Overload UniformScaling to make I(n) create a Diagonal matrix.

Fixes #23156
JuliaLang · Dec 7, 2018 · b103c35 · b103c35
1 parent 1875947
commit b103c35
Show file tree

Hide file tree

Showing 3 changed files with 31 additions and 0 deletions.
diff --git a/NEWS.md b/NEWS.md
@@ -107,6 +107,7 @@ Standard library changes
   * `mul!`, `rmul!` and `lmul!` methods for `UniformScaling` ([#29506]).
   * `Symmetric` and `Hermitian` matrices now preserve the wrapper when scaled with a number ([#29469]).
   * Exponentiation operator `^` now supports raising a `Irrational` to an `AbstractMatrix` power ([#29782]).
+  * `UniformScaling` instances are now callable such that e.g. `I(3)` will produce a `Diagonal` matrix ([#30298]).
 
 #### Random
   * `randperm` and `randcycle` now use the type of their argument to determine the element type of

diff --git a/stdlib/LinearAlgebra/src/uniformscaling.jl b/stdlib/LinearAlgebra/src/uniformscaling.jl
@@ -48,6 +48,31 @@ julia> [1 2im 3; 1im 2 3] * I
 """
 const I = UniformScaling(true)
 
+"""
+    (J::UniformScaling)(n::Integer)
+
+Construct a `Diagonal` matrix from a `UniformScaling`.
+
+!!! compat "Julia 1.1"
+     This method is available as of Julia 1.1.
+
+# Examples
+```jldoctest
+julia> I(3)
+3×3 Diagonal{Bool,Array{Bool,1}}:
+  true    ⋅      ⋅
+   ⋅     true    ⋅
+   ⋅      ⋅     true
+
+julia> (0.7*I)(3)
+3×3 Diagonal{Float64,Array{Float64,1}}:
+ 0.7   ⋅    ⋅
+  ⋅   0.7   ⋅
+  ⋅    ⋅   0.7
+```
+"""
+(J::UniformScaling)(n::Integer) = Diagonal(fill(J.λ, n))
+
 eltype(::Type{UniformScaling{T}}) where {T} = T
 ndims(J::UniformScaling) = 2
 Base.has_offset_axes(::UniformScaling) = false

diff --git a/stdlib/LinearAlgebra/test/uniformscaling.jl b/stdlib/LinearAlgebra/test/uniformscaling.jl
@@ -299,4 +299,9 @@ end
     @test rmul!(copyto!(C, A), J) == target
 end
 
+@testset "Construct Diagonal from UniformScaling" begin
+    @test size(I(3)) === (3,3)
+    @test I(3) isa Diagonal
+end
+
 end # module TestUniformscaling