informaticage

Easy Laplace det

Apr 12th, 2021
635
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  1. #include <iostream>
  2. #include <vector>
  3. #include <cmath>
  4.  
  5. std::vector<std::vector<double>> compute_minor(std::vector<std::vector<double>> matrix, int skip_i, int skip_j) {
  6.   std::vector<std::vector<double>> v_minor(matrix.size() - 1, std::vector<double>(matrix.size() - 1, 0));
  7.  
  8.   for(int i = 0, mi = 0; i < matrix.size(); i++) {
  9.     if(i == skip_i) continue;
  10.     for(int j = 0, mj = 0; j < matrix.size(); j++) {
  11.       if(j == skip_j) continue;
  12.       v_minor.at(mi).at(mj++) = matrix.at(i).at(j);
  13.     }
  14.     mi++;
  15.   }
  16.  
  17.   return v_minor;
  18. }
  19.  
  20. double det(std::vector<std::vector<double>> matrix) {
  21.   if(matrix.size() == 1) {
  22.     return matrix.at(0).at(0);
  23.   }
  24.  
  25.   int determinant = 0;
  26.   for(int j = 0; j < matrix.size(); j++) {
  27.     int sign = j & 1 ? -1 : 1;
  28.     determinant += sign * matrix.at(0).at(j) * det(compute_minor(matrix, 0, j));
  29.   }
  30.  
  31.  
  32.   return determinant;
  33. }
  34.  
  35. int main () {
  36.   std::vector<std::vector<double>> matrix = { { 1, 2, 1, 1}, { 1, 22, 2, 3 }, { 5, 5, 3, 3 }, { 2, 8, 1, 1 } };
  37.   double d = det(matrix);
  38.   std::cout << "Det: " << d << std::endl;
  39.  
  40.   return 0;
  41. }
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