Adding an SMatrix

matthiasbaitsch · June 25, 2025, 2:49pm

Hi,

the following code fails because Ke is immutable after adding the SMatrix.

using StaticArrays
Ke = zeros(3, 3)
Ke += SA[1 2 3; 4 3 2; 4 3 2]
Ke[1, 1] += 3

I wonder whether this is intended behaviour.

barucden · June 25, 2025, 2:56pm

It is intended: adding an AbstractMatrix to a SMatrix results in a SMatrix. If you want to keep the type of Ke unchanged, you could use broadcasting:

using StaticArrays
Ke = zeros(3, 3)
Ke .+= SA[1 2 3; 4 3 2; 4 3 2]
Ke[1, 1] += 3

matthiasbaitsch · June 25, 2025, 3:26pm

Thanks a lot for the clarification!

nsajko · June 25, 2025, 4:23pm

Weird. The implementation is here:

github.com/JuliaArrays/StaticArrays.jl

src/linalg.jl

563adebc2


      
          # Unary ops
          @inline +(a::StaticArray) = map(+, a)
          @inline -(a::StaticArray) = map(-, a)
          
          # Binary ops
          # Between arrays
          @inline +(a::StaticArray, b::StaticArray) = map(+, a, b)
          @inline +(a::AbstractArray, b::StaticArray) = map(+, a, b)
          @inline +(a::StaticArray, b::AbstractArray) = map(+, a, b)
          
          @inline -(a::StaticArray, b::StaticArray) = map(-, a, b)
          @inline -(a::AbstractArray, b::StaticArray) = map(-, a, b)
          @inline -(a::StaticArray, b::AbstractArray) = map(-, a, b)
          
          # Scalar-array
          @inline *(a::Number, b::StaticArray) = map(c->a*c, b)
          @inline *(a::StaticArray, b::Number) = map(c->c*b, a)
          
          @inline /(a::StaticArray, b::Number) = map(c->c/b, a)
          @inline \(a::Number, b::StaticArray) = map(c->a\c, b)

Possibly this is intentional, but it seems like a very bad choice on part of StaticArrays.jl, considering the lack of type stability, and considering how the methods cause method ambiguity for other packages (relevant doc PR of mine: manual: methods: new section on avoiding ambiguity: "playing nicely" by nsajko · Pull Request #58005 · JuliaLang/julia · GitHub).

Furthermore I see StaticArrays.jl doesn’t have promotion with Array defined, which seems like a related bad choice.

I suppose all of this is just technical debt from the early days of Julia.

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Adding an SMatrix

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