# Question about eachindex behavior in DenseAxisArray in JuMP

**URL:** <https://discourse.julialang.org/t/question-about-eachindex-behavior-in-denseaxisarray-in-jump/119333>\
**Category:** Optimization (Mathematical)\
**Tags:** question\
**Created:** [September 12, 2024, 3:33pm UTC](https://discourse.julialang.org/t/question-about-eachindex-behavior-in-denseaxisarray-in-jump/119333 "2024-09-12T15:33:29Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![guilhermebodin](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/guilhermebodin/32/6982_2.png) [@guilhermebodin](https://discourse.julialang.org/u/guilhermebodin)\
**Post date:** [September 12, 2024, 3:33pm UTC](https://discourse.julialang.org/t/question-about-eachindex-behavior-in-denseaxisarray-in-jump/119333/1 "2024-09-12T15:33:29Z")

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This is more of a question of whether this is expected behavior or a corner case that should not work correctly anyway.

When we run this code the access of indices through the use of `eachindex` in dense axis array is not working as I would expect.

```julia
using JuMP
m = Model()
aux = [[1,2,3,4],[2,3],[]]
@variable(m, sparse_var[i = 1:3, j in aux[i]])
@variable(m, dense_var[i = 1:3, j in [1, 2, 4]])
eachindex(sparse_var)
eachindex(dense_var)

(1, 4) in eachindex(sparse_var) # true and I expected it to be true
(1, 4) in eachindex(dense_var) # false but I expected it to be true

# some debug on why it does not work as I was expecting
dense_var[1, 4] == dense_var[CartesianIndex(1, 3)]

```

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<div class="post-metadata">

**Author:** ![odow](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/odow/32/28685_2.png) [@odow](https://discourse.julialang.org/u/odow)\
**Post date:** [September 12, 2024, 9:35pm UTC](https://discourse.julialang.org/t/question-about-eachindex-behavior-in-denseaxisarray-in-jump/119333/2 "2024-09-12T21:35:13Z")

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I don’t know if `eachindex` makes any claims about the eltype of the iterator that it returns.

In this case, `sparse_var` returns the keys of the underlying dictionary:

```julia
julia> eachindex(sparse_var)
KeySet for a OrderedCollections.OrderedDict{Tuple{Int64, Int64}, VariableRef} with 6 entries. Keys:
  (1, 1)
  (1, 2)
  (1, 3)
  (1, 4)
  (2, 2)
  (2, 3)

```

whereas `dense_var` returns the Cartesian indices

```julia
julia> eachindex(dense_var)
CartesianIndices((3, 3))

```

So `(1, 4) in eachindex(dense_var)` is false, because they are different types.

One re-occuring problem is that we’ve tried to make it so that `sparse_var` and `dense_var` behave similarly. But there are actually many cases where there are subtle differences. Trying to write code that works for both `DenseAxisArray` and `SparseAxisArray` is difficult. We probably should have defaulted `sparse_var` to return a `Dict`. That’s what it is under the hood…