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

From:

Renzr <renzhengyong@gmail.com>

Newsgroups:

comp.lang.c++

Date:

Wed, 03 Oct 2007 07:41:04 -0700

Message-ID:

<1191422464.621283.181920@r29g2000hsg.googlegroups.com>

On Oct 3, 10:35 pm, Erik Wikstr=F6m <Erik-wikst...@telia.com> wrote:

On 2007-10-03 16:12, Renzr wrote:

On Oct 3, 9:56 pm, "Victor Bazarov" <v.Abaza...@comAcast.net> wrote:

Renzr wrote:

I have a problem about the std::set<>iterator. After finding a term =

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;

There is no operator - defined for it, that's what the compiler is
telling you. If you need to know how far it's from the start (though
I am not sure why you need that), use 'std::distance' function.

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::operato=

(const std::_Bit_iterator_base&, const std::_Bit_iterator_base&)

V
--
Please remove capital 'A's when replying by e-mail
I do not respond to top-posted replies, please don't ask

Thanks for your solution.
The resean is in my FEM code, I want to get the local position of DOF
in the ith row.
So i design the std::vector<std::set<unsigned int> >.
Maybe std::vector<std::vector<unsigned int> > is prefered.
In my loop to ith row (std::vector[i]), several "unsigned int" will be
added into std::vector[i].
And i need the sorted "unsigned int" in std::vector[i]. So, I have two
ways.
The first is to design the std::vector[i] as std::vector<unsigned
int>. But this way we should involve in std::sort() on
std::vector<unsigned int>. The second way is to design the
std::vector[i] as std::set<unsigned int>.

Actually, it is probably cheaper to insert the elements in the vector
and then sort it that it is to insert them into the set. For large
number of elements the difference can be quite noticeable.

--
Erik Wikstr=F6m

Thanks for your solution.
Your help offer me a right choice on the std::vector<> and std::set<>