How to declare a 2D array dynamically in C++ using new operator - GeeksforGeeks

13 Apr, 2021
Prerequisite: Array BasicsIn 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 typearray_name: Name of the arraysize1, 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:
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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++
// C++ program to dynamically allocate// the memory for 2D array in C++// using new operator#include <iostream>using namespace std; // Driver Codeint 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;    }         //Delete the array created      delete[] arr;     return 0;}
  
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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++
// C++ program to dynamically allocate// the memory for 3D array in C++// using new operator#include <iostream>using namespace std; // Driver Codeint 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;    }         //Delete the array created      for(int i=0;i<m;i++)    //To delete the inner arrays      delete [] a[i];         delete [] a;              //To delete the outer array                              //which contained the pointers                              //of all the inner arrays           return 0;}
  
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