Question

I have two method :

template <class T>
Matrix<T>::Matrix(int rows, int cols, T* data)

{
 this->nRows = rows;
 this->nCols = cols;

for (int i=0; i < nRows; i++)
{

  vector<T> col(nCols);
     for(int j=0;j<nCols;j++)

      col[j]=*(data+i*nCols+j);

   m.push_back(col);

}

}

And for print the matrix:

  template <class T>
  void Matrix<T>::Dump(void)
  {
    cout << "\t[\n";
    for (int row=0; row<nRows; row++)
    {
     for (int col=0; col<nCols; col++)
      cout << "\t\t" << m[row][col] << " ";
     cout << "\n";
    }
     cout << "\t]\n";
   }

And the test case is :

int M[]={1,2,3,3,4,7,2,5,8};
Matrix<int> m(3,3,M);
m.Dump();

Scenarios(In constructor) are :

  for(int j=0;j<nCols;j++)
     {
      T val(*(data+i*nCols+j));
      col.push_back(val)  ;
        cout<<col[j];
     }

The output is all 0.

But

for(int j=0;j<nCols;j++)
     {
      T val(*(data+i*nCols+j));
      col[j]=val ;
        cout<<col[j];
     }

Gives the correct result.

Please explain why pushback behaves different in both cases ?

Answer 1

This

 vector<T> col(nCols);

line already contracts a column with all zeros in it.
In the first case you push back new value and get something like this

[0, 0, 0, 1, 2, 3]

as the first column. So it is not surprising that

cout<<col[j];

outputs zeros, because j is not bigger than 2!