指针对指针的动态二维数组

Question

First timer on this website, so here goes..

I'm a newbie to C++ and I'm currently working through the book "Data structures using C++ 2nd ed, of DS Malik".

In the book Malik offers two ways of creating a dynamic two-dimensional array. In the first method, you declare a variable to be an array of pointers, where each pointer is of type integer. ex.

int *board[4];

..and then use a for-loop to create the 'columns' while using the array of pointers as 'rows'.

The second method, you use a pointer to a pointer.

int **board;
board = new int* [10]; 

etc.

My question is this: which is the better method? The ** method is easier for me to visualize, but the first method can be used in much the same way. Both ways can be used to make dynamic 2-d arrays.

Edit: Wasn't clear enough with the above post. Here's some code I tried:

int row, col;

cout << "Enter row size:";
cin >> row;
cout << "\ncol:";
cin >> col;

int *p_board[row];
for (int i=0; i < row; i++)
    p_board[i] = new int[col];

for (int i=0; i < row; i++)
{
    for (int j=0; j < col; j++)
    {
        p_board[i][j] = j;
        cout << p_board[i][j] << " ";
    }
    cout << endl;
}
cout << endl << endl;

int **p_p_board;
p_p_board = new int* [row];
for (int i=0; i < row; i++)
    p_p_board[i] = new int[col];

for (int i=0; i < row; i++)
{
    for (int j=0; j < col; j++)
    {
        p_p_board[i][j] = j;
        cout << p_p_board[i][j] << " ";
    }
    cout << endl;
}

Answer 1

The first method cannot be used to create dynamic 2D arrays because by doing:

int *board[4];

you essentially allocated an array of 4 pointers to int on stack . Therefore, if you now populate each of these 4 pointers with a dynamic array:

for (int i = 0; i < 4; ++i) {
  board[i] = new int[10];
}

what you end-up with is a 2D array with static number of rows (in this case 4) and dynamic number of columns (in this case 10). So it is not fully dynamic because when you allocate an array on stack you should specify a constant size , ie known at compile-time . Dynamic array is called dynamic because its size is not necessary to be known at compile-time , but can rather be determined by some variable in runtime .

Once again, when you do:

int *board[4];

or:

const int x = 4; // <--- `const` qualifier is absolutely needed in this case!
int *board[x];

you supply a constant known at compile-time (in this case 4 or x ) so that compiler can now pre-allocate this memory for your array, and when your program is loaded into the memory it would already have this amount of memory for the board array, that's why it is called static , ie because the size is hard-coded and cannot be changed dynamically (in runtime).

On the other hand, when you do:

int **board;
board = new int*[10];

or:

int x = 10; // <--- Notice that it does not have to be `const` anymore!
int **board;
board = new int*[x];

the compiler does not know how much memory board array will require, and therefore it does not pre-allocate anything. But when you start your program, the size of array would be determined by the value of x variable (in runtime) and the corresponding space for board array would be allocated on so-called heap - the area of memory where all programs running on your computer can allocate unknown beforehand (at compile-time) amounts memory for personal usage.

As a result, to truly create dynamic 2D array you have to go with the second method:

int **board;
board = new int*[10]; // dynamic array (size 10) of pointers to int

for (int i = 0; i < 10; ++i) {
  board[i] = new int[10];
  // each i-th pointer is now pointing to dynamic array (size 10) of actual int values
}

We've just created an square 2D array with 10 by 10 dimensions. To traverse it and populate it with actual values, for example 1, we could use nested loops:

for (int i = 0; i < 10; ++i) {   // for each row
  for (int j = 0; j < 10; ++j) { // for each column
    board[i][j] = 1;
  }
}

Answer 2

What you describe for the second method only gives you a 1D array:

int *board = new int[10];

This just allocates an array with 10 elements. Perhaps you meant something like this:

int **board = new int*[4];
for (int i = 0; i < 4; i++) {
  board[i] = new int[10];
}

In this case, we allocate 4 int* s and then make each of those point to a dynamically allocated array of 10 int s.

So now we're comparing that with int* board[4]; . The major difference is that when you use an array like this, the number of "rows" must be known at compile-time. That's because arrays must have compile-time fixed sizes. You may also have a problem if you want to perhaps return this array of int* s, as the array will be destroyed at the end of its scope.

The method where both the rows and columns are dynamically allocated does require more complicated measures to avoid memory leaks. You must deallocate the memory like so:

for (int i = 0; i < 4; i++) {
  delete[] board[i];
}
delete[] board;

I must recommend using a standard container instead. You might like to use a std::array<int, std::array<int, 10> 4> or perhaps a std::vector<std::vector<int>> which you initialise to the appropriate size.

Answer 3

In both cases your inner dimension may be dynamically specified (ie taken from a variable), but the difference is in the outer dimension.

This question is basically equivalent to the following:

Is int* x = new int[4]; "better" than int x[4] ?

The answer is: " no , unless you need to choose that array dimension dynamically."

Answer 4

This code works well with very few requirements on external libraries and shows a basic use of int **array .

This answer shows that each array is dynamically sized, as well as how to assign a dynamically sized leaf array into the dynamically sized branch array.

This program takes arguments from STDIN in the following format:

2 2   
3 1 5 4
5 1 2 8 9 3
0 1
1 3

Code for program below...

