如何为动态数组设置特定值？

Question

显然我对此有误解，但我试图简单地将特定数字设置为动态内存数组。

#include <iostream>
#include <cstdlib>
using namespace std;

int main () {
    int *arr = new int [4][5]
    {{3, 6, 9, 23, 16}, 
    {24, 12, 9, 13, 5},
    {37, 19, 43, 17, 11},
    {71, 32, 8, 4, 7}};

    cout<< arr [1][3]<< endl<< endl;
    int *x = new int;
    *x = arr [2][1];
    cout<< x<< endl;
    cout<< *x<< endl << endl; 
    delete x;
    *x = arr [0][3];
    cout<< x<< endl;
    cout<< *x<< endl;
    delete x;
    delete [] arr;
    return;
}

Answer 1

C++ 对多维数组的支持是通过库完成的（例如 boost）。如果没有类，它实际上只能理解一维数组，尤其是当您使用指针时，C/C++ 确实将指针视为指向数组第一个元素的指针。要让您的示例在没有类的情况下工作，您需要定义一个包含一行的类型，然后创建一个该类型的数组，您可以在显示时为其分配值。

#include <iostream>
#include <cstdlib>
using namespace std;

int main () {
    typedef int row_t[5];
    row_t *arr = new row_t[4] {{3, 6, 9, 23, 16}, 
    {24, 12, 9, 13, 5},
    {37, 19, 43, 17, 11},
    {71, 32, 8, 4, 7}};

    cout<< arr[1][3] << endl<< endl;
    int *x = new int;
    *x = arr [2][1];
    cout<< x<< endl;
    cout<< *x<< endl << endl; 

    *x = arr [0][3];
    cout<< x<< endl;
    cout<< *x<< endl;
    delete x;
    delete [] arr;
    return 0;
}

或者，您可以将二维数组投影到一维数组上：

#include <iostream>
#include <cstdlib>
using namespace std;

int main () {
    int *arr = new int[20] {3, 6, 9, 23, 16, 
    24, 12, 9, 13, 5,
    37, 19, 43, 17, 11,
    71, 32, 8, 4, 7};

    cout<< arr[5*1+3] << endl<< endl;
    int *x = new int;
    *x = arr [5*2+1];
    cout<< x<< endl;
    cout<< *x<< endl << endl; 

    *x = arr [5*0+3];
    cout<< x<< endl;
    cout<< *x<< endl;
    delete x;
    delete [] arr;
    return 0;
}

要使用动态数据获取 2D 索引，请使用 boost::multi_array 之类的东西

#include <iostream>
#include <boost/multi_array.hpp>
using namespace std;

int main () {
    boost::multi_array< int, 2 >arr(boost::extents[4][5]);
    int tmp[] { 3, 6, 9, 23, 16, 
      24, 12, 9, 13, 5,
      37, 19, 43, 17, 11,
      71, 32, 8, 4, 7 };
    arr = boost::multi_array_ref< int, 2 >( &tmp[0], boost::extents[4][5] );

    cout<< arr [1][3]<< endl << endl;
    int *x = new int;
    *x = arr [2][1];
    cout<< x<< endl;
    cout<< *x<< endl << endl; 
    *x = arr [0][3];
    cout<< x<< endl;
    cout<< *x<< endl;
    delete x;
    // delete [] arr;
    return 0;
}

Answer 2

使用 operator new 很棘手。您的程序在使用 new 和 delete 方面存在许多错误。有一个动态数组的标准实现，它隐藏了所有的技巧并自行清理，即 std::vector。另外，避免使用“using namespace std;”这样你可能会遇到令人费解的名称冲突。

效果不错--

#include <iostream>
#include <cstdlib>
#include <vector>

using std::cout;
using std::endl;
using std::vector;

int main () {
    vector<vector<int>> arr 
    { { 3, 6, 9, 23, 16 },
    { 24, 12, 9, 13, 5 },
    { 37, 19, 43, 17, 11 },
    { 71, 32, 8, 4, 7 } };

    cout<< arr[1][3]<< endl<< endl;
    int x = arr[2][1];
    cout<< x<<  endl;
    cout<< x<< endl << endl;
    x = arr[0][3];
    cout<< x<< endl;
    return 0;
}

附：您使用 arr 的方式没有什么动态的。您可以改为这样声明：

int arr[4][5] 
{ { 3, 6, 9, 23, 16 },
{ 24, 12, 9, 13, 5 },
{ 37, 19, 43, 17, 11 },
{ 71, 32, 8, 4, 7 } };