C++ min heap with user-defined type.

Question

Hi,

I am trying to implement a min heap in c++ for a struct type that I created. I created a vector of the type, but it crashed when I used make_heap on it, which is understandable because it doesn't know how to compare the items in the heap. How do I create a min-heap (that is, the top element is always the smallest one in the heap) for a struct type?

The struct is below:

struct DOC{

int docid;
double rank;

};

I want to compare the DOC structures using the rank member. How would I do this?

I tried using a priority queue with a comparator class, but that also crashed, and it also seems silly to use a data structure which uses a heap as its underlying basis when what I really need is a heap anyway.

Thank you very much, bsg

Answer 1

Add a comparison operator:

struct DOC{

    int docid;
    double rank;
    bool operator<( const DOC & d ) const {
       return rank < d.rank;
    }
};

Structures can almost always usefully have a constructor, so I would also add:

DOC( int i, double r ) : docid(i), rank(r) {]

to the struct as well.

Answer 2

Simply create your own "functor" for the comparison. Since you want a "min heap" your comparison function should behave like the greater than operator:

#include <iostream>
#include <vector>
#include <algorithm>

struct doc {
    double rank;
    explicit doc(double r) : rank(r) {}
};

struct doc_rank_greater_than {
    bool operator()(doc const& a, doc const& b) const {
        return a.rank > b.rank;
    }
};

int main() {
    std::vector<doc> docvec;
    docvec.push_back( doc(4) );
    docvec.push_back( doc(3) );
    docvec.push_back( doc(2) );
    docvec.push_back( doc(1) );
    std::make_heap(docvec.begin(),docvec.end(),doc_rank_greater_than());
    std::cout << docvec.front().rank << '\n';
}

It's important that you always use the same comparison function in further heap operations.