Taping Array Index Operation: Example and Test
Keywords: taping array index operation: example test tape operation

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Taping Array Index Operation: Example and Test
`

    # include <cppad/cppad.hpp>

    namespace {
    	double Array(const double &index)
    	{	static double array[] = {
    			5.,
    			4.,
    			3.,
    			2.,
    			1.
    		};
    		static size_t number = sizeof(array) / sizeof(array[0]);
    		if( index < 0. )
    			return array[0];

    		size_t i = static_cast<size_t>(index);
    		if( i >= number )
    			return array[number-1];

    		return array[i];
    	}
    	// in empty namespace and outside any other routine
    	CPPAD_DISCRETE_FUNCTION(double, Array)
    }

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

    	// domain space vector
    	size_t n = 2;
    	CPPAD_TESTVECTOR(AD<double>) X(n);
    	X[0] = 2.;   // array index value
    	X[1] = 3.;   // multiplier of array index value

    	// 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] * Array( X[0] );

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

    	// vectors for arguments to the function object f
    	CPPAD_TESTVECTOR(double) x(n);   // argument values
    	CPPAD_TESTVECTOR(double) y(m);   // function values
    	CPPAD_TESTVECTOR(double) w(m);   // function weights
    	CPPAD_TESTVECTOR(double) dw(n);  // derivative of weighted function

    	// check function value
    	x[0] = Value(X[0]);
    	x[1] = Value(X[1]);
    	y[0] = Value(Y[0]);
    	ok  &= y[0] == x[1] * Array(x[0]);

    	// evaluate f where x has different values
    	x[0] = x[0] + 1.;  // new array index value
    	x[1] = x[1] + 1.;  // new multiplier value
    	y    = f.Forward(0, x);
    	ok  &= y[0] == x[1] * Array(x[0]);

    	// evaluate derivaitve of y[0]
    	w[0] = 1.;
    	dw   = f.Reverse(1, w);
    	ok   &= dw[0] == 0.;              // partial w.r.t array index
    	ok   &= dw[1] == Array(x[0]);     // partial w.r.t multiplier

    	return ok;
    }

`

Input File: example/tape_index.cpp