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@(@\newcommand{\W}[1]{ \; #1 \; } \newcommand{\R}[1]{ {\rm #1} } \newcommand{\B}[1]{ {\bf #1} } \newcommand{\D}[2]{ \frac{\partial #1}{\partial #2} } \newcommand{\DD}[3]{ \frac{\partial^2 #1}{\partial #2 \partial #3} } \newcommand{\Dpow}[2]{ \frac{\partial^{#1}}{\partial {#2}^{#1}} } \newcommand{\dpow}[2]{ \frac{ {\rm d}^{#1}}{{\rm d}\, {#2}^{#1}} }@)@
Convert an AD Variable to a Parameter: Example and Test

# 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