Hello,

I have one little problem using eigen.
I declare one matrix

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Eigen::MatrixXd coord(Eigen::DynamicIndex,3);

so with 3 columns. then I want to compute for each column the mean value and the standard deviation value.

I tried something like :

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float col_0_mean = coord.col(0).mean();

but seems not working. And I don't even know if a standard-deviation function exists.

Can you please give me a hand with this.

Thank you for your help

You can use Eigen::Dynamic (and not Eigen::DynamicIndex; I did not even know that constant existed) as a template parameter, to indicate that the number of rows is not known at compile time but the number of columns is, as in

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Eigen::Matrix<double, Eigen::Dynamic, 3> coords;


The number of rows can then be specified later or in the same statement, for instance:

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Eigen::Matrix<double, Eigen::Dynamic, 3> coords(number_of_rows, 3);
// fill coords matrix
double col_0_mean = coord.col(0).mean();


I changed the type of col_0_mean to match the type of the matrix.

There is no function for computing the standard deviation, as far as I know.