Test some untested codepaths so that coverage doesn't drop

Also change some errors to ArgumentErrors

src/Filters/coefficients.jl

@@ -29,7 +29,7 @@ PolynomialRatio{T<:Number}(b::Poly{T}, a::Poly{T}) = PolynomialRatio{T}(b, a)
 # convention is lowest power first.
 function PolynomialRatio{T<:Number,S<:Number}(b::Union(T,Vector{T}), a::Union(S,Vector{S}))
     if findfirst(b) == 0 || findfirst(a) == 0
-        error("filter must have non-zero numerator and denominator")
+        throw(ArgumentError("filter must have non-zero numerator and denominator"))
     end
     PolynomialRatio{promote_type(T,S)}(Poly(b[end:-1:findfirst(b)]), Poly(a[end:-1:findfirst(a)]))
 end
@@ -98,9 +98,9 @@ function Base.convert{T}(::Type{Biquad}, f::PolynomialRatio{T})
     elseif xs == 1
         Biquad(b[0], zero(T), zero(T), zero(T), zero(T))
     elseif xs == 0
-        error("cannot convert an empty PolynomialRatio to Biquad")
+        throw(ArgumentError("cannot convert an empty PolynomialRatio to Biquad"))
     else
-        error("cannot convert a filter of length > 3 to Biquad")
+        throw(ArgumentError("cannot convert a filter of length > 3 to Biquad"))
     end
 end
 
@@ -199,13 +199,13 @@ function Base.convert{Z,P}(::Type{SecondOrderSections}, f::ZeroPoleGain{Z,P})
     p = f.p
     nz = length(z)
     n = length(p)
-    nz > n && error("ZeroPoleGain must not have more zeros than poles")
+    nz > n && throw(ArgumentError("ZeroPoleGain must not have more zeros than poles"))
 
     # Split real and complex poles
     (complexz, realz, matched) = split_real_complex(z)
-    matched || error("complex zeros could not be matched to their conjugates")
+    matched || throw(ArgumentError("complex zeros could not be matched to their conjugates"))
     (complexp, realp, matched) = split_real_complex(p)
-    matched || error("complex poles could not be matched to their conjugates")
+    matched || throw(ArgumentError("complex poles could not be matched to their conjugates"))
 
     # Sort poles according to distance to unit circle (nearest first)
     sort!(complexp, by=x->abs(abs(x) - 1))

test/filt.jl

@@ -14,7 +14,7 @@ for n = 1:6
         if n <= 2
             bq = convert(Biquad, zpk)
         else
-            @test_throws ErrorException convert(Biquad, zpk)
+            @test_throws ArgumentError convert(Biquad, zpk)
         end
         sos = convert(SecondOrderSections, zpk)
 

test/filter_conversion.jl

@@ -130,3 +130,24 @@ for proto in (Butterworth(3), Chebyshev1(3, 1), Chebyshev2(3, 1))
         end
     end
 end
+
+# Test some otherwise untested code paths
+b = Biquad(0.0, 1.0, 2.0, 3.0, 4.0, 5.0, 2)
+@test b.b0 === 0.0
+@test b.b1 === 2*1/3
+@test b.b2 === 2*2/3
+@test b.a1 === 4/3
+@test b.a2 === 5/3
+f = convert(PolynomialRatio, Biquad(2.0, 0.0, 0.0, 0.0, 0.0))
+@test coefb(f) == [2.0]
+@test coefa(f) == [1.0]
+@test convert(Biquad, PolynomialRatio([4.0], [2.0])) == Biquad(2.0, 0.0, 0.0, 0.0, 0.0)
+@test Biquad(2.0, 0.0, 0.0, 0.0, 0.0)*2 == Biquad(4.0, 0.0, 0.0, 0.0, 0.0)
+
+@test_throws ArgumentError PolynomialRatio(Float64[], Float64[])
+f = PolynomialRatio(Float64[1.0], Float64[1.0])
+empty!(f.b.a)
+empty!(f.a.a)
+@test_throws ArgumentError convert(Biquad, f)
+@test_throws ArgumentError convert(SOSFilter, ZPKFilter([0.5 + 0.5im, 0.5 + 0.5im], [0.5 + 0.5im, 0.5 - 0.5im], 1))
+@test_throws ArgumentError convert(SOSFilter, ZPKFilter([0.5 + 0.5im, 0.5 - 0.5im], [0.5 + 0.5im, 0.5 + 0.5im], 1))