How to add distinct pairs into a set? - c++

Question

I am trying to insert a few pairs of int into a set, these pairs don't have any order; that is, (1,2) = (2,1). So simply I do as follows,
typedef pair<int, int> pairs;
set<pairs> Set;
pair <int,int> myPair;
// adding pairs:
myPair=pair<int,int>(0,1);
pathSet.insert(myPair);
myPair=pair<int,int>(0,2);
pathSet.insert(myPair);
myPair=pair<int,int>(1,0);
pathSet.insert(myPair);
So I end up a set like
(0,1), (0,2) , (1,0)
I want to have
(0,1), (0,2)
How can I avoid duplicate? Is there any way? Is there any better ADT like std::unordered_set in terms of efficiency in comparison with 'set'?

Answer 1

You'll need a custom comparison function. In there, make sure that the order of the elements in a pair do not matter when comparing. A simple way would be to let the first element in the pair always be the smaller one (and exchange first and second otherwise).
The code could look like as follows:
int main() {
typedef pair<int, int> pairs;
auto cmp = [](pairs a, pairs b) {
if (a.first > a.second) {
swap(a.first, a.second);
}
if (b.first > b.second) {
swap(b.first, b.second);
}
return a < b;
};
set<pairs, decltype(cmp)> pathSet(cmp);
pairs myPair=pair<int,int>(0,1);
pathSet.insert(myPair);
myPair=pair<int,int>(0,2);
pathSet.insert(myPair);
myPair=pair<int,int>(1,0);
pathSet.insert(myPair);
cout << pathSet.size();
}
Output:
2

Answer 2

You need a custom compare function for std::set as you are using a std::pair as template type. In C++11, you can make it as a lambda as well.
The idea of compare function is to first check if the Pair is Pair.first < Pair.second, if not, swap to make them in order inside the compare function. This will not change the order of original inserted pair elements, but remove the duplicates as you mentioned.
auto compare = [](pairs lhs, pairs rhs)
{
if(lhs.first > lhs.second ) lhs = pairs{lhs.second, lhs.first };
if(rhs.first > rhs.second ) rhs = pairs{rhs.second, rhs.first };
return lhs< rhs;
};
Something like this: See live here
#include <iostream>
#include <set>
typedef std::pair<int, int> pairs;
int main()
{
auto compare = [](pairs lhs, pairs rhs) //custom compare lambda function
{
if(lhs.first > lhs.second ) lhs = pairs{lhs.second, lhs.first };
if(rhs.first > rhs.second ) rhs = pairs{rhs.second, rhs.first };
return lhs< rhs;
};
std::set<pairs, decltype(compare)> Set(compare);
Set.emplace(std::make_pair(0,1)); // use can also emplace to the Set
Set.emplace(pairs{0,2});
Set.emplace(pairs{1,0});
for(const auto& it: Set)
std::cout << it.first << " " << it.second << std::endl;
}
Output:
0 1
0 2