Question

I have this program that is trying to determine how many unique items are within some intersecting sets. The amount of input entirely depends on the the first value n, and then the amount of sets entered afterward. For example, if I start with entering n = 2, I am expected to enter 2 integers. The program then determines how many intersections there are between n items (this is like choosing 2 items from n items). This goes on as k increments. But that’s kind of beyond the point. Just some background info.

My program adapts correctly and accepts the proper amount of input, but it stops working properly before the first for loop that is outside of the while loop. What I have tried to do is make a vector of integer vectors and then add every other row (when index starts at 0 AND index starts at 1). But I am guessing I have constructed my vectors incorrectly. Does anybody see an error in my vector logic?

#include <iostream>
#include <vector>

using namespace std;

int fact (int m) {
        if (m <= 1)
        return 1;
    return m * fact(m - 1);
}

int comb (int n, int k) {
    return fact(n)/(fact(n-k)*fact(k));
}

int main() {
    int n = 0;
    int k = 2;
    int sum = 0;
    int diff = 0;
    int final = 0;

    vector <vector <int> > arr;
    cin >> n;
    while (n > 0) {
        vector <int> row;
        int u;
        for (int i = 0; i < n ; ++i) {
            cin >> u;
            row.push_back(u);
        }
        arr.push_back(row);
        n = comb(row.size(), k);
        k++;
    }
    for (int i = 0; i < arr.size(); i+2)
        for (int j = 0; j < arr[i].size(); ++j)
            sum += arr[i][j];

    for (int i = 1; i < arr.size(); i+2)
        for (int j = 0; j < arr[i].size(); ++j)
            diff += arr[i][j];

    final = sum - diff;
    cout << final;
    return 0;
}

Answer 1

for (int i = 0; i < arr.size(); i+=2)
                                  ^

You want to do i+=2 or i=i+2, else the value of i is never changed, leading to an infinite loop.