如何传递特征矩阵行参考，将其视为向量？

Question

I have a function that operates on a Vector reference, eg

void auto_bias(const Eigen::VectorXf& v, Eigen:Ref<Eigen::VectorXf>> out)
{
  out = ...
}

and at some point I need to have this function operate on a Matrix row. Now, because the default memory layout is column-major, I can't just Map<> the data the row points to into a vector. So, how do I go about passing the row into the above function so that I can operate on it?

The not-so-pretty solution is to have a temp vector, eg

VectorXf tmpVec = matrix.row(5);
auto_bias(otherVector, tmpVec);
matrix.row(5) = tmpVec;

but is there a way of doing it directly?

Answer 1

You can modify your function to take a reference to the row type (which is a vector expression) instead of a vector. This is really only manageable with a template to infer that type for you:

#include <iostream>
#include <Eigen/Core>

template<typename V>
void set_row(V&& v) {
   v = Eigen::Vector3f(4.0f, 5.0f, 6.0f);
}

int main() {
   Eigen::Matrix3f m = Eigen::Matrix3f::Identity();
   set_row(m.row(1));

   std::cout << m;
   return 0;
}

Answer 2

You can allow Ref<> to have a non default inner-stride (also called increment), as follows:

Ref<VectorXf, 0, InnerStride<>>

See the example function foo3 of the Ref's documentation .

The downside is a possible loss of performance even when you are passing a true VectorXf .