Merge pull request #654 from JuliaControl/zerotype

add method for `zero(::Type{<:Discrete})`

src/simplification.jl

@@ -40,7 +40,7 @@ Compute a bit-vector, expressing whether a state of the pair (A, B) is
 structurally controllable based on the location of zeros in the matrices. 
 """
 function struct_ctrb_states(A::AbstractVecOrMat, B::AbstractVecOrMat)
-    size(A,1) > typemax(UInt16) && error("Maximum size of A exceeded. If you encounter this error, please open an issue. This limit is not fundamental and excists for performance reasons only.")
+    size(A,1) > typemax(UInt16) && error("Maximum size of A exceeded. If you encounter this error, please open an issue. This limit is not fundamental and exists for performance reasons only.")
     # UInt16 can only store up to 65535, so if A is completely dense and of size larger than 65535, the computations below might overflow. This is exceedingly unlikely though.
     bitA = UInt16.(.!iszero.(A)) # Convert to Int because mutiplying with a bit matrix is slow
     x = vec(any(B .!= 0, dims=2)) # index vector indicating states that have been affected by input

src/types/StateSpace.jl

@@ -224,7 +224,8 @@ end
 
 ## ADDITION ##
 Base.zero(sys::AbstractStateSpace) = basetype(sys)(zero(sys.D), sys.timeevol)
-Base.zero(::Type{StateSpace{Continuous, F}}) where {F} = ss([zero(F)], Continuous()) # Cannot make a zero of discrete system since sample time is not stored in type.
+Base.zero(::Type{StateSpace{Continuous, F}}) where {F} = ss([zero(F)], Continuous())
+Base.zero(::Type{StateSpace{D, F}}) where {D<:Discrete, F} = ss([zero(F)], undef_sampletime(D))
 
 function +(s1::ST, s2::ST) where {ST <: AbstractStateSpace}
     #Ensure systems have same dimensions

src/types/TransferFunction.jl

@@ -31,8 +31,8 @@ ninputs(G::TransferFunction) = size(G.matrix, 2)
 Base.ndims(::TransferFunction) = 2
 Base.size(G::TransferFunction) = size(G.matrix)
 Base.eltype(::Type{S}) where {S<:TransferFunction} = S
-Base.zero(G::TransferFunction{TE,S}) where {TE,S} = tf(zeros(numeric_type(S), size(G)), G.timeevol) # can not create a zero of a discrete system from the type alone, the sampletime is not stored.
-
+Base.zero(G::TransferFunction{TE,S}) where {TE,S} = tf(zeros(numeric_type(S), size(G)), G.timeevol) 
+Base.zero(::Type{TransferFunction{TE,S}}) where {TE,S} = TransferFunction([zero(S);;], undef_sampletime(TE)) 
 function Base.getproperty(G::TransferFunction, s::Symbol)
     s ∈ fieldnames(typeof(G)) && return getfield(G, s)
     if s === :ny

test/test_connections.jl

@@ -33,6 +33,12 @@ D_022 = ss(4*eye_(2), 0.005)
 @test series(C_111, C_212) == C_212*C_111
 @test parallel(C_111, C_211) == C_111 + C_211
 
+# test that zero(::Type) promotes to correct sample time
+@test zero(D_111) == ss(0, 0.005)
+D_1110 = [D_111 zero(typeof(D_111))]
+@test D_1110.Ts == D_111.Ts
+@test D_1110 == [D_111 zero(D_111)]
+
 # Errors
 @test_throws ErrorException [C_111 D_111]                 # Sampling time mismatch
 @test_throws ErrorException [C_111; D_111]                # Sampling time mismatch
@@ -91,6 +97,11 @@ s = tf("s")
 @test zero(ss(randn(2,3))) == ss(zeros(2,3))
 @test zero(tf(randn(2,3))) == tf(zeros(2,3))
 
+# test that zero(::Type) promotes to correct sample time
+@test zero(Dtf_111) == tf(0, 0.005)
+Dtf_1110 = [Dtf_111 zero(typeof(Dtf_111))]
+@test Dtf_1110.Ts == Dtf_111.Ts
+@test Dtf_1110 == [Dtf_111 zero(Dtf_111)]