数组与 std::array

#include <iostream>
#include <array>
#include <algorithm>
#include <numeric>

// ---- 原生数组的问题 ----
// 这三个函数声明是等价的！都退化为 int*
void process_1(int arr[10]) {   // 10 被忽略
    std::cout << "sizeof(arr) in process_1: " << sizeof(arr) << std::endl;
}

void process_2(int arr[]) {     // 完全等价
    std::cout << "sizeof(arr) in process_2: " << sizeof(arr) << std::endl;
}

void process_3(int* arr) {      // 同上
    std::cout << "sizeof(arr) in process_3: " << sizeof(arr) << std::endl;
}

// 需要额外传递长度参数
double average(const int* arr, size_t size) {
    return std::accumulate(arr, arr + size, 0.0) / size;
}

// ---- std::array 的优势 ----
template <size_t N>
double average_array(const std::array<int, N>& arr) {
    if (N == 0) return 0.0;
    return std::accumulate(arr.begin(), arr.end(), 0.0) / N;
}

// 或者用 std::array 的方法直接访问大小
double average_v2(const std::array<int, 5>& arr) {
    return std::accumulate(arr.begin(), arr.end(), 0.0) / arr.size();
}

int main() {
    // 原生数组
    int raw_arr[5] = {10, 20, 30, 40, 50};
    std::cout << "===== 原生数组 =====" << std::endl;
    std::cout << "sizeof(raw_arr) in main: " << sizeof(raw_arr) << std::endl;
    process_1(raw_arr);  // 打印指针大小（8 字节）
    std::cout << "平均分: " << average(raw_arr, 5) << std::endl;

    // std::array
    std::array<int, 5> std_arr = {10, 20, 30, 40, 50};
    std::cout << "\n===== std::array =====" << std::endl;
    std::cout << "sizeof(std_arr) in main: " << sizeof(std_arr) << std::endl;
    std::cout << "std_arr.size(): " << std_arr.size() << std::endl;
    std::cout << "平均分: " << average_array(std_arr) << std::endl;

    // STL 兼容性：range-based for
    std::cout << "\n遍历: ";
    for (const auto& score : std_arr) {
        std::cout << score << " ";
    }
    std::cout << std::endl;

    // STL 兼容性：算法
    auto [min_it, max_it] = std::minmax_element(std_arr.begin(), std_arr.end());
    std::cout << "最低分: " << *min_it << ", 最高分: " << *max_it << std::endl;

    // 边界检查（at 抛出异常，operator[] 不检查）
    std::cout << "std_arr.at(2) = " << std_arr.at(2) << std::endl;
    try {
        std::cout << std_arr.at(10);  // 抛出 std::out_of_range
    } catch (const std::out_of_range& e) {
        std::cout << "越界访问被捕获: " << e.what() << std::endl;
    }

    return 0;
}

#include <iostream>
#include <array>
#include <iomanip>

// 使用 std::array 实现 3x3 矩阵
using Matrix3 = std::array<std::array<double, 3>, 3>;

// 原生数组版本：退化严重，无法传递
void print_matrix_raw(const double matrix[3][3]) {
    // matrix 是 double (*)[3]，不是 double[3][3]
    // 但第一维的 3 仍被忽略，实际是 double (*)[3]
    for (int i = 0; i < 3; ++i) {
        for (int j = 0; j < 3; ++j) {
            std::cout << std::setw(8) << matrix[i][j];
        }
        std::cout << std::endl;
    }
}

void print_matrix(const Matrix3& matrix) {
    for (const auto& row : matrix) {
        for (double val : row) {
            std::cout << std::setw(8) << std::setprecision(2)
                      << std::fixed << val;
        }
        std::cout << std::endl;
    }
}

Matrix3 add_matrices(const Matrix3& a, const Matrix3& b) {
    Matrix3 result{};
    for (size_t i = 0; i < result.size(); ++i) {
        for (size_t j = 0; j < result[i].size(); ++j) {
            result[i][j] = a[i][j] + b[i][j];
        }
    }
    return result;
}

int main() {
    Matrix3 m1 = {{{1.0, 2.0, 3.0},
                   {4.0, 5.0, 6.0},
                   {7.0, 8.0, 9.0}}};

    Matrix3 m2 = {{{9.0, 8.0, 7.0},
                   {6.0, 5.0, 4.0},
                   {3.0, 2.0, 1.0}}};

    std::cout << "矩阵 m1:" << std::endl;
    print_matrix(m1);

    std::cout << "\n矩阵 m2:" << std::endl;
    print_matrix(m2);

    auto m3 = add_matrices(m1, m2);
    std::cout << "\nm1 + m2:" << std::endl;
    print_matrix(m3);

    // 属性查询
    std::cout << "\n矩阵属性:" << std::endl;
    std::cout << "行数: " << m3.size() << std::endl;
    std::cout << "列数: " << m3[0].size() << std::endl;
    std::cout << "总元素: " << m3.size() * m3[0].size() << std::endl;

    return 0;
}