Re: [boost] [iterator] std::sort on zip_iterator

Subject: Re: [boost] [iterator] std::sort on zip_iterator
From: Max (more4less_at_[hidden])
Date: 2009-05-04 20:44:22

Hello Willcock,

I also hope your code would work as your desire.
That is an expection natural for a lib to the extreme of elegance and power.

But I'm afraid it won't. I have ever similar experience with some iterator
adaptor,
but not zip_iterator, and have a similar question and similar result. But I
just
cannot remember what exactly it was at the present.

I think your suspection is eaxctly what the matter is.

As soon as I get further information that would be of help, I will let you
know.

B/Rgds
Max

> -----Original Message-----
> From: boost-bounces_at_[hidden] [mailto:boost-bounces_at_[hidden]]
> On Behalf Of Jeremiah Willcock
> Sent: Tuesday, May 05, 2009 2:46 AM
> To: boost_at_[hidden]
> Subject: [boost] [iterator] std::sort on zip_iterator
> 
> (Resubmitting to Boost list because I did not receive any answers on
> boost-users.)
> 
> I would like to sort two separate vectors of numbers, treating
corresponding
> elements of the two vectors as tuples and sorting the tuples.  In
particular, I
> am only comparing the first elements of the tuples; that may or may not be
> relevant to my problem.  An example program is:
> 
> #include <vector>
> #include <algorithm>
> #include <iostream>
> #include <boost/tuple/tuple.hpp>
> #include <boost/iterator/zip_iterator.hpp>
> 
> struct compare_first_elements_in_tuples {
>    template <typename Tuple>
>    bool operator()(const Tuple& a, const Tuple& b) const {
>      return (a.template get<0>()) < (b.template get<0>());
>    }
> };
> 
> int main(int, char**) {
>    size_t a_data[5] = {0, 2, 1, 2, 4};
>    size_t b_data[5] = {1, 1, 5, 3, 7};
>    std::vector<size_t> a(a_data, a_data + 5);
>    std::vector<size_t> b(b_data, b_data + 5);
>    std::cout << "Before:";
>    for (size_t i = 0; i < a.size(); ++i) {
>      std::cout << " (" << a[i] << ", " << b[i] << ")";
>    }
>    std::cout << std::endl;
>    typedef std::vector<size_t>::iterator vec_iter;
>    typedef boost::zip_iterator<boost::tuple<vec_iter, vec_iter> >
zip_iter;
>    std::sort(zip_iter(boost::make_tuple(a.begin(), b.begin())),
>              zip_iter(boost::make_tuple(a.end(), b.end())),
>              compare_first_elements_in_tuples());
>    std::cout << "After:";
>    for (size_t i = 0; i < a.size(); ++i) {
>      std::cout << " (" << a[i] << ", " << b[i] << ")";
>    }
>    std::cout << std::endl;
>    return 0;
> }
> 
> On gcc 4.0.1, the sort operation results in the element (2, 1) (a pair of
> corresponding elements in a and b) being duplicated, while the element (1,
5)
> does not appear in the result at all:
> 
> Before: (0, 1) (2, 1) (1, 5) (2, 3) (4, 7)
> After: (0, 1) (2, 1) (2, 1) (2, 3) (4, 7)
> 
> I suspect the problem is related to the fact that zip_iterator does not
return
> a "real" reference when dereferenced.  Am I mistaken about this? Is the
code
> above supposed to work?  If not, is there an alternative way to sort one
> sequence while swapping elements in another in a corresponding way?  I am
not
> able to join the two input sequences into a single sequence of pairs
because of
> other factors in the use case for this code.  Thank you for your help.
> 
> -- Jeremiah Willcock
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