# Unification of \`SparseMatrixCSC\` and \`SparseVector\`

**URL:** <https://discourse.julialang.org/t/unification-of-sparsematrixcsc-and-sparsevector/20091>\
**Category:** Internals & Design\
**Created:** [January 25, 2019, 1:28pm UTC](https://discourse.julialang.org/t/unification-of-sparsematrixcsc-and-sparsevector/20091 "2019-01-25T13:28:07Z")\
**Posts on this page:** 5\
**Page:** 1

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**Author:** ![abraunst](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/abraunst/32/6880_2.png) [@abraunst](https://discourse.julialang.org/u/abraunst)\
**Post date:** [January 25, 2019, 1:28pm UTC](https://discourse.julialang.org/t/unification-of-sparsematrixcsc-and-sparsevector/20091/1 "2019-01-25T13:28:07Z")

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Are there plans (proposals, PRs, discussions) for unifying `SparseMatrixCSC` and `SparseVector`? Something like a new type `SparseArrayCSC{Tv,Ti,D}` where D is the dimension (`1` for `SparseVector`, `2` for `SparseMatrixCSC`). It seems like there is a large quantity of repeated code between these two types (as `SparseVector` is very close to an `Mx1` `SparseMatrixCSC`). Bonus point is that it would provide `D>2` types for free.

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**Author:** ![kristoffer.carlsson](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/kristoffer.carlsson/32/22_2.png) [@kristoffer.carlsson](https://discourse.julialang.org/u/kristoffer.carlsson)\
**Post date:** [January 25, 2019, 2:10pm UTC](https://discourse.julialang.org/t/unification-of-sparsematrixcsc-and-sparsevector/20091/2 "2019-01-25T14:10:50Z")

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While it might be possible to share more code between `SparseVector` and `SparseMatrixCSC`, I don’t think it will give `D>2` “for free” (since all code is written for either D=1 or D = 2).

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**Author:** ![abraunst](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/abraunst/32/6880_2.png) [@abraunst](https://discourse.julialang.org/u/abraunst)\
**Post date:** [January 25, 2019, 2:27pm UTC](https://discourse.julialang.org/t/unification-of-sparsematrixcsc-and-sparsevector/20091/3 "2019-01-25T14:27:53Z")

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Maybe I’m too optimistic, but it seems reasonable to me to imagine that code that is general enough for both `D=1` and `D=2` would work also for `D>2`. My naive idea would be to think about general `D` as a `D-1` dimension collection of sparse columns. So essentially it is only the iteration over `colptr` that needs to change. I think that the easiest would be to have `S.colptr::Vector{Ti}` with `length(S.colptr) == prod(S.dims[2:end])+1` and `size(S) = S.dims`. (alternatively, `S.colptr` could be an Array of dimension `D-1` but this has some drawbacks). But I don’t know if any of this has been already been discussed (and/or discarded!) and if there is potential interest in this.

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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:** [January 25, 2019, 3:45pm UTC](https://discourse.julialang.org/t/unification-of-sparsematrixcsc-and-sparsevector/20091/4 "2019-01-25T15:45:08Z")

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It’s something that I’ve considered this in the past, too: it’s effectively embedding a reshape into the CSC structure itself. I’ve tried to find our previous discussions but came up empty — perhaps it was on Slack. It’d work for some basic structural functions, but I think even the general indexing implementation wouldn’t generalize effectively.

Changing the structure of SparseMatrixCSC would be breaking, but adding general CSC-like-property-accessors would be a feature: [https://github.com/JuliaLang/julia/issues/26613](https://github.com/JuliaLang/julia/issues/26613). It’d then be easy to add those definitions for the compatible `reshape(::SparseMatrixCSC,...)`s.

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**Author:** ![abraunst](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/abraunst/32/6880_2.png) [@abraunst](https://discourse.julialang.org/u/abraunst)\
**Post date:** [January 26, 2019, 12:25pm UTC](https://discourse.julialang.org/t/unification-of-sparsematrixcsc-and-sparsevector/20091/5 "2019-01-26T12:25:35Z")

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Thanks @mbauman for the explanation. I’m not sure I understand though, you propose to add another type, instead of unifying those two?