initializeMatrix

#include <iostream>
#include <fstream>
#include <cmath>
#include <cstdlib>

void initializeMatrix(int s, int n, double *A, const char *filename) {
    if (s == 0) {
        std::ifstream file(filename);
        if (!file) {
            std::cerr << "Error: Cannot open file " << filename << std::endl;
            std::exit(1);
        }
        for (int i = 0; i < n * n; i++) {
            if (!(file >> A[i])) {
                std::cerr << "Error: Invalid data in file" << std::endl;
                std::exit(1);
            }
        }
        file.close();
    } else {
        for (int i = 1; i <= n; i++) {
            for (int j = 1; j <= n; j++) {
                int idx = (i - 1) * n + (j - 1);
                switch (s) {
                    case 1: A[idx] = n - std::max(i, j) + 1; break;
                    case 2: A[idx] = std::max(i, j); break;
                    case 3: A[idx] = std::abs(i - j); break;
                    case 4: A[idx] = 1.0 / (i + j - 1); break;
                    default: std::cerr << "Error: Invalid formula number" << std::endl; std::exit(1);
                }
            }
        }
    }
}