(C++) Two-dimensional std::vectors

Like arrays, vectors can also be two-dimensional. 2-D vectors can be used as matrices.

Create and resize example

The code shows how to create and use them. 2D vectors can be created in either multiple lines (using the resize method or using its flexible constructor. The latter yields a more complex-looking line.

include <iostream>

include <cassert>

include <vector>


int main()
{
  { //Multi-line construction
    const int maxx = 10;
    const int maxy = 5;
    std::vector<std::vector<int> > vec;
    vec.resize(maxx);
    for (int i=0; i<maxx; ++i)
    {
      vec[i].resize(maxy);
    }
    vec.at(maxx-1).at(maxy-1) = 10;
    std::cout << vec[maxx-1][maxy-1] << std::endl;
    assert(vec.size() == maxx);
    assert(vec[0].size() ==maxy);
  }
  { //One-line construction
    const int maxx = 10;
    const int maxy = 5;
    std::vector<std::vector<int> > vec(maxx,std::vector<int>(maxy,0));
    vec.at(maxx-1).at(maxy-1) = 10;
    std::cout << vec[maxx-1][maxy-1] << std::endl;
    assert(vec.size() == maxx);
    assert(vec[0].size() == maxy);
  }
  { //One-line construction
    const int maxx = 5;
    const int maxy = 10;
    std::vector<std::vector<int> > vec(maxx,std::vector<int>(maxy,0));
    vec.at(maxx-1).at(maxy-1) = 10;
    std::cout << vec[maxx-1][maxy-1] << std::endl;
    assert(vec.size() ==maxx);
    assert(vec[0].size() ==maxy);
  }
  std::cout << "Done" << std::endl;
}

std::cout on a 2D std::vector

Can be done overloading the stream out operator.

include <vector>

include <iostream>


//Works for std::vector that are saved [x][y]
template <class T>
std::ostream& operator<<(std::ostream& os, const std::vector<std::vector<T> >& v)
{
  const int maxx = v.size();
  if (maxx == 0)
  {
    os << "{empty std::vector}";
  }
  else
  {
    const int maxy = v[0].size();
    for (int y=0; y!=maxy; ++y)
    {
      for (int x=0; x!=maxx; ++x)
      {
        os  << static_cast<int>(v[x][y]);
      }
      os << '\n';
    }
  }
  return os;
}
//Works for std::vector that are saved [y][x]
template <class T>
std::ostream& operator<<(std::ostream& os, const std::vector<std::vector<T> >& v)
{
  if (v.size() == 0)
  {
    os << "{empty std::vector}";
  }
  else
  {
    for (
      std::vector<std::vector<T> >::const_iterator y=v.begin();
      y!=v.end();
      ++y)
    {
      for (
        std::vector<T>::const_iterator x=(*y).begin();
        x!=(*y).end();
        ++x)
      {
        os  << static_cast<int>(*x);
      }
      os << '\n';
    }
  }
  return os;
}
int main()
{
  const std::vector<std::vector<unsigned char> > image(20,std::vector<unsigned char>(10,127));
  std::cout << std::hex << image << std::dec << '\n';
}

Code links

last edited (June 12, 2007) by bilderbikkel, Number of views: 5886, Current Rev: 9 (Diff)

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