How to Declare 2D Array Using new in C++

1. Declare 2D Array to Access Elements With arr[x][y] Notation
2. Declare 2D Array to Access Elements With arr[] Notation
3. Use vector Container to Implicitly Allocate Dynamic 2D Array

This article introduces multiple C++ methods to declare a 2D array dynamically using new.

Declare 2D Array to Access Elements With arr[x][y] Notation

This solution utilizes new keyword so that the generated matrix structure can be accessed using array notation - [x][y]. At first, we declare pointer to pointer to integer (int **) variable and allocate int pointer array of row size in the array. Next, we loop over this pointer array and allocate the int array of column size each iteration. Lastly, when we finish the 2D array operation, we need to free up the allocated memory. Notice, that deallocation is done in the reverse order loop.

#include <iomanip>
#include <iostream>

using std::cin;
using std::cout;
using std::endl;
using std::setw;

constexpr int ROW = 4;
constexpr int COL = 6;

int main() {
  int **matrix = new int *[ROW];
  for (int i = 0; i < ROW; ++i) {
    matrix[i] = new int[COL];
  }

  for (int i = 0; i < ROW; ++i) {
    for (int j = 0; j < COL; ++j) {
      matrix[i][j] = rand() % 100;
      cout << setw(2) << matrix[i][j] << "; ";
    }
    cout << endl;
  }

  for (int i = 0; i < ROW; ++i) {
    delete matrix[i];
  }
  delete[] matrix;
  return EXIT_SUCCESS;
}

Output:

83; 86; 77; 15; 93; 35;
86; 92; 49; 21; 62; 27;
90; 59; 63; 26; 40; 26;
72; 36; 11; 68; 67; 29;

Declare 2D Array to Access Elements With arr[] Notation

This method uses a single array allocation, making it a more efficient method in terms of program execution speed. Consequently, we should use the [] notation and some arithmetic to access the elements. This method is the recommended way when the array size gets bigger, and calculations on its elements are quite intensive. Note also that deallocation becomes easier this way.

#include <iomanip>
#include <iostream>

using std::cin;
using std::cout;
using std::endl;
using std::setw;

constexpr int ROW = 4;
constexpr int COL = 6;

int main() {
  int *matrix = new int[ROW * COL];

  for (int i = 0; i < ROW; ++i) {
    for (int j = 0; j < COL; ++j) {
      matrix[j * ROW + i] = rand() % 100;
      cout << setw(2) << matrix[j * ROW + i] << "; ";
    }
    cout << endl;
  }
  delete[] matrix;

  return EXIT_SUCCESS;
}

Use vector Container to Implicitly Allocate Dynamic 2D Array

Alternatively, we can construct a dynamic 2D array using a vector container and without manual memory allocation/deallocation. In this example, we initialize a 4x6 vector_2d array with each element of the value - 0. The best part of this method is the flexibility of the iteration with the range-based loop.

#include <iomanip>
#include <iostream>
#include <vector>

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

constexpr int ROW = 4;
constexpr int COL = 6;

int main() {
  vector<vector<int> > vector_2d(ROW, vector<int>(COL, 0));

  for (auto &item : vector_2d) {
    for (auto &item1 : item) {
      item1 = rand() % 100;
      cout << setw(2) << item1 << "; ";
    }
    cout << endl;
  }
  return EXIT_SUCCESS;
}