convert to banded matrix types from AbstractMatrixes

src/bidiag.jl

@@ -109,8 +109,8 @@ julia> Bidiagonal(A, :L) # contains the main diagonal and first subdiagonal of A
  ⋅  ⋅  4  4
 ```
 """
-function Bidiagonal(A::AbstractMatrix, uplo::Symbol)
-    Bidiagonal(diag(A, 0), diag(A, uplo === :U ? 1 : -1), uplo)
+function (::Type{Bi})(A::AbstractMatrix, uplo::Symbol) where {Bi<:Bidiagonal}
+    Bi(diag(A, 0), diag(A, uplo === :U ? 1 : -1), uplo)
 end
 
 
@@ -220,7 +220,12 @@ promote_rule(::Type{<:Tridiagonal}, ::Type{<:Bidiagonal}) = Tridiagonal
 AbstractMatrix{T}(A::Bidiagonal) where {T} = Bidiagonal{T}(A)
 AbstractMatrix{T}(A::Bidiagonal{T}) where {T} = copy(A)
 
-convert(::Type{T}, m::AbstractMatrix) where {T<:Bidiagonal} = m isa T ? m : T(m)::T
+function convert(::Type{T}, A::AbstractMatrix) where T<:Bidiagonal
+    checksquare(A)
+    isbanded(A, -1, 1) || throw(InexactError(:convert, T, A))
+    iszero(diagview(A, 1)) ? T(A, :L) :
+        iszero(diagview(A, -1)) ? T(A, :U) : throw(InexactError(:convert, T, A))
+end
 
 similar(B::Bidiagonal, ::Type{T}) where {T} = Bidiagonal(similar(B.dv, T), similar(B.ev, T), B.uplo)
 similar(B::Bidiagonal, ::Type{T}, dims::Union{Dims{1},Dims{2}}) where {T} = similar(B.dv, T, dims)

src/tridiag.jl

@@ -107,18 +107,21 @@ julia> SymTridiagonal(B)
  [1 2; 3 4]  [1 2; 2 3]
 ```
 """
-function SymTridiagonal(A::AbstractMatrix)
+function (::Type{SymTri})(A::AbstractMatrix) where {SymTri <: SymTridiagonal}
     checksquare(A)
     du = diag(A, 1)
     d  = diag(A)
     dl = diag(A, -1)
-    if all(((x, y),) -> x == transpose(y), zip(du, dl)) && all(issymmetric, d)
-        SymTridiagonal(d, du)
+    if _checksymmetric(d, du, dl)
+        SymTri(d, du)
     else
         throw(ArgumentError("matrix is not symmetric; cannot convert to SymTridiagonal"))
     end
 end
 
+_checksymmetric(d, du, dl) = all(((x, y),) -> x == transpose(y), zip(du, dl)) && all(issymmetric, d)
+_checksymmetric(A::AbstractMatrix) = _checksymmetric(diagview(A), diagview(A, 1), diagview(A, -1))
+
 SymTridiagonal{T,V}(S::SymTridiagonal{T,V}) where {T,V<:AbstractVector{T}} = S
 SymTridiagonal{T,V}(S::SymTridiagonal) where {T,V<:AbstractVector{T}} =
     SymTridiagonal(convert(V, S.dv)::V, convert(V, S.ev)::V)
@@ -128,6 +131,11 @@ SymTridiagonal{T}(S::SymTridiagonal) where {T} =
                     convert(AbstractVector{T}, S.ev)::AbstractVector{T})
 SymTridiagonal(S::SymTridiagonal) = S
 
+function convert(::Type{T}, A::AbstractMatrix) where T<:SymTridiagonal
+    checksquare(A)
+    _checksymmetric(A) && isbanded(A, -1, 1) ? T(A) : throw(InexactError(:convert, T, A))
+end
+
 AbstractMatrix{T}(S::SymTridiagonal) where {T} = SymTridiagonal{T}(S)
 AbstractMatrix{T}(S::SymTridiagonal{T}) where {T} = copy(S)
 
