Eigen 3 Square Array Divided by Column Array

Question

I want to divide each row of a square array(b) by the transpose of a column array (a).

Test code is also at godbolt.org, which has an installation of the eigen 3 library.

Expected Result:

// before (a)
2
2
2
// before (b)
2 2 2
4 4 4
6 6 6
// after
1 1 1
2 2 2
3 3 3

Test:

#include <Eigen/Eigen>
#include <iostream>
using namespace Eigen;

int main() {
    ArrayXXf a(3, 1);
    a << 2, 2, 2;
    ArrayXXf b(3, 3);
    b << 2, 2, 2, 4, 4, 4, 6, 6, 6;
    std::cout << a << "\n";
    std::cout << b << "\n";
    b.rowwise() /= a.transpose();
    std::cout << b << "\n";
}

Compilation Error

...
error: static assertion failed: YOU_TRIED_CALLING_A_VECTOR_METHOD_ON_A_MATRIX
...

Related Question:

Eigen: Divide each row by last row

The accepted answer is using rowwise on Eigen matrices converted to Eigen arrays. I just don't get why it doesn't work in my case...

Answer 1

The assertion tells you that you are using a matrix (aka 2D array), whereas a compile-time vector (aka 1D array) is expected. So the solution is to define a as a compile-time 1D array:

ArrayXf a(3);

You can also see this operation in terms of standard linear algebra:

VectorXd a(3);
MatrixXd b(3,3);
b = b * a.asDiagonal().inverse();