# How best to maintain collection of indices into 2-d array
**URL:** https://discourse.julialang.org/t/how-best-to-maintain-collection-of-indices-into-2-d-array/29921
**Category:** New to Julia
**Created:** [October 15, 2019, 3:53am UTC](https://discourse.julialang.org/t/how-best-to-maintain-collection-of-indices-into-2-d-array/29921 "2019-10-15T03:53:36Z")
**Posts on this page:** 4
**Page:** 1
### Author:  [@mpf01](https://discourse.julialang.org/u/mpf01)
#### Post date: [October 15, 2019, 3:53am UTC](https://discourse.julialang.org/t/how-best-to-maintain-collection-of-indices-into-2-d-array/29921/1 "2019-10-15T03:53:36Z")
I’d like to maintain a collection of index pairs (i,j) into a 2-dimensional array. The list must not contain any repeated elements, and elements can be added and deleted. `Set` seems to do the trick:
```julia
A = Set([(1,1),(3,3)])
union!(A, [(2,2),(3,3)]) # add only new element (2,2)
setdiff!(A, [(3,3)]) # remove item
B = rand(3,3)
let t = 0
for ind in A
i,j = ind
t += B[i,j]
end
end
```
Is this the best way to do this sort of thing in Julia?
---
### Author:  [@Tamas\_Papp](https://discourse.julialang.org/u/Tamas_Papp)
#### Post date: [October 15, 2019, 4:50am UTC](https://discourse.julialang.org/t/how-best-to-maintain-collection-of-indices-into-2-d-array/29921/2 "2019-10-15T04:50:43Z")
Probably yes. If you need other features of the collection (eg ordering, fallback defaults), check out
> **[GitHub - JuliaCollections/DataStructures.jl: Julia implementation of Data...](https://github.com/JuliaCollections/DataStructures.jl)**
>
> Julia implementation of Data structures. Contribute to JuliaCollections/DataStructures.jl development by creating an account on GitHub.
---
### Author:  [@Per](https://discourse.julialang.org/u/Per)
#### Post date: [October 15, 2019, 12:50pm UTC](https://discourse.julialang.org/t/how-best-to-maintain-collection-of-indices-into-2-d-array/29921/3 "2019-10-15T12:50:39Z")
Depending on the sparsity and what you’re using the indices for, it may be more efficient to use a `SparseMatrix{Bool}` than a `Set`.
```julia
using SparseArrays
A = sparse([1, 3], [1, 3], ones(Bool, 2))
A[2,2] |= true # add element
A[3,3] &= false # remove element
B = rand(3,3)
t = sum(B .* A) # will have a better memory access pattern than the above for-loops
```
---
### Author:  [@mpf01](https://discourse.julialang.org/u/mpf01)
#### Post date: [October 16, 2019, 12:09am UTC](https://discourse.julialang.org/t/how-best-to-maintain-collection-of-indices-into-2-d-array/29921/4 "2019-10-16T00:09:17Z")
Thanks for this suggestion. Your reply also pointed me to `|=` and `&=`, which are new to me.