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copied!<p>I mostly agree with Earwicker in that you can define a range. Now, I am in favor of implementing operators with the real meaning (do what basic types do: two ranges compare equal if both ranges ARE equal). Then you can use the third map parameter to pass it a comparison functor (or function) that solves your particular problem with this map.</p> <pre><code>// Generic range, can be parametrized for any type (double, float, int...) template< typename T > class range { public: typedef T value_type; range( T const & center ) : min_( center ), max_( center ) {} range( T const & min, T const & max ) : min_( min ), max_( max ) {} T min() const { return min_; } T max() const { return max_; } private: T min_; T max_; }; // Detection of outside of range to the left (smaller values): // // a range lhs is left (smaller) of another range if both lhs.min() and lhs.max() // are smaller than rhs.min(). template <typename T> struct left_of_range : public std::binary_function< range<T>, range<T>, bool > { bool operator()( range<T> const & lhs, range<T> const & rhs ) const { return lhs.min() < rhs.min() && lhs.max() <= rhs.min(); } }; int main() { typedef std::map< range<double>, std::string, left_of_range<double> > map_type; map_type integer; // integer part of a decimal number: integer[ range<double>( 0.0, 1.0 ) ] = "zero"; integer[ range<double>( 1.0, 2.0 ) ] = "one"; integer[ range<double>( 2.0, 3.0 ) ] = "two"; // ... std::cout << integer[ range<double>( 0.5 ) ] << std::endl; // zero std::cout << integer[ range<double>( 1.0 ) ] << std::endl; // one std::cout << integer[ 1.5 ] << std::endl; // one, again, implicit conversion kicks in } </code></pre> <p>You must be careful with equality and comparisons among double values. Different ways of getting to the same value (in the real world) can yield slightly different floating point results.</p>

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