Re: std::make_heap - compilation issues ?

"Tom Widmer [VC++ MVP]" <>
Thu, 01 Feb 2007 14:20:16 +0000
Lady_A wrote:


I have a set with a custom ordering operator defined on it.
My goal is get 10 max elements using another ordering.
To do that I am thinking to use make_heap algorithm on the set,
supplying the begin and end iterators of the set and a new ordering

You can't do that - the ordering of a set is a fundamental invariant of
the set, and you can't alter it once you have created the set. Instead,
you're going to have to copy the set to another container (I'd recommend
a std::vector). e.g.

set<A*, op1>* my_set; //why a pointer?

void B:my_func() {
   std::vector<A*> v;
   v.insert(v.end(), my_set->begin(), my_set()->end());

   make_heap(v.begin(), v.end(), op2());

However, if you want the 10 maximum elements, use this:

typedef std::vector<A*>::const_iterator it;

std::vector<A*> B::getTopN(size_t N) {
   std::vector<A*> v;
   v.insert(v.end(), my_set->begin(), my_set()->end());
   int pos = std::min(v.size(), N);
   std::nth_element(v.begin(), v.begin() + pos, v.end(), op2());
   return v; //v contains top N elements.

or similar.

Why do I need operator- ? Operator for comparing is not enough ?

make_heap needs random access iterators.


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