How can I create a diagonal matrix in Eigen-CodePudding

I was reading this post and when I was replicating it using

#include <iostream>
#include <eigen3/Eigen/Core>
#include <eigen3/Eigen/Dense>

int main(){
    Eigen::DiagonalMatrix<double, 3> M(3.0, 8.0, 6.0);
    std::cout << M << std::endl;
    return 0;
}

I get the error

error: invalid operands to binary expression ('std::ostream' (aka 'basic_ostream<char>') and 'Eigen::DiagonalMatrix<double, 3>')
  std::cout << M << std::endl;
  ~~~~~~~~~ ^  ~
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX13.0.sdk/usr/include/c  /v1/cstddef:143:3: note: candidate function template not viable: no known conversion from 'std::ostream' (aka 'basic_ostream<char>') to 'std::byte' for 1st argument
  operator<< (byte  __lhs, _Integer __shift) noexcept
  ^
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX13.0.sdk/usr/include/c  /v1/ostream:748:1: note: candidate function template not viable: no known conversion from 'Eigen::DiagonalMatrix<double, 3>' to 'char' for 2nd argument
operator<<(basic_ostream<_CharT, _Traits>& __os, char __cn)
^
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX13.0.sdk/usr/include/c  /v1/ostream:781:1: note: candidate function template not viable: no known conversion from 'Eigen::DiagonalMatrix<double, 3>' to 'char' for 2nd argument
operator<<(basic_ostream<char, _Traits>& __os, char __c)
^

and a few more lines like this. I complied it using the CMakeLists.txt file

cmake_minimum_required(VERSION 3.0)

project(ExternalLib CXX)

set(CMAKE_CXX_STANDARD 17)

find_package(Eigen3 REQUIRED)

add_executable(out main.cpp)
target_link_libraries(main PUBLIC)

Can anyone help me understand what might be the issue here? Thanks in advance!

CodePudding user response：

In order to print the matrix you have to cast your DiagonalMatrix to a DenseMatrixType, for example by doing:

std::cout << static_cast<Eigen::Matrix3d>(M) << std::endl;

Or by using the toDenseMatrix method:

std::cout << M.toDenseMatrix() << std::endl;