How to put NULL in all cells of a matrix vector?

Question

Hello, i'm trying to initialize all cells of a matrix with NULL values, but something is wrong here.
the code :

vector<vector<Distance*> > distanceMatrix;  
    for (int i = 0; i < 7 ; i++)
        for (int j = 0; j < 7 ; j++)
            distanceMatrix[i][j].push_back(NULL);  

i bet it's something stupid, thanks for the help.

Answer 1

From the std::vector reference page:

Vectors can be constructed with some values in them.

You may try:

vector<vector<Distance*> > distanceMatrix(7, vector<Distance*>(7, NULL));

---

Also, regarding your problem:

vector<vector<Distance*> > distanceMatrix;  
    for (int i = 0; i < 7 ; i++)
        for (int j = 0; j < 7 ; j++)
            distanceMatrix[i][j].push_back(NULL); //1

When you code first reach //1, distanceMatrix[i] resolves to distanceMatrix[0] but you did not call distanceMatrix.push_back(vector<Distance*>()) so you are referring to a non initialized cell.

To correct code would have been:

vector<Distance*> vec;

for (int j = 0; j < 7 ; j++)
  vec.push_back(NULL);

vector<vector<Distance*> > distanceMatrix;

for (int i = 0; i < 7 ; i++)
{
    distanceMatrix.push_back(vec);
}

Which is still far worse than my first suggestion.

Answer 2

Since the matrix is empty to begin with, you need to push_back every value and every row, not depending on its position in the matrix:

vector<Distance*> row;
for(int j=0; j < 7; j++)
    row.push_back(NULL);

vector<vector<Distance*> > distanceMatrix; 
for(int i=0; i < 7; i++)
    distanceMatrix.push_back(row);