#include <iostream>

int main()
{
    int **array_of_arrays;

    int num_arrays, num_queries;
    num_arrays = num_queries = 0;
    std::cin >> num_arrays >> num_queries;
    //std::cout << num_arrays << " " << num_queries;

    //Process the Arrays
    array_of_arrays = new int*[num_arrays];
    int size_current_array = 0;

    for (int i = 0; i < num_arrays; i++)
    {
        std::cin >> size_current_array;
        int *tmp_array = new int[size_current_array];
        for (int j = 0; j < size_current_array; j++)
        {
            int tmp = 0;
            std::cin >> tmp;
            tmp_array[j] = tmp;
        }
        array_of_arrays[i] = tmp_array;
    }


    //Process the Queries
    int x, y;
    x = y = 0;
    for (int q = 0; q < num_queries; q++)
    {
        std::cin >> x >> y;
        //std::cout << "Current x & y: " << x << ", " << y << "\n";
        std::cout << array_of_arrays[x][y] << "\n";
    }

    return 0;
}

It's a very simple implementation of int main and relies solely on std::cin and std::cout . Barebones, but good enough to show how to work with simple multidimensional arrays.

Answer 5

this can be done this way

1. I have used Operator Overloading
2. Overloaded Assignment

3. Overloaded Copy Constructor

   /* * Soumil Nitin SHah * Github: https://github.com/soumilshah1995 */ #include <iostream> using namespace std; class Matrix{ public: /* * Declare the Row and Column * */ int r_size; int c_size; int **arr; public: /* * Constructor and Destructor */ Matrix(int r_size, int c_size):r_size{r_size},c_size{c_size} { arr = new int*[r_size]; // This Creates a 2-D Pointers for (int i=0;i < r_size; i++) { arr[i] = new int[c_size]; } // Initialize all the Vector to 0 initially for (int row=0; row<r_size; row ++) { for (int column=0; column < c_size; column ++) { arr[row][column] = 0; } } std::cout << "Constructor -- creating Array Size::" << r_size << " " << c_size << endl; } ~Matrix() { std::cout << "Destructpr -- Deleting Array Size::" << r_size <<" " << c_size << endl; } Matrix(const Matrix &source):Matrix(source.r_size, source.c_size) { for (int row=0; row<source.r_size; row ++) { for (int column=0; column < source.c_size; column ++) { arr[row][column] = source.arr[row][column]; } } cout << "Copy Constructor " << endl; } public: /* * Operator Overloading */ friend std::ostream &operator<<(std::ostream &os, Matrix & rhs) { int rowCounter = 0; int columnCOUNTER = 0; int globalCounter = 0; for (int row =0; row < rhs.r_size; row ++) { for (int column=0; column < rhs.c_size; column++) { globalCounter = globalCounter + 1; } rowCounter = rowCounter + 1; } os << "Total There are " << globalCounter << " Elements" << endl; os << "Array Elements are as follow -------" << endl; os << "\n"; for (int row =0; row < rhs.r_size; row ++) { for (int column=0; column < rhs.c_size; column++) { os << rhs.arr[row][column] << " "; } os <<"\n"; } return os; } void operator()(int row, int column, int Data) { arr[row][column] = Data; } int &operator()(int row, int column) { return arr[row][column]; } Matrix &operator=(Matrix &rhs) { cout << "Assingment Operator called " << endl;cout <<"\n"; if(this == &rhs) { return *this; } else { delete [] arr; arr = new int*[r_size]; // This Creates a 2-D Pointers for (int i=0;i < r_size; i++) { arr[i] = new int[c_size]; } // Initialize all the Vector to 0 initially for (int row=0; row<r_size; row ++) { for (int column=0; column < c_size; column ++) { arr[row][column] = rhs.arr[row][column]; } } return *this; } } }; int main() { Matrix m1(3,3); // Initialize Matrix 3x3 cout << m1;cout << "\n"; m1(0,0,1); m1(0,1,2); m1(0,2,3); m1(1,0,4); m1(1,1,5); m1(1,2,6); m1(2,0,7); m1(2,1,8); m1(2,2,9); cout << m1;cout <<"\n"; // print Matrix cout << "Element at Position (1,2): " << m1(1,2) << endl; Matrix m2(3,3); m2 = m1; cout << m2;cout <<"\n"; print(m2); return 0; }

Answer 6

int row, col;

cout << "Enter row size:";
cin >> row;
cout << "\ncol:";
cin >> col;

    int *p_board[row];
    for (int i=0; i < row; i++)
    p_board[i] = new int[col];

    for (int i=0; i < row; i++)
    {
    for (int j=0; j < col; j++)
    {
        p_board[i][j] = j;
        cout << p_board[i][j] << " ";
    }
    cout << endl;
    }
    cout << endl << endl;

/ / in this method of declaring 2d array using new..first you have created an array of pointers locally not using new so it will be created on stack not on heap then you have created an array using new on every index of p_board, these all arrays are created on heap but due to locally created p_board will make these array of no use becoz when you will use this method in any function and will try to return the p_board pointer from that function..at that time p_board will be vanished from stack because it was created on stack...but 2nd method is preferable to use /

 int **p_p_board;
    p_p_board = new int* [row];
    for (int i=0; i < row; i++)
    p_p_board[i] = new int[col];

   for (int i=0; i < row; i++)
   {
   for (int j=0; j < col; j++)
   {
        p_p_board[i][j] = j;
        cout << p_p_board[i][j] << " ";
    }
    cout << endl;
    }

//in this method both array will be created on heap