how to find the intersection of two std::set in C++?

Question

I have been trying to find the intersection between two std::set in C++, but I keep getting an error.

I created a small sample test for this

#include <iostream> #include <vector> #include <algorithm> #include <set> using namespace std;  int main() {   set<int> s1;   set<int> s2;    s1.insert(1);   s1.insert(2);   s1.insert(3);   s1.insert(4);    s2.insert(1);   s2.insert(6);   s2.insert(3);   s2.insert(0);    set_intersection(s1.begin(),s1.end(),s2.begin(),s2.end());   return 0; } 

The latter program does not generate any output, but I expect to have a new set (let's call it s3) with the following values:

s3 = [ 1 , 3 ] 

Instead, I'm getting the error:

test.cpp: In function ‘int main()’: test.cpp:19: error: no matching function for call to ‘set_intersection(std::_Rb_tree_const_iterator<int>, std::_Rb_tree_const_iterator<int>, std::_Rb_tree_const_iterator<int>, std::_Rb_tree_const_iterator<int>)’ 

What I understand out of this error, is that there's no definition in set_intersection that accepts Rb_tree_const_iterator<int> as a parameter.

Furthermore, I suppose the std::set.begin() method returns an object of such type,

Is there a better way to find the intersection of two std::set in C++? Preferably a built-in function?

Answer 1

You haven't provided an output iterator for set_intersection

template <class InputIterator1, class InputIterator2, class OutputIterator> OutputIterator set_intersection ( InputIterator1 first1, InputIterator1 last1,                                   InputIterator2 first2, InputIterator2 last2,                                   OutputIterator result ); 

Fix this by doing something like

...; set<int> intersect; set_intersection(s1.begin(), s1.end(), s2.begin(), s2.end(),                  std::inserter(intersect, intersect.begin())); 

You need a std::insert iterator since the set is as of now empty. We cannot use std::back_inserter or std::front_inserter since set doesn't support those operations.

Answer 2

Have a look at the sample in the link: http://en.cppreference.com/w/cpp/algorithm/set_intersection

You need another container to store the intersection data, below code suppose to work:

std::vector<int> common_data; set_intersection(s1.begin(),s1.end(),s2.begin(),s2.end(), std::back_inserter(common_data));