reverse iterator

[1vf9z8AR]

Reverse iterator is giving weird errors without showing which line has the error.
const_ won't work as given on stack overflow.

Part of code which I think has the problem.

  map<int,int>::reverse_iterator it;
    for(it=m.rbegin();it!=m.rend();++it)
    {
        if(it->second<pos)
        {
            count+=pos-it->second;
            map<int,int>::reverse_iterator i;
            for(i=it+1;i!=m.rend();++i)
            {
                if(i->second-pos>0)
                    count+=i->second-pos;
            }
            cout<<count;
            return 0;
        }
        if(it->second>pos)
            pos=it->second;
    }

Error message-

In file included from /usr/include/c++/5/bits/stl_algobase.h:67:0, from /usr/include/c++/5/bits/char_traits.h:39, from /usr/include/c++/5/ios:40, from /usr/include/c++/5/ostream:38, from /usr/include/c++/5/iostream:39, from /hackerearth/CPP14_f89e_0633_f143_e667/s_0dbf_a7df_dcc3_6244.cpp:1: /usr/include/c++/5/bits/stl_iterator.h: In instantiation of ‘std::reverse_iterator<_Iterator> std::reverse_iterator<_Iterator>::operator+(std::reverse_iterator<_Iterator>::difference_type) const [with _Iterator = std::_Rb_tree_iterator<std::pair<const int, int> >; std::reverse_iterator<_Iterator>::difference_type = long int]’: /hackerearth/CPP14_f89e_0633_f143_e667/s_0dbf_a7df_dcc3_6244.cpp:23:22: required from here /usr/include/c++/5/bits/stl_iterator.h:233:41: error: no match for ‘operator-’ (operand types are ‘const std::_Rb_tree_iterator<std::pair<const int, int> >’ and ‘std::reverse_iterator<std::_Rb_tree_iterator<std::pair<const int, int> > >::difference_type {aka long int}’) { return reverse_iterator(current - __n); } ^ /usr/include/c++/5/bits/stl_iterator.h:328:5: note: candidate: template<class _Iterator> typename std::reverse_iterator<_Iterator>::difference_type std::operator-(const std::reverse_iterator<_Iterator>&, const std::reverse_iterator<_Iterator>&) operator-(const reverse_iterator<_Iterator>& __x, ^ /usr/include/c++/5/bits/stl_iterator.h:328:5: note: template argument deduction/substitution failed: /usr/include/c++/5/bits/stl_iterator.h:233:41: note: ‘const std::_Rb_tree_iterator<std::pair<const int, int> >’ is not derived from ‘const std::reverse_iterator<_Iterator>’ { return reverse_iterator(current - __n); } ^ /usr/include/c++/5/bits/stl_iterator.h:380:5: note: candidate: template<class _IteratorL, class _IteratorR> decltype ((__y.base() - __x.base())) std::operator-(const std::reverse_iterator<_Iterator>&, const std::reverse_iterator<_IteratorR>&) operator-(const reverse_iterator<_IteratorL>& __x, ^ /usr/include/c++/5/bits/stl_iterator.h:380:5: note: template argument deduction/substitution failed: /usr/include/c++/5/bits/stl_iterator.h:233:41: note: ‘const std::_Rb_tree_iterator<std::pair<const int, int> >’ is not derived from ‘const std::reverse_iterator<_Iterator>’ { return reverse_iterator(current - __n); } ^ /usr/include/c++/5/bits/stl_iterator.h:1138:5: note: candidate: template<class _IteratorL, class _IteratorR> decltype ((__x.base() - __y.base())) std::operator-(const std::move_iterator<_Iterator>&, const std::move_iterator<_IteratorR>&) operator-(const move_iterator<_IteratorL>& __x, ^ /usr/include/c++/5/bits/stl_iterator.h:1138:5: note: template argument deduction/substitution failed: /usr/include/c++/5/bits/stl_iterator.h:233:41: note: ‘const std::_Rb_tree_iterator<std::pair<const int, int> >’ is not derived from ‘const std::move_iterator<_Iterator>’ { return reverse_iterator(current - __n); } ^ /usr/include/c++/5/bits/stl_iterator.h:1145:5: note: candidate: template<class _Iterator> decltype ((__x.base() - __y.base())) std::operator-(const std::move_iterator<_Iterator>&, const std::move_iterator<_Iterator>&) operator-(const move_iterator<_Iterator>& __x, ^ /usr/include/c++/5/bits/stl_iterator.h:1145:5: note: template argument deduction/substitution failed: /usr/include/c++/5/bits/stl_iterator.h:233:41: note: ‘const std::_Rb_tree_iterator<std::pair<const int, int> >’ is not derived from ‘const std::move_iterator<_Iterator>’ { return reverse_iterator(current - __n); } ^

[helios]

std::map iterators (reverse or otherwise) are bidirectional. As such, they only support increments and decrements. They don't support + or -.

std::map<int, int> m;
//...
auto it = m.begin();
it++; //OK
++it; //OK
it--; //OK
--it; //OK
auto &kv = *it; //OK.
auto it2 = it + 3; //Error.
auto &kv2 = it[3]; //Error.
auto it3 = it - 3; //Error. 

[dutch]

It's "i = it + 1" causing the problem. Try this for lines 7 and 8 above:

            map<int,int>::reverse_iterator i = it;
            for(++i; i != m.rend(); ++i)

Note that it would probably be better to post the error outside of an output tag so you get some line wrapping!