Re: [ublas] ublas::vector in std::map followup

From: johanngoetz (jgoetz_at_[hidden])
Date: 2008-05-31 12:22:31

martux wrote:
> 
> Hi, I tried using a ublas vector as key type in a std::map along the lines
> of
> 
> boost::ublas::vector<double> x(2);
> std::map<boost::ublas::vector<double>, int> vm;
> vm[x] = 1;
> 
> which wont compile (gcc 4.0.3) due to
> 
> /usr/lib/gcc/x86_64-linux-gnu/4.0.3/../../../../include/c++/4.0.3/bits/stl_function.h:227:
> error: no match 
> for ‘operator<’ in ‘__x < __y’.
> 
> Adding an implementation of
> 
> bool operator<(boost::ublas::vector<double> const& A,
> boost::ublas::vector<double> const& B) {...}
> 
> unfortunately doesnt help. Could anyone please point out what I am
> missing?
> 

I am working on a fieldmap object based on std::map of
ublas::bounded_vectors so that I can take advantage of matrix multiplication
of ublas as well as the storage/iterators of std::map. However,
ublas::vector (and ublas::bounded_vector) do not have the necessary
properties to comply with the requirements of the Key value of a std::map.

I have a workaround, though I am sure it could be written in a much more
general way, I just don't know how to do it. Since I am working with
bounded_vector<int, dimension> where dimensions are small, I created a
operator< and a functor bouded_vector_less<T> and pass this to the third
template parameter of std::map. Thus:

template<typename _Tp, size_t _Dim>
inline bool
operator<(
        const bounded_vector<_Tp, _Dim>& __x,
        const bounded_vector<_Tp, _Dim>& __y
) {
        return std::lexicographical_compare(
                __x.begin(),
                __x.end(),
                __y.begin(),
                __y.end()
        );
}
template <typename _Tp>
struct my_less {
        bool
        operator()(
                const _Tp& __x,
                const _Tp& __y
        ) const {
                return __x < __y;
        }
};

This is more or less a copy of what is in the standard template library.
Therefore, when I create the map I can do this:

std::map<
    ublas::bounded_vector<int,    3>,
    ublas::bounded_vector<float, 3>,
    my_less<
        ublas::bounded_vector<int,    3>
    >
> fieldmap;

Any suggestions on making this more general and/or more efficient would be
greatly appreciated!

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