Question

I am calculating intersection, union and differences of sets.
I have a typedef of my set type:

typedef set<node_type> node_set;

When it is replaced with

typedef hash_set<node_type> node_set;

The results are different. It’s a complicated program, and before I start debugging – am I doing it right? When I use functions like this:

set_intersection(v_higher.begin(), v_higher.end(), neighbors[w].begin(), neighbors[w].end(), 
            insert_iterator<node_set>(tmp1, tmp1.begin()));

• should they work seamlessly with both set and hash_set?

Answer 1

I don’t think so.

One of the pre-condition of set_intersection is:

• [first1, last1) is ordered in ascending order according to operator<. That is, for every pair of iterators i and j in [first1, last1) such that i precedes j, *j < *i is false.

The hash_set (and unordered_set) is unordered, so the ordered condition cannot be satisfied.

See tr1::unordered_set union and intersection on how to intersect unordered_sets.