如何在 c++ 中构建 3x3 平移矩阵

Question

I am new to c++. I am looking to create a 3x3 matrix that looks like this:

I want theta to be a parameter that I can define outside of the matrix and plug it in when doing matrix multiplication. How do I do this?

If it were python, I'd want something like this:

define matrix(theta):
    row1 = np.array([cos(theta), -sin(theta), 0])
    row2 = np.array([sin(theta), cos(theta), 0])
    row3 = np.array([0,0,1])
    matrix = np.stack([row1, row2, row3])

    return matrix

In[] matrix(2)
Out[] 

[[cos(2), -sin(2), 0],
 [sin(2),  cos(2), 0],
 [0,            0, 1]]
 

Answer 1

Something like this:

class RotationMatrix
{
  public: 
    RotationMatrix(const double radians) 
     : m({{cos(radians, -sin(radians), 0}, {sin(radians), cos(radians), 0}, {0, 0, 1}}) {}
  private:
    std::array<std::array<double, 3>, 3> m;
};

Answer 2

You don't necessarily need to define a dedicated class if you want to go a bit quick and dirty:

#include <iostream>
#include <cmath>
#include <array>

using Mat3x3 = std::array<std::array<double, 3>, 3>;
auto rotationMatrix(double theta)
{
    Mat3x3 mat
    { std::array{ cos(theta), -sin(theta), 0. },
      std::array{ sin(theta), cos(theta), 0. },
      std::array{ 0., 0., 1. } };
    return mat;
}

int main()
{
    auto mat = rotationMatrix(0.1);
    for (const auto& row : mat)
      {
        for (const auto& col : row)
            std::cout << col << " ";
        std::cout << "\n";
      }
}
/* prints:
    0.995004 -0.0998334 0
    0.0998334 0.995004 0
    0 0 1
*/

-- EDIT --

Here is C++11 version due to popular demand. Compiled with "g++ mat3.cc --std=c++11"

#include <iostream>
#include <cmath>
#include <array>

using Mat3x3 = std::array<std::array<double, 3>, 3>;
Mat3x3 translationMatrix(double theta)
{
    Mat3x3 mat
    { std::array<double, 3> { cos(theta), -sin(theta), 0. },
      std::array<double, 3> { sin(theta), cos(theta), 0. },
      std::array<double, 3> { 0., 0., 1. } };
    return mat;
}

int main()
{
    Mat3x3 mat = translationMatrix(0.1);
    for (const auto& row : mat)
      {
        for (const auto& col : row)
            std::cout << col << " ";
        std::cout << "\n";
      }
}

Answer 3

I think this would a better solution:

#include <array>    
#include <math.h>
#include <iostream>

typedef std::array<std::array<int,3>,3> Matrix3X3;

Matrix3X3 getMatrix(int x) {
    Matrix3X3 matrix;
    matrix[0][0] = //sin of x;
    //...and so on
    return matrix;
}

int main() {
    Matrix3X3 matrix = getMatrix(3);
    return 0;
}