Question

In this code, for vector size, n >=32767, it gives segmentation fault, but upto 32766, it runs fine. What could be the error? This is full code.

#include<cstdio>
#include<cstring>
#include<cmath>
#include<queue>
#include<utility>
#include<algorithm>
#include<sys/time.h>
using namespace std;
#define MAX 100000

bool compare(pair<int,int> p1,pair<int,int> p2) {
    if(p1.second < p2.second)
        return 1;
    else if(p1.second > p2.second)
        return 0;
    if(p1.first <= p2.first)
        return 1;
    else
        return 0;
}

int main() {
    freopen("randomin.txt","r",stdin);
    int n;
    scanf("%d",&n);
    vector< pair<int,int> > p(n);
    for(int i=0;i<n;i++)
        scanf("%d%d",&p[i].first,&p[i].second);
    **printf("%d\n",(int)p.max_size()); // prints 536870911**
    sort(p.begin(),p.begin()+n,compare);

    //for(int i=0;i<n;i++)
        //printf("%d %d\n",p[i].first,p[i].second);
        printf("%.6f\n",(p[n-1].second+p[n-2].second)/(20.0+p[n-1].first+p[n-2].first));

    return 0;
}

Answer 1

In C++, your compare predicate must be a strict weak ordering. In particular, compare(X,X)
must return "false" for any X. In your compare function, if both pairs are identical, you hit the test (p1.first <= p2.first) , and return true. Therefore, this compare predicate does not impose a strict weak ordering, and the result of passing it to sort is undefined.