如何创建二维矩阵 c++ 的水平镜像逆？

Question

For example if the input is:

21 26 31 36
22 27 32 37
23 28 33 38
24 29 34 39
25 30 35 40

the output should be:

25 30 35 40
24 29 34 39
23 28 33 38
22 27 32 37
21 26 31 36

Al the rows are swapped until the middle row.

The first row should be swapped with the last one, the second with the before last one row, etc...

This is my code yet:

int A[100][100] = { 0 };
int n, m;
std::cin >> n >> m;


for (int k = 0, j = n - 1; k < j; k++, --j)
     std::swap(A[k], A[j]);

number of rows(n), columns(m), for n, m < 100 It shows me the same matrix as before.

Answer 1

You can swap a two-dimensional array either using the standard algorithm std::swap_ranges or by writing "manually" a loop.

Here is a demonstrative program.

#include <iostream>
#include <iterator>
#include <algorithm>

int main() 
{
    const size_t M = 5, N = 4;
    int a[M][N] = 
    {
        { 21, 26, 31, 36 },
        { 22, 27, 32, 37 },
        { 23, 28, 33, 38 },
        { 24, 29, 34, 39 },
        { 25, 30, 35, 40 }
    };

    for ( const auto &row : a )
    {
        for ( const auto &item : row )
        {
            std::cout << item << ' ';
        }
        std::cout << '\n';
    }
    std::cout << '\n';

    std::swap_ranges( std::begin( a ), std::next( std::begin( a ), M / 2 ), 
                      std::rbegin( a ) );

    for ( const auto &row : a )
    {
        for ( const auto &item : row )
        {
            std::cout << item << ' ';
        }
        std::cout << '\n';
    }
    std::cout << '\n';

    for ( std::size_t i = 0; i < N / 2; i++ )
    {
        std::swap( a[i], a[M - i - 1] );
    }

    for ( const auto &row : a )
    {
        for ( const auto &item : row )
        {
            std::cout << item << ' ';
        }
        std::cout << '\n';
    }
    std::cout << '\n';

    return 0;
}

Its output is

21 26 31 36 
22 27 32 37 
23 28 33 38 
24 29 34 39 
25 30 35 40 

25 30 35 40 
24 29 34 39 
23 28 33 38 
22 27 32 37 
21 26 31 36 

21 26 31 36 
22 27 32 37 
23 28 33 38 
24 29 34 39 
25 30 35 40 

Pay attention to that variable length arrays is not a standard C++ feature. It means that the sizes of an array shall be known at compile time. If you want that it is the user who will enter the sizes then use the standard class template std::vector .

If you have an already defined array with some maximum values of its sizes and the user may specify its sub-array then the array should be at least zero-initialized

Answer 2

You can swap the two dimensional array using std::reverse :

#include <algorithm>
#include <iostream>

int main()
{
    int A[100][100] = { {21, 26, 31, 36},
                        {22, 27, 32, 37},
                        {23, 28, 33, 38},
                        {24, 29, 34, 39},
                        {25, 30, 35, 40} };

    std::reverse(&A[0], &A[5]);  // Starting and ending ranges are &A[0] and &A[5]

    // Output results
    for (int i = 0; i < 5; ++i)
    {
        for (int j = 0; j < 4; ++j)
            std::cout << A[i][j] << " ";
        std::cout << "\n";
    }
}

Output:

25 30 35 40 
24 29 34 39 
23 28 33 38 
22 27 32 37 
21 26 31 36