\(\newcommand{\W}[1]{ \; #1 \; }\) \(\newcommand{\R}[1]{ {\rm #1} }\) \(\newcommand{\B}[1]{ {\bf #1} }\) \(\newcommand{\D}[2]{ \frac{\partial #1}{\partial #2} }\) \(\newcommand{\DD}[3]{ \frac{\partial^2 #1}{\partial #2 \partial #3} }\) \(\newcommand{\Dpow}[2]{ \frac{\partial^{#1}}{\partial {#2}^{#1}} }\) \(\newcommand{\dpow}[2]{ \frac{ {\rm d}^{#1}}{{\rm d}\, {#2}^{#1}} }\)

compare.cpp

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AD Binary Comparison Operators: Example and Test

# include <cppad/cppad.hpp>

bool Compare(void)
{   bool ok = true;
    using CppAD::AD;
    using CppAD::NearEqual;
    double eps99 = 99.0 * std::numeric_limits<double>::epsilon();

    // declare independent variables and start tape recording
    size_t n  = 2;
    double x0 = 0.5;
    double x1 = 1.5;
    CPPAD_TESTVECTOR(AD<double>) x(n);
    x[0]      = x0;
    x[1]      = x1;
    CppAD::Independent(x);

    // some binary comparison operations
    AD<double> p;
    if( x[0] < x[1] )
        p = x[0];   // values in x choose this case
    else
        p = x[1];
    if( x[0] <= x[1] )
        p *= x[0];  // values in x choose this case
    else
        p *= x[1];
    if( x[0] >  x[1] )
        p *= x[0];
    else
        p *= x[1];  // values in x choose this case
    if( x[0] >= x[1] )
        p *= x[0];
    else
        p *= x[1];  // values in x choose this case
    if( x[0] == x[1] )
        p *= x[0];
    else
        p *= x[1];  // values in x choose this case
    if( x[0] != x[1] )
        p *= x[0];  // values in x choose this case
    else
        p *= x[1];

    // dependent variable vector
    size_t m = 1;
    CPPAD_TESTVECTOR(AD<double>) y(m);
    y[0] = p;

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

    // check value
    ok &= NearEqual(y[0] , x0*x0*x1*x1*x1*x0, eps99, eps99);

    // forward computation of partials w.r.t. x[0]
    CPPAD_TESTVECTOR(double) dx(n);
    CPPAD_TESTVECTOR(double) dy(m);
    dx[0] = 1.;
    dx[1] = 0.;
    dy    = f.Forward(1, dx);
    ok   &= NearEqual(dy[0], 3.*x0*x0*x1*x1*x1, eps99, eps99);

    // forward computation of partials w.r.t. x[1]
    dx[0] = 0.;
    dx[1] = 1.;
    dy    = f.Forward(1, dx);
    ok   &= NearEqual(dy[0], 3.*x0*x0*x1*x1*x0, eps99, eps99);

    // reverse computation of derivative of y[0]
    CPPAD_TESTVECTOR(double)  w(m);
    CPPAD_TESTVECTOR(double) dw(n);
    w[0]  = 1.;
    dw    = f.Reverse(1, w);
    ok   &= NearEqual(dw[0], 3.*x0*x0*x1*x1*x1, eps99, eps99);
    ok   &= NearEqual(dw[1], 3.*x0*x0*x1*x1*x0, eps99, eps99);

    return ok;
}