map comparator for pair of objects in c++

Question

I want to use a map to count pairs of objects based on member input vectors. If there is a better data structure for this purpose, please tell me. My program returns a list of int vectors. Each int vector is the output of a comparison between two int vectors ( a pair of int vectors). It is, however, possible, that the output of the comparison differs, though the two int vectors are the same (maybe in different order). I want to store how many different outputs (int vectors) each pair of int vectors has produced.

Assuming that I can access the int vector of my object with .inp()

Two pairs (a1,b1) and (a2,b2) should be considered equal, when (a1.inp() == a2.inp() && b2.inp() == b1.inp()) or (a1.inp() == b2.inp() and b1.inp() == a2.inp()).

This answer says:

The keys in a map a and b are equivalent by definition when neither a < b nor b < a is true.

class SomeClass
{
    vector <int> m_inputs;
public:
    //constructor, setter...
    vector<int> inp() {return m_inputs};
}

typedef pair < SomeClass, SomeClass > InputsPair;
typedef map < InputsPair, size_t, MyPairComparator > InputsPairCounter;

So the question is, how can I define equivalency of two pairs with a map comparator. I tried to concatenate the two vectors of a pair, but that leads to (010,1) == (01,01), which is not what I want.

struct MyPairComparator
{
    bool operator() (const InputsPair & pair1, const InputsPair pair2) const
    {
        vector<int> itrc1 = pair1.first->inp();
        vector<int> itrc2 = pair1.second->inp();
        vector<int> itrc3 = pair2.first->inp();
        vector<int> itrc4 = pair2.second->inp();
        // ?
        return itrc1 < itrc3;
    }
};

In your `operator<` order each pair such that the lowest is 1st. Then it's the normal lexical less-than compare. — Richard Critten, Oct 26 '17 at 14:45
This may be an XY problem, can you elaborate on why you need this comparison, how does it help you to "count pairs of input vectors"? What are you really trying to do? — Fantastic Mr Fox, Oct 26 '17 at 14:45

Slava · Accepted Answer · 2017-10-26T15:22:31.550

I want to use a map to count pairs of input vectors. If there is a better data structure for this purpose, please tell me.

Using std::unordered_map can be considered instead due to 2 reasons:

if hash implemented properly it could be faster than std::map
you only need to implement hash and operator== instead of operator<, and operator== is trivial in this case

Details on how implement hash for std::vector can be found here. In your case possible solution could be to join both vectors into one, sort it and then use that method to calculate the hash. This is straightforward solution, but can produce to many hash collisions and lead to worse performance. To suggest better alternative would require knowledge of the data used.

score 0 · Answer 2 · answered Oct 26 '17 at 15:13

As I understand, you want:

struct MyPairComparator
{
    bool operator() (const InputsPair& lhs, const InputsPair pair2) const
    {
        return std::minmax(std::get<0>(lhs), std::get<1>(lhs))
            < std::minmax(std::get<0>(rhs), std::get<1>(rhs));
    }
};

we order the pair {a, b} so that a < b, then we use regular comparison.

map comparator for pair of objects in c++

2 Answers2