# Store a zero in an sparse matrix
**URL:** https://discourse.julialang.org/t/store-a-zero-in-an-sparse-matrix/68515
**Category:** New to Julia
**Tags:** sparse
**Created:** [September 21, 2021, 9:37am UTC](https://discourse.julialang.org/t/store-a-zero-in-an-sparse-matrix/68515 "2021-09-21T09:37:55Z")
**Posts on this page:** 6
**Page:** 1
### Author:  [@carlomontec](https://discourse.julialang.org/u/carlomontec)
#### Post date: [September 21, 2021, 9:37am UTC](https://discourse.julialang.org/t/store-a-zero-in-an-sparse-matrix/68515/1 "2021-09-21T09:37:55Z")
Hello,
do you know how can I store a zero in an sparse matrix? it seems that it automatically neglects the zero allocation: `A[1,1]=0.0` is neglected. I know is far from efficient, but the need of it is to debug and replicate an old FORTAN code I am working with.
Thank you.
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### Author:  [@Sukera](https://discourse.julialang.org/u/Sukera)
#### Post date: [September 21, 2021, 9:44am UTC](https://discourse.julialang.org/t/store-a-zero-in-an-sparse-matrix/68515/2 "2021-09-21T09:44:18Z")
The point of sparse matrices is not to store zeros, so it’s surprising to me that you’d get a difference based on whether fortran sets an explicit zero - maybe fortran is storing explicitly set zeros? Can you maybe show some more info about what you’re trying to replicate?
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### Author:  [@ettersi](https://discourse.julialang.org/u/ettersi)
#### Post date: [September 21, 2021, 9:59am UTC](https://discourse.julialang.org/t/store-a-zero-in-an-sparse-matrix/68515/3 "2021-09-21T09:59:47Z")
It turns out that `getindex(::SparseMatrixCSC, v, i,j)` first checks whether `A[i,j]` is structurally nonzero, and if it isn’t then it turns it into a structural nonzero only if `v` is nonzero, see [https://github.com/JuliaLang/julia/blob/4f3a89e3abecb96bbe221d027278c55dccf23da7/stdlib/SparseArrays/src/sparsematrix.jl#L2634-L2665](https://github.com/JuliaLang/julia/blob/4f3a89e3abecb96bbe221d027278c55dccf23da7/stdlib/SparseArrays/src/sparsematrix.jl#L2634-L2665).
For debugging purposes, you can work around this by first making `A[i,j]` nonzero and then zeroing it out again:
```julia
julia> A = spzeros(3,3);
julia> A[2,2] = 0.0; # This doesn't work
julia> A
3×3 SparseMatrixCSC{Float64, Int64} with 0 stored entries:
⋅ ⋅ ⋅
⋅ ⋅ ⋅
⋅ ⋅ ⋅
julia> A[2,2] = 1.0; # This will make A[2,2] structurally nonzero
julia> A
3×3 SparseMatrixCSC{Float64, Int64} with 1 stored entry:
⋅ ⋅ ⋅
⋅ 1.0 ⋅
⋅ ⋅ ⋅
julia> A[2,2] = 0.0; # Now we get where we wanted
julia> A
3×3 SparseMatrixCSC{Float64, Int64} with 1 stored entry:
⋅ ⋅ ⋅
⋅ 0.0 ⋅
⋅ ⋅ ⋅
```
* * *
To add my unsolicited five cents, I’m not sure whether special-casing `v == 0` in `setindex()` is good design. But it is what it is and is highly unlikely to change anytime soon.
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### Author:  [@carlomontec](https://discourse.julialang.org/u/carlomontec)
#### Post date: [September 21, 2021, 10:00am UTC](https://discourse.julialang.org/t/store-a-zero-in-an-sparse-matrix/68515/4 "2021-09-21T10:00:32Z")
I am aware that is the whole point of a sparse matrix, but in this case after some coordinate transformations I have as a side product some zeros. My intention is to replicate and compare some sparsity maps as I have in the FORTRAN code.
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### Author:  [@carlomontec](https://discourse.julialang.org/u/carlomontec)
#### Post date: [September 21, 2021, 10:03am UTC](https://discourse.julialang.org/t/store-a-zero-in-an-sparse-matrix/68515/5 "2021-09-21T10:03:22Z")
Thanks for the idea!
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### Author:  [@Bardo](https://discourse.julialang.org/u/Bardo)
#### Post date: [September 22, 2021, 1:59pm UTC](https://discourse.julialang.org/t/store-a-zero-in-an-sparse-matrix/68515/6 "2021-09-22T13:59:12Z")
You might use `eps()` as your structural nonzero.