Saturday, April 16, 2011

STL multiset = sorted Bag

(Q: Have you ever wondered why set/multiset are grouped with map/multimap as Associative containers? Answer revealed at the end.)

http://en.wikipedia.org/wiki/Set_(abstract_data_type)#Multiset points out

It is possible for distinct objects to be considered "equivalent" under some equivalence relation but still distinct under another relation. Some types of multiset implementations will store distinct equivalent objects as separate items in the data structure; while others will collapse it down to one version (the first one encountered) and keep a positive integer count of the multiplicity of the element.

STL multiset is the first type. See also P299 [[STL Tutorial and Reference Guide]] written by one of the STL inventors.

STL multiset is a reliable store keeper -- never "loses" items that look alike. In contrast, the counting version discards items as _unwanted_duplicate_. STL multiset associates objects to invisible keys -- associative containers.

int main() {
    float floats[3] = {1.1,1.2,2};
    multiset<float, less<int> > cont;
    cont.insert(floats, floats+3);
    copy(cont.begin(), cont.end(), ostream_iterator<float>(cout, "   "));
}

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