vectors, declaring and initialising

Hi i would like to know how to declare a multi dimensional vector and then later initialize its size.

normally id declare and init a vector like :

vector< vector<Tile> > tiles(50, vector<Tile>(50));

creating a multidimensional vector with 50 rows and 50 cols.

i know how to declare a vector and not init its size :

vector< vector<Tile> > tiles;

but how can i later declare the number of rows and columns.

cheers

rob.

Code:

vector< vector<Tile> > tiles;

...

tiles.resize(50);
for( vector<vector<Tile> >::iterator it = tiles.begin(); it != tiles.end(); ++it )
    it->resize(50);

amazing, thank you very much