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Count of parallelograms in a plane

  • Difficulty Level : Medium
  • Last Updated : 07 Jan, 2017

Given some points on a plane, which are distinct and no three of them lie on the same line. We need to find number of Parallelograms with the vertices as the given points.

Input : points[] = {(0, 0), (0, 2), (2, 2), (4, 2), 
                                   (1, 4), (3, 4)}
Output : 2
Two Parallelograms are possible by choosing above
given point as vertices, which are shown in below 

We can solve this problem by using a special property of parallelograms that diagonals of a parallelogram intersect each other in the middle. So if we get such a middle point which is middle point of more than one line segment, then we can conclude that a parallelogram exists, more accurately if a middle point occurs x times, then diagonals of possible parallelograms can be chosen in xC2 ways, i.e. there will be x*(x-1)/2 parallelograms corresponding to this particular middle point with a frequency x. So we iterate over all pair of points and we calculate their middle point and increase frequency of middle point by 1. At the end, we count number of parallelograms according to the frequency of each distinct middle point as explained above. As we just need frequency of middle point, division by 2 is ignored while calculating middle point for simplicity.

// C++ program to get number of Parallelograms we
// can make by given points of the plane
#include <bits/stdc++.h>
using namespace std;
// Returns count of Parallelograms possible
// from given points
int countOfParallelograms(int x[], int y[], int N)
    // Map to store frequency of mid points
    map<pair<int, int>, int> cnt;
    for (int i=0; i<N; i++)
        for (int j=i+1; j<N; j++)
            // division by 2 is ignored, to get
            // rid of doubles
            int midX = x[i] + x[j];
            int midY = y[i] + y[j];
            // increase the frequency of mid point
            cnt[make_pair(midX, midY)]++;
    // Iterating through all mid points
    int res = 0;
    for (auto it = cnt.begin(); it != cnt.end(); it++)
        int freq = it->second;
        // Increase the count of Parallelograms by
        // applying function on frequency of mid point
        res += freq*(freq - 1)/2
    return res;
//  Driver code to test above methods
int main()
    int x[] = {0, 0, 2, 4, 1, 3};
    int y[] = {0, 2, 2, 2, 4, 4};
    int N = sizeof(x) / sizeof(int);
    cout << countOfParallelograms(x, y, N) << endl;
    return 0;



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