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SparseArrays: specialize zero(...) so result has nnz(...) = 0 #34209

Merged
merged 1 commit into from
Dec 28, 2019
Merged

SparseArrays: specialize zero(...) so result has nnz(...) = 0 #34209

merged 1 commit into from
Dec 28, 2019

Commits on Dec 28, 2019

  1. SparseArrays: specialize zero(...) so result has nnz(...) = 0 

    Before this commit, the result of

    zero(x::AbstractSparseArray)

is filled with stored zeros exactly where `x` has a stored value. That is a
wasteful artifact of the default fallback implementation for `AbstractArray`.
After this commit, we return a sparse array of the same dimensions as `x`
without any stored values.

This change is backwards incompatible where it relates to object identity.
Before this commit, for mutable objects, every stored zero has the same
identity. Example:

    julia> using SparseArrays

    julia> y = zero(BigInt[1,2,3]); y[1] === y[2]
    true

    julia> y = zero(sparse(BigInt[1,2,3])); y[1] === y[2]
    true

    julia> Base.zero(a::AbstractSparseArray) = spzeros(eltype(a), size(a)...)

    julia> y = zero(sparse(BigInt[1,2,3])); y[1] === y[2]
    false

But this behaviour should be classified as a performance bug and can therefore
be fixed in a minor (but not a patch) release.
    @tkluck
    tkluck committed Dec 28, 2019
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