Question

I need to resize a two dimensional vector (of unknown size) as follows:

//Creating the vector of vectors: 

vector< vector< long > > v;

//Resizing the vectors:

/*Needing help here: 

my current assumption is: 
v.resize(1);
v[0].resize(1);
*/

//Adding elements:

v[0][0].push_back(0); 
v[0][1].push_back(-1); 

The compiler is reporting error upon applying the push_back. I think i have a problem in resizing the vector.

The objective is creating a two dimensional vector that allocates memory dynamically according to the added values.

Thanks in advance for your help

Answer 1

Your assumption about resizing the two dimensions is correct, but you are getting an error on push_back because the type of v[0][0] is long &, not vector<long> &.

I think you want:

 v.resize(1);
 //Don't resize the second dimension because you want it to grow dynamically
 //v[0].resize(1);
 v[0].push_back(0);
 v[0].push_back(-1);

Keep in mind though that push_back only resizes that particular row/column. If you’re not consistent about adding the same number of elements to v[0], v[1], etc, you’ll end up with a “jagged” array. You may want to wrap this whole thing up in a class to enforce that consistency.