@@ -583,7 +591,7 @@ julia> Tridiagonal(A)
  ⋅  ⋅  3  4
 ```
 """
-Tridiagonal(A::AbstractMatrix) = Tridiagonal(diag(A,-1), diag(A,0), diag(A,1))
+(::Type{Tri})(A::AbstractMatrix) where {Tri<:Tridiagonal} = Tri(diag(A,-1), diag(A,0), diag(A,1))
 
 Tridiagonal(A::Tridiagonal) = A
 Tridiagonal{T}(A::Tridiagonal{T}) where {T} = A
@@ -605,6 +613,11 @@ function Tridiagonal{T,V}(A::Tridiagonal) where {T,V<:AbstractVector{T}}
     end
 end
 
+function convert(::Type{T}, A::AbstractMatrix) where T<:Tridiagonal
+    checksquare(A)
+    isbanded(A, -1, 1) ? T(A) : throw(InexactError(:convert, T, A))
+end
+
 size(M::Tridiagonal) = (n = length(M.d); (n, n))
 axes(M::Tridiagonal) = (ax = axes(M.d,1); (ax, ax))
 

test/bidiag.jl

@@ -1158,4 +1158,21 @@ end
     @test opnorm(B, Inf) == opnorm(Matrix(B), Inf)
 end
 
+@testset "convert to Bidiagonal" begin
+    M = diagm(0 => [1,2,3], 1=>[4,5])
+    B = convert(Bidiagonal, M)
+    @test B == Bidiagonal(M, :U)
+    M = diagm(0 => [1,2,3], -1=>[4,5])
+    B = convert(Bidiagonal, M)
+    @test B == Bidiagonal(M, :L)
+    B = convert(Bidiagonal{Int8}, M)
+    @test B == M
+    @test B isa Bidiagonal{Int8, Vector{Int8}}
+    B = convert(Bidiagonal{Int8, OffsetVector{Int8, Vector{Int8}}}, M)
+    @test B == M
+    @test B isa Bidiagonal{Int8, OffsetVector{Int8, Vector{Int8}}}
+    M = diagm(-1 => [1,2], 1=>[4,5])
+    @test_throws InexactError convert(Bidiagonal, M)
+end
+
 end # module TestBidiagonal

test/special.jl

@@ -43,7 +43,7 @@ Random.seed!(1)
        @test Matrix(convert(newtype, A)) == Matrix(A)
     end
     for newtype in [Diagonal, Bidiagonal]
-       @test_throws ArgumentError convert(newtype,A)
+       @test_throws Union{ArgumentError,InexactError} convert(newtype,A)
     end
     A = SymTridiagonal(a, zeros(n-1))
     @test Matrix(convert(Bidiagonal,A)) == Matrix(A)
@@ -57,7 +57,7 @@ Random.seed!(1)
        @test Matrix(convert(newtype, A)) == Matrix(A)
     end
     for newtype in [Diagonal, Bidiagonal]
-        @test_throws ArgumentError convert(newtype,A)
+        @test_throws Union{ArgumentError,InexactError} convert(newtype,A)
     end
     A = Tridiagonal(zeros(n-1), [1.0:n;], fill(1., n-1)) #not morally Diagonal
     @test Matrix(convert(Bidiagonal, A)) == Matrix(A)
@@ -79,7 +79,7 @@ Random.seed!(1)
     end
     A = UpperTriangular(triu(rand(n,n)))
     for newtype in [Diagonal, Bidiagonal, Tridiagonal, SymTridiagonal]
-        @test_throws ArgumentError convert(newtype,A)
+        @test_throws Union{ArgumentError,InexactError} convert(newtype,A)
     end
 
 

test/tridiag.jl

@@ -1099,4 +1099,38 @@ end
     @test opnorm(S, Inf) == opnorm(Matrix(S), Inf)
 end
 
+@testset "convert to Tridiagonal/SymTridiagonal" begin
+    @testset "Tridiagonal" begin
+        for M in [diagm(0 => [1,2,3], 1=>[4,5]),
+                diagm(0 => [1,2,3], 1=>[4,5], -1=>[6,7]),
+                diagm(-1 => [1,2], 1=>[4,5])]
+            B = convert(Tridiagonal, M)
+            @test B == Tridiagonal(M)
+            B = convert(Tridiagonal{Int8}, M)
+            @test B == M
+            @test B isa Tridiagonal{Int8}
+            B = convert(Tridiagonal{Int8, OffsetVector{Int8, Vector{Int8}}}, M)
+            @test B == M
+            @test B isa Tridiagonal{Int8, OffsetVector{Int8, Vector{Int8}}}
+        end
+        @test_throws InexactError convert(Tridiagonal, fill(5, 4, 4))
+    end
+    @testset "SymTridiagonal" begin
+        for M in [diagm(0 => [1,2,3], 1=>[4,5], -1=>[4,5]),
+            diagm(0 => [1,2,3]),
+            diagm(-1 => [1,2], 1=>[1,2])]
+            B = convert(SymTridiagonal, M)
+            @test B == SymTridiagonal(M)
+            B = convert(SymTridiagonal{Int8}, M)
+            @test B == M
+            @test B isa SymTridiagonal{Int8}
+            B = convert(SymTridiagonal{Int8, OffsetVector{Int8, Vector{Int8}}}, M)
+            @test B == M
+            @test B isa SymTridiagonal{Int8, OffsetVector{Int8, Vector{Int8}}}
+        end
+        @test_throws InexactError convert(SymTridiagonal, fill(5, 4, 4))
+        @test_throws InexactError convert(SymTridiagonal, diagm(0=>fill(NaN,4)))
+    end
+end
+
 end # module TestTridiagonal