Introduction to dynamic two dimensional arrays in C++

Introduction

A dynamic array is an array data structure that can be resized and which allows elements to be added or removed.

There are many ways of creating two dimensional dynamic arrays in C++.

1. Pointer to pointer

First, we will allocate memory for an array which contains a set of pointers. Next, we will allocate memory for each array which is pointed by the pointers. The deallocation of memory is done in the reverse order of memory allocation.

int **dynamicArray = 0;

//memory allocated for elements of rows.


dynamicArray = new int *[ROWS] ;

//memory allocated for  elements of each column.


for( int i = 0 ; i < ROWS ; i++ )
dynamicArray[i] = new int[COLUMNS];

//free the allocated memory


for( int i = 0 ; i < ROWS ; i++ )
delete [] dynamicArray[i] ;
delete [] dynamicArray ;

The above code is for integer values. We can use the template to operate with generic types. In the below example, for the memory allocation, the AllocateDynamicArray function templates are used, and to free the memory, FreeDynamicArray is used.

template <typename T> 
T **AllocateDynamicArray( int nRows, int nCols)
{
      T **dynamicArray;

      dynamicArray = new T*[nRows];
      for( int i = 0 ; i < nRows ; i++ )
      dynamicArray[i] = new T [nCols];

      return dynamicArray;
}

template <typename T>
void FreeDynamicArray(T** dArray)
{
      delete [] *dArray;
      delete [] dArray;
}

int main()
{
      int **my2dArr = AllocateDynamicArray<int>(4,4);
      my2dArr[0][0]=5;
      my2dArr[2][2]=8;
      cout << my2dArr[0][0] << my2dArr[0][1] << endl;
      cout << my2dArr[1][1] <<  my2dArr[2][2]<< endl;
  
      FreeDynamicArray<int>(my2dArr);
      return 0;
}

2. Vector of vector

The above code can be done in one line of code by using a vector of vector, i.e., a vector containing an array of vectors.

vector<vector<T> > dynamicArray(ROWS, vector<T>(COLUMNS));

#include <vector>

using namespace std;

#define ROWS  4
#define COLUMNS  4

//vector<int > vec(5, 7); create a vector with 
//5 elements and each element will have the value 7.


vector<vector<int> > dynamicArray(ROWS, vector<int>(COLUMNS));

for(int i = 0;i < dynamicArray.size();++i)
{
      for(int j = 0;j < dynamicArray[i].size();++j)
      {
           dynamicArray[i][j] = i*j;
      }
}
 
for(int i = 0;i < dynamicArray.size();++i)
{           
     for(int j = 0;j < dynamicArray[i].size();++j)
     {
          cout << dynamicArray[i][j] << endl;
     }
}

3.Vector wrapper class

We have created a wrapper class DynamicArray with a vector<vector<T> > dArray data member. Then, we pass the number of rows and columns as arguments in the constructor.

template <typename T>
class DynamicArray
{
public:
    DynamicArray(){};

    DynamicArray(int rows, int cols): dArray(rows, vector<T>(cols)){}

    vector<T> & operator[](int i) 
    { 
      return dArray[i];
    }
    const vector<T> & operator[] (int i) const 
    { 
      return dArray[i];
    }
    void resize(int rows, int cols)//resize the two dimentional array .


    {
        dArray.resize(rows);
        for(int i = 0;i < rows;++i) dArray[i].resize(cols);
    }
private:
    vector<vector<T> > dArray;  
};

void Matrix(int x, int y)
{
    DynamicArray<int> my2dArr(x, y);
    my2dArr[0][0] = -1;
    my2dArr[0][1] = 5;
    cout << my2dArr[0][0] << endl;
    cout << my2dArr[0][1]  << endl;
}

int main(){

 Matrix(2,2);
 return 0;
}