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How to declare a 2D array dynamically in C++ using new operator
  • Difficulty Level : Expert
  • Last Updated : 11 Dec, 2020

Prerequisite: Array Basics
In C/C++, multidimensional arrays in simple words as an array of arrays. Data in multidimensional arrays are stored in tabular form (in row major order). Below is the general form of declaring N-dimensional arrays:

Syntax of a Multidimensional Array:

data_type array_name[size1][size2]….[sizeN];

data_type: Type of data to be stored in the array. 
Here data_type is valid C/C++ data type
array_name: Name of the array
size1, size2, …, sizeN: Sizes of the dimensions

2D arrays are arrays of single-dimensional arrays.



Syntax of a 2D array:

data_type array_name[x][y];
data_type: Type of data to be stored. Valid C/C++ data type.

Below is the diagrammatic representation of 2D arrays:

For more details on multidimensional and 2D arrays, please refer to Multidimensional arrays in C++ article.

Problem: Given a 2D array, the task is to dynamically allocate memory for a 2D array using new in C++.

Solution: Following 2D array is declared with 3 rows and 4 columns with the following values:

1 2 3 4
5 6 7 8
9 10 11 12

Note: Here M is the number of rows and N is the number of columns.



Method 1: using a single pointer – In this method, a memory block of size M*N is allocated and then the memory blocks are accessed using pointer arithmetic. Below is the program for the same:

C++

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// C++ program to dynamically allocate
// the memory for 2D array in C++
// using new operator
#include <iostream>
using namespace std;
  
// Driver Code
int main()
{
    // Dimensions of the 2D array
    int m = 3, n = 4, c = 0;
  
    // Declare a memory block of
    // size m*n
    int* arr = new int[m * n];
  
    // Traverse the 2D array
    for (int i = 0; i < m; i++) {
        for (int j = 0; j < n; j++) {
  
            // Assign values to
            // the memory block
            *(arr + i * n + j) = ++c;
        }
    }
  
    // Traverse the 2D array
    for (int i = 0; i < m; i++) {
        for (int j = 0; j < n; j++) {
  
            // Print values of the
            // memory block
            cout << *(arr + i * n + j)
                 << " ";
        }
        cout << endl;
    }
  
    return 0;
}

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Output:

1 2 3 4 
5 6 7 8 
9 10 11 12

Method 2: using an array of pointer: Here an array of pointers is created and then to each memory block. Below is the diagram to illustrate the concept:

Below is the program for the same:

C++

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// C++ program to dynamically allocate
// the memory for 3D array in C++
// using new operator
#include <iostream>
using namespace std;
  
// Driver Code
int main()
{
    // Dimensions of the array
    int m = 3, n = 4, c = 0;
  
    // Declare memory block of size M
    int** a = new int*[m];
  
    for (int i = 0; i < m; i++) {
  
        // Declare a memory block
        // of size n
        a[i] = new int[n];
    }
  
    // Traverse the 2D array
    for (int i = 0; i < m; i++) {
        for (int j = 0; j < n; j++) {
  
            // Assign values to the
            // memory blocks created
            a[i][j] = ++c;
        }
    }
  
    // Traverse the 2D array
    for (int i = 0; i < m; i++) {
        for (int j = 0; j < n; j++) {
  
            // Print the values of
            // memory blocks created
            cout << a[i][j] << " ";
        }
        cout << endl;
    }
    return 0;
}

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Output:

1 2 3 4 
5 6 7 8 
9 10 11 12

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