数组的类型

• 
• 

指定数组内部的数值实例

#include <Eigen/Dense>
#include <iostream>
using namespace Eigen;
using namespace std;
int main()
{
  ArrayXXf  m(2,2);
  
  // assign some values coefficient by coefficient
  m(0,0) = 1.0; m(0,1) = 2.0;
  m(1,0) = 3.0; m(1,1) = m(0,1) + m(1,0);
  
  // print values to standard output
  cout << m << endl << endl;
 
  // using the comma-initializer is also allowed
  m << 1.0,2.0,
       3.0,4.0;
     
  // print values to standard output
  cout << m << endl;
}

加法与减法操作实例

#include <Eigen/Dense>
#include <iostream>
using namespace Eigen;
using namespace std;
int main()
{
  ArrayXXf a(3,3);
  ArrayXXf b(3,3);
  a << 1,2,3,
       4,5,6,
       7,8,9;
  b << 1,2,3,
       1,2,3,
       1,2,3;
       
  // Adding two arrays
  cout << "a + b = " << endl << a + b << endl << endl;
  // Subtracting a scalar from an array
  cout << "a - 2 = " << endl << a - 2 << endl;
}

数组的乘法运算

• 两个数组在做乘法时必须要具有相同的维度

#include <Eigen/Dense>
#include <iostream>
using namespace Eigen;
using namespace std;
int main()
{
  ArrayXXf a(2,2);
  ArrayXXf b(2,2);
  a << 1,2,
       3,4;
  b << 5,6,
       7,8;
  cout << "a * b = " << endl << a * b << endl;
}

常用的系数运算函数举例

#include <Eigen/Dense>
#include <iostream>
using namespace Eigen;
using namespace std;
int main()
{
  ArrayXf a = ArrayXf::Random(5);
  a *= 2;
  cout << "a =" << endl 
       << a << endl;
  cout << "a.abs() =" << endl 
       << a.abs() << endl;
  cout << "a.abs().sqrt() =" << endl 
       << a.abs().sqrt() << endl;
  cout << "a.min(a.abs().sqrt()) =" << endl 
       << a.min(a.abs().sqrt()) << endl;
}

矩阵与数组的转化

实例一：

#include <Eigen/Dense>
#include <iostream>
using namespace Eigen;
using namespace std;
int main()
{
  MatrixXf m(2,2);
  MatrixXf n(2,2);
  MatrixXf result(2,2);
  m << 1,2,
       3,4;
  n << 5,6,
       7,8;
  result = m * n;
  cout << "-- Matrix m*n: --" << endl << result << endl << endl;
  result = m.array() * n.array();
  cout << "-- Array m*n: --" << endl << result << endl << endl;
  result = m.cwiseProduct(n);
  cout << "-- With cwiseProduct: --" << endl << result << endl << endl;
  result = m.array() + 4;
  cout << "-- Array m + 4: --" << endl << result << endl << endl;
}

实例二

#include <Eigen/Dense>
#include <iostream>
using namespace Eigen;
using namespace std;
int main()
{
  MatrixXf m(2,2);
  MatrixXf n(2,2);
  MatrixXf result(2,2);
  m << 1,2,
       3,4;
  n << 5,6,
       7,8;
  
  result = (m.array() + 4).matrix() * m;
  cout << "-- Combination 1: --" << endl << result << endl << endl;
  result = (m.array() * n.array()).matrix() * m;
  cout << "-- Combination 2: --" << endl << result << endl << endl;
}