Tutorial :Two dimensional array using vector


I want to create 2D array using vector. But, when I do this, I get seg fault. Can anyone please explain what I am doing wrong, and possible solution for this problem.

I made everything public since I dont want to deal with getters and setters now. I want to get the concept of 2D array clear.

#include <iostream>  #include <vector>  using namespace std;    class point  {         public:          point():x(0),y(0){}          ~point(){}          point(float xx,float yy):x(xx),y(yy){}          float x,y;  };    int main()  {      vector<vector<point> > a; // 2D array      point p(2,3);      a[0][0] = p; // error here      return 0;  }  


Your vector is empty. So you can't use [0][0].

Here is how you declare it:

a.push_back(vector<point>());  a[0].push_back(p);  

If you know how many items you will have from the start, you can do :

vector<vector<point> > a(10, vector<point>(10));  

It's a vector containing 10 vectors containing 10 point. Then you can use

a[4][4] = p;  

However, I believe that using vector of vectors is confusing. If you want an array, consider using uBLAS http://www.boost.org/doc/libs/1_41_0/libs/numeric/ublas/doc/index.htm

#include <boost/numeric/ublas/matrix.hpp>  #include <boost/numeric/ublas/io.hpp>    int main () {      using namespace boost::numeric::ublas;      matrix<double> m (3, 3);      for (unsigned i = 0; i < m.size1 (); ++ i)          for (unsigned j = 0; j < m.size2 (); ++ j)              m (i, j) = 3 * i + j;      std::cout << m << std::endl;  }  


Here's another suggestion. What you're trying to accomplish has been done before and can be found within the Boost Multi-Array.


You have constructed a vector of vectors that is empty, and have tried to dereference the first element without adding any elements to it.

Vectors don't work like (some) associative arrays, where attempting to access a value that's missing will add it to the collection. You need to ensure the vectors have an appropriate number of entries before you try to access them by using the appropriate form of the vector constructor or by using push_back.


You're creating your 2D array just fine. The problem is that when you create it, it's an empty array -- it doesn't hold any points at all yet. You try to use the point at [0][0] before you've actually created a point there. Normally, to put a new element into a vector, you use resize() to set the size of the vector, or push_back() to add items one at a time. In this case, the latter will probably be a bit clumsy -- since you have a vector of vectors of point, you need to create a vector of point, push a point onto that vector, then push that vector onto your array.


Managed to get it working. Picked up the idea for the 'typedef' from somewhere else. Try the below code, it works:

#include <iostream>  #include <stdlib.h>  #include <stdio.h>  #include <vector>      using namespace std;    int main()  {      int i = 0;      int j = 0;    ///////////////////////////////////////////////////////////        typedef vector<string> vecRow;      typedef vector<vecRow> vecCol;        vecRow vr;      vecCol vc;  ///////////////////////////////////////////////////////////  // Assigning string elements to the 2d array        for(i=0;i<10;i++)      {              for(j=0;j<5;j++)              {                  vr.push_back("string ["+to_string(i)+"]["+to_string(j)+"]");              }              vecRow vr_temp = vecRow(vr);              vc.push_back(vr_temp);              vr.clear();      }    ///////////////////////////////////////////////////////////  // Printing back the elements from the 2D array        for(auto element : vc)      {              for(unsigned int ictr = 0;ictr < element.size() ; ictr++)              {                  cout<<element[ictr]<<"\t";              }              cout<<endl;      }        getchar();      return 0;  }  


You can define vectorMatrix[][], which is a matrix of vectors as follows.


class vectorMatrix  {    std::vector<object> **cell;      int columns;    int rows;      public:    vectorMatrix(int columns, int rows);    virtual ~vectorMatrix();      void addCellAt(int row, int column, const object& entry);      virtual std::vector<object>* getCell(int row, int column);      void clearMatrix();  };  

Define constructor:

vectorMatrix::vectorMatrix(int columns, int rows)  {     this->columns = columns;     this->rows = rows;       cell = new std::vector<object>* [columns];       for (int i = 0; i < columns; i++)     {         cell[i] = new std::vector<object>[rows];     }  }  

A method for adding an entry:

void vectorMatrix::addCellAt(int row, int column, const object& entry)  {         cell[channel][timeSlot].push_back(entry);  }  

Getting a pointer to the vector in a given row and column:

std::vector<object>* vectorMatrix::getCell(int row, int column)  {      return &cell[row][column];  }  

Clearing all the matrix:

void vectorMatrix::clearMatrix()  {      for (int tmpRow = 0; tmpRow < columns; tmpRow ++)      {          for(int tmpColumn = 0; tmpColumn < rows; tmpColumn ++)          {              cell[tmpRow][tmpColumn].clear();          }      }  }  


You can use resize(); e.g., here I resize a to a 100 x 200 array:

  vector<vector<point> > a; // 2D array                                             a.resize(100);    for_each(a.begin(),a.end(),[](vector<point>& v){v.resize(200);});    point p(2,3);    a[0][0] = p; // ok now                                                      


The simplest way would be to use resize() method as follow:

vector <vector<int>> v;  cin>>n>>m; //n is rows and m is columns  v.resize(n,vector<int>(m));  for(i=0;i<n;i++)      // inserts elements into the vector v   for(j=0;j<m;j++)    cin>>v[i][j];     for(i=0;i<n;i++)      //accesses elements of vector v   for(j=0;j<m;j++)     cout<<v[i][j]<<" ";  


The simplest way to use vector as C-style array

int z =0;  vector<vector<int>> vec(5,5);  for(int i =0; i < 5; i++)  {             for(int j=0; j<5; j++)      {          vec[i][j] = ++z;      }  }      for(int i =0; i < x; i++)  {        for(int j=0; j<x; j++)      {          cout<<vec[i][j]<<" ";      }    }  

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