Add constructors for Matrix{T} and SparseMatrixCSC{T} from UniformScaling

Author: martinholters

README.md

@@ -180,6 +180,8 @@ Currently, the `@compat` macro supports the following syntaxes:
 
 * `*(::Union{Char,AbstractString},::Union{Char,AbstractString})` concatenation. ([#22512])
 
+* Constructors for `Matrix{T}` and `SparseMatrixCSC{T}` from `UniformScaling` ([#24372])
+
 ## Renaming
 
 
@@ -347,3 +349,4 @@ includes this fix. Find the minimum version from there.
 [#23931]: https://github.com/JuliaLang/julia/issues/23931
 [#24282]: https://github.com/JuliaLang/julia/issues/24282
 [#22512]: https://github.com/JuliaLang/julia/issues/22532
+[#24372]: https://github.com/JuliaLang/julia/issues/24372

src/Compat.jl

@@ -729,6 +729,26 @@ end
     export BitSet
 end
 
+if VERSION < v"0.7.0-DEV.2377"
+    (::Type{Matrix{T}}){T}(s::UniformScaling, dims::Dims{2}) = setindex!(zeros(T, dims), T(s.λ), diagind(dims...))
+    (::Type{Matrix{T}}){T}(s::UniformScaling, m::Integer, n::Integer) = Matrix{T}(s, Dims((m, n)))
+
+    (::Type{SparseMatrixCSC{Tv,Ti}}){Tv,Ti}(s::UniformScaling, m::Integer, n::Integer) = SparseMatrixCSC{Tv,Ti}(s, Dims((m, n)))
+    (::Type{SparseMatrixCSC{Tv}}){Tv}(s::UniformScaling, m::Integer, n::Integer) = SparseMatrixCSC{Tv}(s, Dims((m, n)))
+    (::Type{SparseMatrixCSC{Tv}}){Tv}(s::UniformScaling, dims::Dims{2}) = SparseMatrixCSC{Tv,Int}(s, dims)
+    function (::Type{SparseMatrixCSC{Tv,Ti}}){Tv,Ti}(s::UniformScaling, dims::Dims{2})
+        @boundscheck first(dims) < 0 && throw(ArgumentError("first dimension invalid ($(first(dims)) < 0)"))
+        @boundscheck last(dims) < 0 && throw(ArgumentError("second dimension invalid ($(last(dims)) < 0)"))
+        iszero(s.λ) && return spzeros(Tv, Ti, dims...)
+        m, n, k = dims..., min(dims...)
+        nzval = fill!(Vector{Tv}(k), Tv(s.λ))
+        rowval = copy!(Vector{Ti}(k), 1:k)
+        colptr = copy!(Vector{Ti}(n + 1), 1:(k + 1))
+        for i in (k + 2):(n + 1) colptr[i] = (k + 1) end
+        SparseMatrixCSC{Tv,Ti}(dims..., colptr, rowval, nzval)
+    end
+end
+
 include("deprecated.jl")
 
 end # module Compat

test/runtests.jl

@@ -863,6 +863,16 @@ end
 # 0.7
 @test 1 in BitSet(1:10)
 
+# 0.7
+let a = [1 0 0; 0 1 0; 0 0 1]
+    @test Matrix{Int}(I, 3, 3) == a
+    @test Matrix{Float64}(I, 3, 2) == a[:,1:2]
+    @test SparseMatrixCSC{Int}(I, 3, 3) == a
+    @test SparseMatrixCSC{Float64}(I, 3, 2) == a[:,1:2]
+    @test SparseMatrixCSC{Bool,Int16}(I, 3, 3) == a
+    @test SparseMatrixCSC{Complex128,Int8}(I, 3, 2) == a[:,1:2]
+end
+
 if VERSION < v"0.6.0"
     include("deprecated.jl")
 end