How can I store the return value of 'derivativex' as a double?

[GaffaSnobb]

I wish to store the result of the numerical differentiation as a double in an armadillo matrix, as illustrated in the code below:

autodiff::var func(autodiff::var x)
{
    return autodiff::reverse::sin(x);
}

int main()
{
      int n = 20;
      arma::Mat<double> M(n, 2);
      arma::vec scope = arma::linspace(0, 3.141592, n);
      autodiff::var x;
      autodiff::var u;
      std::array<autodiff::reverse::Variable<double>, 1> ux;
      std::array<autodiff::reverse::Variable<double>, 1> uxx;
      
      for (int i = 0; i < n; i++)
      {   
            x = scope(i);
            u = func(x);
            ux = autodiff::reverse::derivativesx(u, autodiff::reverse::wrt(x));
            uxx = autodiff::reverse::derivativesx(ux[0], autodiff::reverse::wrt(x));
            
            M(i, 0) = ux[0];   // This is what I want to do.
            M(i, 1) = uxx[0];  // This is what I want to do.
      }
}

If I run the code I get the error

error: cannot convert 'std::array<autodiff::reverse::Variable<double>, 1>::value_type' {aka 'autodiff::reverse::Variable<double>'} to 'double'

How can I achieve this? The examples on your website only couts the results, no storing.

[allanleal]

This is because var cannot be converted to double implicitly. Try:

M(i, 0) = double(ux[0]);   // This is what I want to do.
M(i, 1) = double(uxx[0]);  // This is what I want to do.

or

M(i, 0) = autodiff::val(ux[0]);   // This is what I want to do.
M(i, 1) = autodiff::val(uxx[0]);  // This is what I want to do.

[GaffaSnobb]

Thanks for the quick help! Appreciate it!