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Convert an AD Variable to a Parameter: Example and Test
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    # include <cppad/cppad.hpp>

    bool Var2Par(void)
    {     bool ok = true;
         using CppAD::AD;
         using CppAD::Value;
         using CppAD::Var2Par;

         // domain space vector
         size_t n = 2;
         CPPAD_TESTVECTOR(AD<double>) x(n);
         x[0] = 3.;
         x[1] = 4.;

         // declare independent variables and start tape recording
         CppAD::Independent(x);

         // range space vector
         size_t m = 1;
         CPPAD_TESTVECTOR(AD<double>) y(m);
         y[0] = - x[1] * Var2Par(x[0]);    // same as y[0] = -x[1] * 3.;

         // cannot call Value(x[j]) or Value(y[0]) here (currently variables)
         ok &= ( Value( Var2Par(x[0]) ) == 3. );
         ok &= ( Value( Var2Par(x[1]) ) == 4. );
         ok &= ( Value( Var2Par(y[0]) ) == -12. );

         // create f: x -> y and stop tape recording
         CppAD::ADFun<double> f(x, y);

         // can call Value(x[j]) or Value(y[0]) here (currently parameters)
         ok &= (Value(x[0]) ==  3.);
         ok &= (Value(x[1]) ==  4.);
         ok &= (Value(y[0]) == -12.);

         // evaluate derivative of y w.r.t x
         CPPAD_TESTVECTOR(double) w(m);
         CPPAD_TESTVECTOR(double) dw(n);
         w[0] = 1.;
         dw   = f.Reverse(1, w);
         ok  &= (dw[0] == 0.);  // derivative of y[0] w.r.t x[0] is zero
         ok  &= (dw[1] == -3.); // derivative of y[0] w.r.t x[1] is 3

         return ok;
    }

`

Input File: example/general/var2par.cpp