This problem : https://mirror.codeforces.com/problemset/problem/514/B, gives different verdicts according to different change in epsilon values used for comparing two double quantities. Why is this so? For example it passes for epsilon from 1e-9 to 1e-13.

During contest how am I supposed to know which epsilon value will pass?

Solution:

#include <bits/stdc++.h>
using namespace std;
#define fast ios::sync_with_stdio(0); cin.tie(0); cout.tie(0);
#define ll  long long
#define ld long double
#define all(v) v.begin(),v.end()
#define ff first
#define ss second
#define pb push_back
#define mp make_pair
#define pi pair<int,int>
const ll maxn=1e3+50;
const ll mod=1e9+7;
const double epsilon=1e-12;
bool deq(double a, double b)
{
	return std::abs(a - b) < epsilon;
}
int main() {
  	fast;
  	int n,x,y,u,v,i,j;
  	cin>>n>>x>>y;
  	pi p[n];
  	for(i=0;i<n;i++){
  		cin>>u>>v;
  		p[i]={u,v};	
	}
	bool mark[n];
	memset(mark,false,sizeof(mark));
	int ans=0;
	for(i=0;i<n;i++){
		if(mark[i])
			continue;
		double del,f=0;
		if((x-p[i].ff)!=0)
			del=(1.0*(y-p[i].ss))/(x-p[i].ff);
		else{
			f=1;
		}
		for(j=i;j<n;j++){
			if(mark[j])
				continue;
			if((x-p[j].ff)!=0){
				if(deq(del,(1.0*(y-p[j].ss))/(x-p[j].ff)))
					mark[j]=true;
			}
			else if(f)
				mark[j]=true;
		}
		++ans;
	}
	cout<<ans;
}

Please dont downvote unnecessarily.

Any help will be appreciated.