Drop variables not in any substitution (#95)

src/subs.jl

@@ -94,7 +94,7 @@ function _subs(::MP.Eval, p::Polynomial{C, T}, vals::AbstractVector{S}) where {C
 end
 function _subs(::MP.Subs, p::Polynomial{C, T}, vals::AbstractVector{S}) where {C, T, S}
     Tout = MA.promote_operation(*, T, MP.coefficienttype(S))
-    q = zero_with_variables(Polynomial{C, Tout}, variables(p))
+    q = zero_with_variables(Polynomial{C, Tout}, mergevars_of(PolyVar{C}, vals)[1])
     for i in 1:length(p.a)
         MA.mutable_operate!(+, q, p.a[i] * monoeval(p.x.Z[i], vals))
     end

src/var.jl

@@ -104,3 +104,8 @@ function mergevars(varsvec::Vector{Vector{PV}}) where {PV<:PolyVar}
     maps = mergevars_to!(vars, varsvec)
     return vars, maps
 end
+function mergevars_of(::Type{PolyVar{C}}, polys::AbstractVector) where {C}
+    varsvec = Vector{PolyVar{C}}[variables(p) for p in polys if p isa PolyType]
+    # TODO avoid computing `maps`
+    return mergevars(varsvec)
+end

test/runtests.jl

@@ -16,7 +16,7 @@ end
     @polyvar x[1:2]
     p1 = x[1] * 0.0 + x[2] * 0
     p2 = ( x[1] + x[2] ) * 0.0
-    @test variables(p1) == x 
+    @test variables(p1) == x
     @test variables(p1) == variables(p2)
 
     @polyvar χ[1:4]
@@ -33,6 +33,10 @@ end
     # This is for Issue #92
     p3 = subs(p1, x => x)
     @test variables(p3) == x
+
+    p4 = x[1] + x[2]
+    @test variables(subs(p4, x[2] => 1)) == [x[1]]
+    @test variables(subs(p4, x[1] => 1)) == [x[2]]
 end
 
 include("mono.jl")