Givens rotation / Поворот Гивенса

/*
 * basis for python code: https://github.com/AndrosovAS/QR-decomposition/blob/main/QR-decomposition.py
 * example: https://www.youtube.com/watch?v=MxZy0LLEDLY
 */

#include <iostream>

using namespace std;

int main() {
    const int eps = 10000;
    const int N = 3;
    double A[N][N] = {
        {0., -1., 1.},
        {4., 2., 0.},
        {3., 4., 0.}
    };

    for (int col = 0; col < N; col++) {
        for (int row = col+1; row < N; row++) {
            if (A[row][col] != 0) {
                double r = sqrt(A[col][col] * A[col][col] + A[row][col] * A[row][col]);
                double c = A[col][col] / r;
                double s = -A[row][col] / r;

                for (int i = 0; i < N; i++) {
                    double row_i, col_i;
                    col_i = A[col][i] * c + A[row][i] * (-s);
                    row_i = A[col][i] * s + A[row][i] * c;
                    A[col][i] = col_i, A[row][i] = row_i;
                }
            }
        }
    }

    for (int i = 0; i < N; i++) {
        for (int j = 0; j < N; j++) {
            cout << floor(A[i][j] * eps) / eps << '\t';
        }
        cout << '\n';
    }

    return 0;
}