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Number of Variables That Can be Skipped: Example and Test
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    # include <cppad/cppad.hpp>
    bool number_skip(void)
    {     bool ok = true;
         using CppAD::AD;

         // independent variable vector
         CppAD::vector< AD<double> > ax(2);
         ax[0] = 0.;
         ax[1] = 1.;
         Independent(ax);

         // Use a conditional expression
         CppAD::vector< AD<double> > ay(1);

         // variable that gets optimized out
         AD<double> az = ax[0] * ax[0];

         // conditional expression
         ay[0] = CondExpLt(ax[0], ax[1], ax[0] + ax[1], ax[0] - ax[1]);

         // create function object F : x -> ay
         CppAD::ADFun<double> f;
         f.Dependent(ax, ay);

         // use zero order to evaluate F[ (3, 4) ]
         CppAD::vector<double>  x( f.Domain() );
         CppAD::vector<double>  y( f.Range() );
         x[0]    = 3.;
         x[1]    = 4.;
         y   = f.Forward(0, x);
         ok &= (y[0] == x[0] + x[1]);

         // before call to optimize
         ok &= f.number_skip() == 0;
         size_t n_var = f.size_var();

         // now optimize the operation sequence
         f.optimize();

         // after optimize, check forward mode result
         x[0]    = 4.;
         x[1]    = 3.;
         y   = f.Forward(0, x);
         ok &= (y[0] == x[0] - x[1]);

         // after optimize, check amount of optimization
         ok &= f.size_var() == n_var - 1;
         ok &= f.number_skip() == 1;

         return ok;
    }

`

Input File: example/general/number_skip.cpp