 4), Origin(0))
4×4 OffsetArray(::Matrix{Float64}, 0:3, 0:3) with eltype Float64 with indices 0:3×0:3:
 0.555166 0.773755 0.441709 0.337798
 0.201427 0.542714 0.626299 0.518176
 0.998011 0.735479 0.894207 0.99043
 0.826478 0.261973 0.890299 0.484077

julia> for j in axes(a, 2)
           @show CartesianIndex(firstindex(a, 1), j)
       end
CartesianIndex(firstindex(a, 1), j) = CartesianIndex(0, 0)
CartesianIndex(firstindex(a, 1), j) = CartesianIndex(0, 1)
CartesianIndex(firstindex(a, 1), j) = CartesianIndex(0, 2)
CartesianIndex(firstindex(a, 1), j) = CartesianIndex(0, 3)

julia> for i in axes(a, 1)
           @show CartesianIndex(i, lastindex(a, 2))
       end
CartesianIndex(i, lastindex(a, 2)) = CartesianIndex(0, 3)
CartesianIndex(i, lastindex(a, 2)) = CartesianIndex(1, 3)
CartesianIndex(i, lastindex(a, 2)) = CartesianIndex(2, 3)
CartesianIndex(i, lastindex(a, 2)) = CartesianIndex(3, 3)

```

Also, using `CartesianIndices` seems to be simpler and more consistent with my example:

```julia
julia> CartesianIndices(a)[begin, :]
4-element OffsetArray(::Vector{CartesianIndex{2}}, 0:3) with eltype CartesianIndex{2} with indices 0:3:
 CartesianIndex(0, 0)
 CartesianIndex(0, 1)
 CartesianIndex(0, 2)
 CartesianIndex(0, 3)

julia> CartesianIndices(a)[:, end]
4-element OffsetArray(::Vector{CartesianIndex{2}}, 0:3) with eltype CartesianIndex{2} with indices 0:3:
 CartesianIndex(0, 3)
 CartesianIndex(1, 3)
 CartesianIndex(2, 3)
 CartesianIndex(3, 3)

```

Am I using the correct way to do it?

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