function_taking_ref.cpp
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	#include <iostream>
	#include <Eigen/SVD>
	using namespace Eigen;
	using namespace std;

	float inv_cond(const Ref<const MatrixXf>& a)
	{
	const VectorXf sing_vals = a.jacobiSvd().singularValues();
	return sing_vals(sing_vals.size()-1) / sing_vals(0);
	}

	int main()
	{
	Matrix4f m = Matrix4f::Random();
	cout << "matrix m:" << endl << m << endl << endl;
	cout << "inv_cond(m): " << inv_cond(m) << endl;
	cout << "inv_cond(m(1:3,1:3)): " << inv_cond(m.topLeftCorner(3,3)) << endl;
	cout << "inv_cond(m+I): " << inv_cond(m+Matrix4f::Identity()) << endl;
	}