How to declare a 2D array dynamically in C++ using new operator (original) (raw)

Last Updated : 14 Sep, 2022

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++ `

// C++ program to dynamically allocate // the memory for 2D array in C++ // using new operator #include 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;
}

  //Delete the array created
  delete[] arr;

return 0;

}

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++ `

// C++ program to dynamically allocate // the memory for 2D array in C++ // using new operator #include 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;
}

// 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;

}

Output:

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