set-intersection

I have been using the highly concise and intuitive C++ syntax for finding the intersection of two sorted vector s and putting the result in a third vector : vector<bar> a,b,c; //... std::set_intersection(a.begin(),a.end(),b.begin(),b.end(), std::back_inserter(c)); This should set c to intersection( a , b ), assuming a and b are sorted. But what if I just use c.begin() (I thought I saw an example somewhere of this, which is why I did): std::set_intersection(a.begin(),a.end(),b.begin(),b.end(), c.begin()); set_intersection expects an OutputIterator at that parameter. The standard I believe

I have come across some special purpose implementation of set operations, but nothing for the general case. What is the general case for performing set operations (specifically intersection, union, symmetric difference). This is easier to figure out using javascript in a $where or map reduce, but I want to know how to do this in aggregation in order to get native performance. The better way to illustrate this question is with an example. Say I have a record with 2 arrays/sets: db.colors.insert({ _id: 1, left : ['red', 'green'], right : ['green', 'blue'] }); I want to find the union,

First of all, my purpose is to randomly get only one element in both known sets. So my original method is firstly intersect two sets. And then randomly pick up a element from the intersected set. But this is foolish, because that I only need a elements but a intersected set. So I need to find the algorithm of set.intersection(). I compare the cost time between the methods of 'set.intersection()' and 'for{for{}}'. Set.intersection() is more faster than other one(100 times). So using 'for{for{}}' to pick up a randomly elements is not a wise idea. What's the algorithm behind set.intersection() in

Is there a method in java.util.Map or any util to perform an intersection on two maps? (To intersect two maps by the "keys") I am not able to find any. I can always implement my own intersection logic, but I was hoping there is already some operation in one of the java.util.* classes that would do this. How about: Map map1 = ...; Map map2 = ...; Map result = new ...(map1); result.keySet().retainAll(map2.keySet()); or: Map map1 = ...; Map map2 = ...; Set result = new ...(map1.keySet()); result.retainAll(map2.keySet()); Louis Wasserman If you're using Guava, you can use Maps.difference to get a

I have two arrays list1 and list2 which have objects with some properties; userId is the Id or unique property: list1 = [ { userId: 1234, userName: 'XYZ' }, { userId: 1235, userName: 'ABC' }, { userId: 1236, userName: 'IJKL' }, { userId: 1237, userName: 'WXYZ' }, { userId: 1238, userName: 'LMNO' } ] list2 = [ { userId: 1235, userName: 'ABC' }, { userId: 1236, userName: 'IJKL' }, { userId: 1252, userName: 'AAAA' } ] I'm looking for an easy way to execute the following three operations: list1 operation list2 should return the intersection of elements: [ { userId: 1235, userName: 'ABC' }, {

Given two lists (not necessarily sorted), what is the most efficient non-recursive algorithm to find the intersection of those lists? You could put all elements of the first list into a hash set. Then, iterate the second one and, for each of its elements, check the hash to see if it exists in the first list. If so, output it as an element of the intersection. You might want to take a look at Bloom filters. They are bit vectors that give a probabilistic answer whether an element is a member of a set. Set intersection can be implemented with a simple bitwise AND operation. If you have a large