How to expand a 2D vector?

[northfly]

for example, I defined an empty 2D vector

vector<vector<size_t> > PI;

now I want to expand this 2D vector to Nx rows and Ny columns one by one and assign different value for each;

How can I do this? using push_back()?

[coder777]

One approach could look like this:

vector<vector<size_t> > PI;
...

const size_t cur_y = PI.size();
PI.resize(cur_y + Ny);
for(size_t y = cur_y; y < PI.size(); ++y)
{
  const size_t cur_x = PI[y].size();
  PI[y].resize(cur_x + Nx);
  for(size_t x = cur_x; x < PI[y].size(); ++x)
  {
    PI[y][x] = ...;
  }
}