Question : Finding the bandwidth of a matrix about bandedmatrices.jl HOT 7 OPEN

julia> using SparseArrays

julia> sparse(A)
6×6 SparseMatrixCSC{Float64, Int64} with 12 stored entries:
 2.0  5.0  5.0   ⋅    ⋅    ⋅ 
 2.0  5.0  5.0   ⋅    ⋅    ⋅ 
 2.0  5.0  5.0   ⋅    ⋅    ⋅ 
  ⋅    ⋅    ⋅   8.0   ⋅    ⋅ 
  ⋅    ⋅    ⋅    ⋅   8.0   ⋅ 
  ⋅    ⋅    ⋅    ⋅    ⋅   8.0

julia> sparse(A) |> bandwidths
(2, 2)

Why are you expecting (3,3) btw?

from bandedmatrices.jl.

yeah, I agree it must be (2, 2) not (3,3).

It is working for SparseArray type but not with dense matrices as I have mentioned in the question.
I am not sure, if it is intentionally designed to work in that way for any advantage.

I confirm the same behavior with the new version of the package.

from bandedmatrices.jl.

bandwidths should be “fast”. Checking every entry is an O(n^2) operation

from bandedmatrices.jl.

How can you get the bandwidths of a dense matrix without checking for the zeros?

from bandedmatrices.jl.

bandwidths is about structural zeros not actual zeros. So you just assume it’s full

from bandedmatrices.jl.

Maybe it needs info in the docs because, for example, rank of a dense matrix is slow, based on svdvals, but it could have been "generically" full. Contrast that to rank of a LowRankMatrix which is fast and structural by the type.

from bandedmatrices.jl.

If you make a PR to the docs I’ll merge it

from bandedmatrices.jl.