Re: A question about the std::set<>::iterator

I have a problem about the std::set<>iterator. After finding a term in
the std::set<>, i want to know the distance from
the current term to the begin(). But i have got a error. Please offer
me help, thank you. I am a freshman about the STL. The following is
the code.
#include <set>
#include <iostream>
#include <vector>
int main()
{
 std::set<unsigned int> a;
 a.insert(1); a.insert(2); a.insert(3); a.insert(4); a.insert(5);
 std::set<unsigned int> b;
 b.insert(6); b.insert(7); b.insert(8); b.insert(9); b.insert(10);
 std::vector<std::set<unsigned int> > ab(2);
 ab[0]= a; ab[1]=b;
 {
   const unsigned int i=0, j=5;
   std::set<unsigned int>::iterator it=ab[i].find(5);
   if(it!=ab[i].end()) std::cout<<it-ab[i].begin()<<std::endl;

   else { std::cout<"There is no:"<<j<<" in "<<i<<std::endl;
   }
 }
 return 0;
}

This is the error:
error: no match for 'operator-' in 'it - (+(&ab)->std::vector<_Tp,
_Alloc>::operator[] [with _Tp = std::set<unsigned int,
std::less<unsigned int>, std::allocator<unsigned int> >, _Alloc =
std::allocator<std::set<unsigned int, std::less<unsigned int>,
std::allocator<unsigned int> > >](0u))->std::set<_Key, _Compare,
_Alloc>::begin [with _Key = unsigned int, _Compare =
std::less<unsigned int>, _Alloc = std::allocator<unsigned int>]()'
/usr/lib/gcc/i386-redhat-linux/3.4.2/../../../../include/c++/3.4.2/
bits/stl_bvector.h:180: note: candidates are: ptrdiff_t std::operator-
(const std::_Bit_iterator_base&, const std::_Bit_iterator_base&)