Iterating over 2-dimensional STL vector c++

Question

I'm currently trying to print out a history of movements for players in a game I am working on. At the end of each round every player has moved some amount in the positive or negative direction and this gets recorded as an int in the movement vector. Eventually I'm wanting to plot the directions moved vs time for each player but I'm having trouble extracting the data out of the 2d vector.

So the first thing I tried was to just iterate and print all the elements, however this doesn't compile:

void output_movement(const std::vector< std::vector<int> > & movement){

    std::vector< std::vector<int> >::iterator row;
    std::vector<int>::iterator col;
    for (row = movement.begin(); row != movement.end(); ++row) {
         for (col = row->begin(); col != row->end(); ++col) {
            std::cout << **col;
         }
    }

}

The compiler gives this error message which I don't really understand:

hg_competition.cpp:45: error: no match for ‘operator=’ in ‘row = ((const std::vector<std::vector<int, std::allocator<int> >, std::allocator<std::vector<int, std::allocator<int> > > >*)money_movement)->std::vector<_Tp, _Alloc>::begin [with _Tp = std::vector<int, std::allocator<int> >, _Alloc = std::allocator<std::vector<int, std::allocator<int> > >]()’
/usr/include/c++/4.4/bits/stl_iterator.h:669: note: candidates are: __gnu_cxx::__normal_iterator<std::vector<int, std::allocator<int> >*, std::vector<std::vector<int, std::allocator<int> >, std::allocator<std::vector<int, std::allocator<int> > > > >& __gnu_cxx::__normal_iterator<std::vector<int, std::allocator<int> >*, std::vector<std::vector<int, std::allocator<int> >, std::allocator<std::vector<int, std::allocator<int> > > > >::operator=(const __gnu_cxx::__normal_iterator<std::vector<int, std::allocator<int> >*, std::vector<std::vector<int, std::allocator<int> >, std::allocator<std::vector<int, std::allocator<int> > > > >&)

Any help is greatly appreciated!

Answer 1

You need to use a const_iterator if the vector is a const reference. Also, to output col you only need to dereference it once.

void output_movement(const std::vector< std::vector<int> > & movement){

    std::vector< std::vector<int> >::const_iterator row;
    std::vector<int>::const_iterator col;
    for (row = movement.begin(); row != movement.end(); ++row) {
         for (col = row->begin(); col != row->end(); ++col) {
            std::cout << *col;
         }
    }
}

Edit: using typedefs will make your code more readable

typedef std::vector<int> Vector;
typedef std::vector<Vector> DoubleVector;

void output_movement(
    const DoubleVector& movement
)
{
    for (DoubleVector::const_iterator row = movement.begin(); row != movement.end(); ++row) {
         for (Vector::const_iterator col = row->begin(); col != row->end(); ++col) {
            std::cout << *col;
         }
         std::cout << std::endl;
    }
}