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  1. #include <array>
  2. #include <cassert>
  3. #include <iostream>
  4. #include <map>
  5. #include <stdexcept>
  6.  
  7. template <typename T>
  8. class SparseMatrix;
  9.  
  10. template <typename T>
  11. class SparseMatrixProxy {
  12. 	SparseMatrix<T> &matrix;
  13. 	const std::array<int, 2> indices;
  14.  
  15. 	public:
  16. 	SparseMatrixProxy(SparseMatrix<T> &matrix, const std::array<int, 2> indices)
  17. 		: matrix{matrix}
  18. 		, indices{indices} {}
  19. 	//for reading an element:
  20. 	operator T() const {
  21. 		// Check if given indices are within the size of the matrix
  22. 		if (indices[0] >= matrix.size[0] || indices[1] >= matrix.size[1])
  23. 			throw std::invalid_argument("Indices out of bounds");
  24. 		auto map_it = matrix.data.find(indices);
  25. 		if (map_it == matrix.data.end()) {
  26. 			return T{};
  27. 		}
  28. 		return map_it->second;
  29. 	}
  30. 	//for writing an element:
  31. 	auto operator=(const T &t) {
  32. 		//optional: when setting a value to 0 erase it from the map
  33. 		if (t == T{}) {
  34. 			matrix.data.erase(indices);
  35. 		} else {
  36. 			matrix.data[indices] = t;
  37. 		}
  38. 		return *this;
  39. 	}
  40. };
  41.  
  42. template <typename T>
  43. class SparseMatrix {
  44. 	std::map<std::array<int, 2>, T> data;
  45. 	const std::array<int, 2> size;
  46.  
  47. 	public:
  48. 	SparseMatrix(const int rows, const int cols)
  49. 		: size({{rows, cols}}) {}
  50.  
  51. 	// []-operator to set and get values from matrix
  52. 	SparseMatrixProxy<T> operator[](const std::array<int, 2> indices) {
  53. 		return {*this, indices};
  54. 	}
  55. 	auto mapsize() const {
  56. 		return data.size();
  57. 	}
  58. 	friend class SparseMatrixProxy<T>;
  59. };
  60.  
  61. int main() {
  62. 	SparseMatrix<double> M(2, 2); // Create a new sparse matrix with 2 rows and 2 columns
  63. 	M[{{1, 1}}] = 3.1;            // Sets element {1,1} to 3.1
  64. 	std::cout << M[{{1, 1}}] << '\n';
  65. 	assert(M.mapsize() == 1); //1 element for index {1,1}
  66. 	std::cout << M[{{0, 0}}] << '\n';
  67. 	assert(M.mapsize() == 1); //still 1 element because reading doesn't insert an element
  68. 	M[{{1, 1}}] = 0;
  69. 	assert(M.mapsize() == 0); //0 elements because we set the only non-0 element to 0
  70. }
  71.  
Success #stdin #stdout 0s 16056KB
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3.1
0
https://ideone.com/Z93ri5
language:
C++14 (gcc 8.3)
created:
8 years ago
visibility:
secret

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