Restructure the rhoi derivatives

include/teqp/derivs.hpp

@@ -15,17 +15,17 @@ derivT(const FuncType& f, TType T, const ContainerType& rho) {
     return f(std::complex<TType>(T, h), rho).imag() / h;
 }
 
-/// Given a function, use complex step derivatives to calculate the derivative with respect to the first composition variable
+/// Given a function, use complex step derivatives to calculate the derivative with respect to the given composition variable
 template <typename TType, typename ContainerType, typename FuncType, typename Integer>
 typename std::enable_if<is_container<ContainerType>::value, typename ContainerType::value_type>::type
-derivrhoi(const FuncType& f, TType T, const ContainerType& rho, Integer j) {
+derivrhoi(const FuncType& f, TType T, const ContainerType& rho, Integer i) {
     double h = 1e-100;
     using comtype = std::complex<ContainerType::value_type>;
     std::valarray<comtype> rhocom(rho.size());
-    for (auto i = 0; i < rho.size(); ++i) {
-        rhocom[i] = comtype(rho[i], 0.0);
+    for (auto j = 0; j < rho.size(); ++j) {
+        rhocom[j] = comtype(rho[j], 0.0);
     }
-    rhocom[j] = comtype(rho[j], h);
+    rhocom[i] = comtype(rho[i], h);
     return f(T, rhocom).imag() / h;
 }