CODEWITHSUNDEEP
c++matrixlinear-algebraeigen3
Frank-Rene Schäfer
Frank-Rene Schäfer

Reputation: 3352

Eigen3/C++: MatrixXd multiply one row with another

Using the Eigen3/C++ Library, given a MatrixXd 

    / x0   ...  y0  \
    | x1   ...  y1  |
M = | ...  ...  ... |
    |               |
    \ xN   ...  yN  /

what is the fasted method to achieve a modified version as shown below?

     / x0 * y0   ...  y0  \
     | x1 * y1   ...  y1  |
M' = | ...  ...  ...      |
     |                    |
     \ xN * yN   ...  yN  /

That is, one column (the one with the x-s) is replaced by itself multiplied with another column (that with the y-s).

Upvotes: 0

Views: 122

Answers (1)

Massimiliano Janes
Massimiliano Janes

Reputation: 5624

do you mean how to coefficient-wise assign-multiply the first and last column vectors ? there are many ways of doing it, but the easiest/fastest might be

Eigen::MatrixXd M2 = M;

M2.leftCols<1>().array() *= M2.rightCols<1>().array();

an alternative might be constructing an uninitialized matrix with a given number of rows/cols and then block-assign like

Eigen::MatrixXd M2{ M.rows(), M.cols() };

M2.rightCols( M.cols() - 1 ) = M.rightCols( M.cols() - 1 );
M2.leftCols<1>() = M.leftCols<1>().cwiseProduct( M.rightCols<1>() );

which is faster I don't know ( but your preferred profiler does ).

for future questions, here is the official Eigen quick reference ;)

Upvotes: 1

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