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Incorrect conversion from band matrix to sparse matrix #482
Description
The following illustrates the issue:
julia> sparse(BandedMatrix(1=>1:3))
4×4 SparseMatrixCSC{Int64, Int64} with 4 stored entries:
0 1 ⋅ ⋅
⋅ ⋅ 2 ⋅
⋅ ⋅ ⋅ 3
⋅ ⋅ ⋅ ⋅
The expected result is
julia> sparse(Matrix(BandedMatrix(1=>1:3)))
4×4 SparseMatrixCSC{Int64, Int64} with 3 stored entries:
⋅ 1 ⋅ ⋅
⋅ ⋅ 2 ⋅
⋅ ⋅ ⋅ 3
⋅ ⋅ ⋅ ⋅
Similarly:
julia> a = BandedMatrix(1=>1:3,-1=>3:5)
4×4 BandedMatrix{Int64} with bandwidths (1, 1):
0 1 ⋅ ⋅
3 0 2 ⋅
⋅ 4 0 3
⋅ ⋅ 5 0
julia> sparse(a)
4×4 SparseMatrixCSC{Int64, Int64} with 10 stored entries:
0 1 ⋅ ⋅
3 0 2 ⋅
⋅ 4 0 3
⋅ ⋅ 5 0
julia> sparse(Matrix(a))
4×4 SparseMatrixCSC{Int64, Int64} with 6 stored entries:
⋅ 1 ⋅ ⋅
3 ⋅ 2 ⋅
⋅ 4 ⋅ 3
⋅ ⋅ 5 ⋅
Activity
jecs commentedon Apr 24, 2026
It's even worse when bands span more of the matrix. For example,
dlfivefifty commentedon Apr 24, 2026
A PR to correct this would be very much appreciated!