eigen/doc/examples/class_CwiseUnaryOp.cpp

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

// define a custom template unary functor
template <typename Scalar>
struct CwiseClampOp {
  CwiseClampOp(const Scalar& inf, const Scalar& sup) : m_inf(inf), m_sup(sup) {}
  const Scalar operator()(const Scalar& x) const { return x < m_inf ? m_inf : (x > m_sup ? m_sup : x); }
  Scalar m_inf, m_sup;
};

int main(int, char**) {
  Eigen::Matrix4d m1 = Eigen::Matrix4d::Random();
  std::cout << m1 << std::endl
            << "becomes: " << std::endl
            << m1.unaryExpr(CwiseClampOp<double>(-0.5, 0.5)) << std::endl;
  return 0;
}