the difference between using a reference (&) and a regular member variable

Let me explain the difference between using a reference (&) and a regular member variable in this context, and why a regular member variable is preferred.

References in C++ In C++, a reference is an alias for another variable. When you declare a reference, it must be initialized to refer to an existing object, and it cannot be reassigned to refer to another object after initialization.

Using a Reference When you declare a member variable as a reference, you are essentially saying that this member variable will always refer to some other existing variable. Here is what it looks like in your context:

cpp Copy code class NumMatrix { public: vector<vector>& pre_sum;

NumMatrix(vector<vector<int>>& matrix) : pre_sum(matrix) { // Implementation }

}; Here, pre_sum is a reference to matrix. This means pre_sum will always refer to matrix and cannot be reassigned to refer to something else. This is not suitable for your use case because you want pre_sum to be an independent data structure that holds the prefix sums, not just a reference to the input matrix.

Using a Regular Member Variable A regular member variable is an independent variable that is part of the object and can be initialized and modified as needed. Here is what it looks like in your context:

cpp Copy code class NumMatrix { public: vector<vector> pre_sum;

NumMatrix(vector<vector<int>>& matrix) { // Check if the input matrix is empty if (matrix.empty() || matrix[0].empty()) return; int m = matrix.size(); int n = matrix[0].size(); // Initialize pre_sum with the same dimensions as matrix pre_sum = vector<vector<int>>(m, vector<int>(n, 0)); // Calculate prefix sums pre_sum[0][0] = matrix[0][0]; for(int i = 1; i < m; ++i) { pre_sum[i][0] = pre_sum[i-1][0] + matrix[i][0]; } for(int j = 1; j < n; ++j) { pre_sum[0][j] = pre_sum[0][j-1] + matrix[0][j]; } for(int i = 1; i < m; ++i) { for(int j = 1; j < n; ++j) { pre_sum[i][j] = pre_sum[i][j-1] + pre_sum[i-1][j] - pre_sum[i-1][j-1] + matrix[i][j]; } } // Debug output for(int i = 0; i < m; ++i) { for(int j = 0; j < n; ++j) { cout << pre_sum[i][j] << ", "; } cout << endl; } } int sumRegion(int row1, int col1, int row2, int col2) { if (row1 == 0 && col1 == 0) return pre_sum[row2][col2]; else if (row1 == 0) return pre_sum[row2][col2] - pre_sum[row2][col1-1]; else if (col1 == 0) return pre_sum[row2][col2] - pre_sum[row1-1][col2]; return pre_sum[row2][col2] - pre_sum[row1-1][col2] - pre_sum[row2][col1-1] + pre_sum[row1-1][col1-1]; }

}; Here, pre_sum is an independent vector<vector> that is part of the NumMatrix object. It is initialized in the constructor and then filled with the prefix sums. This way, pre_sum is completely separate from the input matrix and can be modified independently.

Why a Regular Member Variable is Preferred Independence: pre_sum should be an internal data structure that stores the prefix sums independently of the input matrix. Flexibility: Using a regular member variable allows you to reinitialize or modify pre_sum as needed, whereas a reference must always refer to the same object it was initialized with. Encapsulation: It helps in encapsulating the logic and data within the NumMatrix class, which is a good practice in object-oriented programming. Summary In your case, pre_sum needs to be an independent data structure that the NumMatrix class manages internally. Using a regular member variable for pre_sum achieves this, while using a reference would incorrectly tie pre_sum to the input matrix, limiting its functionality and independence.