可变大小的二维数组作为参数 C++

Question

我打算创建一个函数 Swap() 来交换 n 乘以 m 的矩阵 A 和矩阵 B 的两个最大元素。但是我有 error: declaration of 'A' as multidimensional array must have bounds for all dimensions except the first

void Swap(int A[][], int B[][], int n, int m)
{
    int max_A = A[0][0];
    int max_B = B[0][0];
    int index_Ai, index_Aj, index_Bi, index_Bj;
    for(int i = 0; i < n; i++)
    {
        for(int j = 0; j < m; j++)
        {
            if(max_A < A[i][j])
            {
                max_A = A[i][j];
                index_Ai = i;
                index_Aj = j;
            }
        }
    }
    for(int i = 0; i < n; i++)
    {
        for(int j = 0; j < m; j++)
        {
            if(max_B < B[i][j])
            {
                max_B = B[i][j];
                index_Bi = i;
                index_Bj = j;
            }
        }
    }
    int temp;
     temp = A[index_Ai][index_Aj];
     A[index_Ai][index_Aj] = B[index_Bi][index_Bj];
     B[index_Bi][index_Bj] = temp;
}

我该如何处理这个问题？或者我应该只传递两个矩阵作为参数，然后在函数中找到它们的大小？感谢任何帮助。

Answer 1

假设您的二维数组是 "regular" C++ 二维数组（不是向量），请注意数组将其数据存储在连续内存中。

鉴于这一事实，要找到任何二维数组中的最大元素，只需知道维度，然后使用 std::max_element to get a pointer to the maximum element of each array, and call std::swap 交换找到的元素。

#include <algorithm>
#include <iostream>
//...
int main()
{
    const int m = 10;
    const int n = 20;

    int A[m][n] = {0};
    int B[m][n] = {0};

    // Test
    A[4][5] = 20;
    B[3][2] = 50;
    std::cout << "Before:\n" << A[4][5] << " " << B[3][2];                  

    // Swap the maximum elements  
    std::swap(*std::max_element(&A[0][0], &A[m-1][n]), 
              *std::max_element(&B[0][0], &B[m-1][n]));

    std::cout << "\n\nAfter:\n" << A[4][5] << " " << B[3][2];                  
}

输出：

Before:
20 50

After:
50 20

无需创建单独的函数。

如果您必须创建一个函数，那么模板函数可能就是您正在寻找的：

#include <algorithm>
#include <iostream>
//...

template <int M, int N>
void Swap(int A[M][N], int B[M][N])
{
    std::swap(*std::max_element(&A[0][0], &A[M-1][N]), 
              *std::max_element(&B[0][0], &B[M-1][N]));
}


int main()
{
    const int m = 10;
    const int n = 20;

    int A[m][n] = {0};
    int B[m][n] = {0};

    A[4][5] = 20;
    B[3][2] = 50;
    std::cout << "Before:\n" << A[4][5] << " " << B[3][2];                  

    Swap<10,20>(A, B);

    std::cout << "\n\nAfter:\n" << A[4][5] << " " << B[3][2];                  
}

输出：

Before:
20 50

After:
50 20

Answer 2

根据您的要求，这里可能会使用向量的向量（也称为 2D 向量）实现您的代码，并附注：

Live demo

#include <iostream>
#include <vector>
#include <exception>

//pass vectors by reference so the changes are reflected in the passed arguments
void Swap(std::vector<std::vector<int>>& A, std::vector<std::vector<int>>& B) {

    int max_A = A.at(0).at(0); //if indexing A[0][0], exception would not be thrown
    int max_B = B.at(0).at(0);
    int index_Ai, index_Aj, index_Bi, index_Bj;

    for (size_t i = 0; i < A.size(); i++) {
        for (size_t j = 0; j < A.at(0).size(); j++) {
            if (max_A < A[i][j]) { //indexing A[i][j] safe, cycle limited to vector size
                max_A = A[i][j];
                index_Ai = i;
                index_Aj = j;
            }
        }
    }
    for (size_t i = 0; i < B.size(); i++) {
        for (size_t j = 0; j < B.at(0).size(); j++) {
            if (max_B < B[i][j]) {
                max_B = B[i][j];
                index_Bi = i;
                index_Bj = j;
            }
        }
    }
    //standard library swap function
    std::swap(A.at(index_Ai).at(index_Aj), B.at(index_Bi).at(index_Bj));
}

int main() {

    std::vector<std::vector<int>> A = {{1, 2, 300}, {4, 9, 10, 56, 5, 6}};
    std::vector<std::vector<int>> B = {{10, 45, 2, 12, 20, 80}, {40, 45, 500, 60}};
    try{
        Swap(A, B);
    } catch(std::exception& e){ //if vectors accessed out of bounds throws exception
        std::cout << "ERROR: " << e.what() << std::endl; //we catch it here
        return EXIT_FAILURE;    
    }

    for (auto &v : A) { //test print A
        for (auto i : v) {
            std::cout << i << " ";
        }
        std::cout << std::endl;
    }
    std::cout << std::endl;
    for (auto &v : B) { //test print B
        for (auto i : v)
        {
            std::cout << i << " ";
        }
        std::cout << std::endl;
    }
    return EXIT_SUCCESS;
}

初始数组：

1 2 300
4 9 10 56 5 6

10 45 2 12 20 80
40 45 500 60

交换后：

1 2 500
4 9 10 56 5 6

10 45 2 12 20 80
40 45 300 60

可变大小的二维数组作为参数 C++

2D arrays with variable sizes as arguments C++

c++

swap

multidimensional-array