# Replace diagonals in existing spdiagm array

**URL:** <https://discourse.julialang.org/t/replace-diagonals-in-existing-spdiagm-array/102202>\
**Category:** General Usage\
**Tags:** sparsearrays\
**Created:** [July 28, 2023, 3:05pm UTC](https://discourse.julialang.org/t/replace-diagonals-in-existing-spdiagm-array/102202 "2023-07-28T15:05:41Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![dalarev](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/dalarev/32/9881_2.png) [@dalarev](https://discourse.julialang.org/u/dalarev)\
**Post date:** [July 28, 2023, 3:05pm UTC](https://discourse.julialang.org/t/replace-diagonals-in-existing-spdiagm-array/102202/1 "2023-07-28T15:05:41Z")

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Hi, wondering if there is an equivalent to the MATLAB method `spdiags(Bin, d, A)`, where the diagonals in `A` are replaced by those in `d` with the columns of `Bin`.

As far as I can tell, `spdiagsm` does not support a method that allows as input an array `A` to modify.

The context is finite-difference derivative matrices: one can initialize these to satisfy Dirichlet boundary conditions; implementing Periodic boundary conditions then requires only modifying one diagonal of this array.

Appreciate the support!

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**Author:** ![jishnub](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/jishnub/32/33620_2.png) [@jishnub](https://discourse.julialang.org/u/jishnub)\
**Post date:** [July 31, 2023, 2:42pm UTC](https://discourse.julialang.org/t/replace-diagonals-in-existing-spdiagm-array/102202/2 "2023-07-31T14:42:39Z")

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This won’t be the most efficient, but perhaps you may do something like

```julia
julia> S = spdiagm(-1 => [1,2,3,4], 1 => [4,3,2,1], 0=>1:5)
5×5 SparseMatrixCSC{Int64, Int64} with 13 stored entries:
 1 4 ⋅ ⋅ ⋅
 1 2 3 ⋅ ⋅
 ⋅ 2 3 2 ⋅
 ⋅ ⋅ 3 4 1
 ⋅ ⋅ ⋅ 4 5

julia> S[diagind(S)] .= reverse(1:5)
5-element view(reshape(::SparseMatrixCSC{Int64, Int64}, 25), 1:6:25) with eltype Int64:
 5
 4
 3
 2
 1

julia> S
5×5 SparseMatrixCSC{Int64, Int64} with 13 stored entries:
 5 4 ⋅ ⋅ ⋅
 1 4 3 ⋅ ⋅
 ⋅ 2 3 2 ⋅
 ⋅ ⋅ 3 2 1
 ⋅ ⋅ ⋅ 4 1

```

If you use `BandedMatrices`, you may use a `Band` for the indexing.

```julia
julia> S = spdiagm(-1 => [1,2,3,4], 1 => [4,3,2,1], 0=>1:5);

julia> S[Band(1)] = fill(2,4);

julia> S
5×5 SparseMatrixCSC{Int64, Int64} with 13 stored entries:
 1 2 ⋅ ⋅ ⋅
 1 2 2 ⋅ ⋅
 ⋅ 2 3 2 ⋅
 ⋅ ⋅ 3 4 2
 ⋅ ⋅ ⋅ 4 5

```

Then again, perhaps you may want to work with a `BandedMatrix` directly. You may convert this to a sparse matrix when necessary.

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<div class="post-metadata">

**Author:** ![dalarev](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/dalarev/32/9881_2.png) [@dalarev](https://discourse.julialang.org/u/dalarev)\
**Post date:** [July 31, 2023, 4:41pm UTC](https://discourse.julialang.org/t/replace-diagonals-in-existing-spdiagm-array/102202/3 "2023-07-31T16:41:50Z")

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Thanks for the reply, @jishnub. In my case, I need to add content to a new diagonal, so perhaps the best method right now is to simply redefine the sparse array in full.