如何在二维向量上使用std :: max_element

Question

I've looked around for the solution to this problem and was unable to turn up anything. I am sure it is a quick fix, hopefully someone can spot my error and show me.

Here is what I've got

#include<algorithm>
#include<vector>

void myFunction ( cont std::vector<std::vector<int> > &counts){
    std::vector<int>::iterator max_it;
    int index;
    for ( int i = 0 ; i < counts.size() ; i ++ ){
        max_it = std::max_element(counts[i].begin(), counts[i].end());
        index = std::distance(counts[i].begin(),max_it);
    }
}

When compiling the above code I get the following error for the line where max_element is called.

error: no match for ‘operator=’ in ‘it = std::max_element [with _ForwardIterator = __gnu_cxx::__normal_iterator<const int*, std::vector<int, std::allocator<int> > >](((const std::vector<int, std::allocator<int> >*)((const std::vector<std::vector<int, std::allocator<int> >, std::allocator<std::vector<int, std::allocator<int> > > >*)counts)->std::vector<_Tp, _Alloc>::operator[] [with _Tp = std::vector<int, std::allocator<int> >, _Alloc = std::allocator<std::vector<int, std::allocator<int> > >](((long unsigned int)i)))->std::vector<_Tp, _Alloc>::begin [with _Tp = int, _Alloc = std::allocator<int>](), ((const std::vector<int, std::allocator<int> >*)((const std::vector<std::vector<int, std::allocator<int> >, std::allocator<std::vector<int, std::allocator<int> > > >*)counts)->std::vector<_Tp, _Alloc>::operator[] [with _Tp = std::vector<int, std::allocator<int> >, _Alloc = std::allocator<std::vector<int, std::allocator<int> > >](((long unsigned int)i)))->std::vector<_Tp, _Alloc>::end [with _Tp = int, _Alloc = std::allocator<int>]())’

/usr/include/c++/4.2.1/bits/stl_iterator.h:637: note: candidates are: __gnu_cxx::__normal_iterator > >& __gnu_cxx::__normal_iterator > >::operator=(const __gnu_cxx::__normal_iterator > >&)

I have tried several solutions, mostly involving changing how I declare the max_it iterator. Nothing has worked so far and I am unable to decipher the error message.

Answer 1

Your iterator should be a constant iterator, since you are invoking begin() and end() through a reference to const :

std::vector<int>::const_iterator max_it;
//                ^^^^^^