generating a set of sets that appear in every set - c++

Question

I have an array of arrays of things
typedef std::vector<thing> group;
std::vector<group> groups;
things could be compared like so
int comparison(thing a, thing b);
where the return value is 0, 1 or 2
0 means that the things are not alike
1 means that they are alike and a is more specific or equal to b
2 means that they are alike and b is more specific or equal to a
and I am looking for a function that would return me a group that contains all things that appear in every group.
std::getgroup(groups.begin(), groups.end(), myComparisonFunction);
the problem is I have no idea what this function may be called, if it does even exist, or what the closest thing to it would be.

Answer 1

Eventually, what you want is an intersection. Luckily, there is std::set_intersection which almost does what you need. Here's a simple example on std::vector<std::vector<int>>. You can easily change it to work with your thing:
#include <iostream>
#include <vector>
#include <algorithm>
std::vector<int> getGroup(const std::vector<std::vector<int>>& groups) {
std::vector<int> group;
std::vector<int> temp = groups[0];
std::sort(temp.begin(), temp.end());
for ( unsigned i = 1; i < groups.size(); ++i ) {
group = std::vector<int>();
std::vector<int> temp2 = groups[i];
std::sort(temp2.begin(), temp2.end());
std::set_intersection(temp2.begin(), temp2.end(),
temp.begin(), temp.end(),
std::back_inserter(group));
temp = group;
}
return group;
}
int main() {
std::vector<std::vector<int>> groups = { {1, 2, 3, 4, 5, 6, 7, 8, 9, 10},
{1, 2, 3, 5, 6, 7, 8, 10},
{1, 2, 3, 4, 5, 6, 7, 8, 9, 10},
{1, 3, 4, 5, 6, 9, 10},
{1, 2, 6, 7, 8, 9, 10},
{1, 2, 3, 4, 5, 6, 7, 8, 9, 10} };
for ( auto g : getGroup(groups) )
std::cout << g << "\n";
return 0;
}
This will print:
1
6
10