Language: C++

#include <cstdio>
#include <cstdlib>
#include <cmath>
#include <climits>
#include <cfloat>
#include <map>
#include <utility>
#include <set>
#include <iostream>
#include <memory>
#include <string>
#include <vector>
#include <algorithm>
#include <functional>
#include <sstream>
#include <complex>
#include <stack>
#include <queue>
using namespace std;
static const double EPS = 1e-10;
typedef long long ll;
#define rep(i,n) for(int i=0;i<n;i++)
#define rev(i,n) for(int i=n-1;i>=0;i--)
#define sz(a) a.size()
#define all(a) a.begin(),a.end()
#define mp(a,b) make_pair(a,b)
#define pb(a) push_back(a)
#define SS stringstream
#define DBG1(a) rep(_X,sz(a)){printf("%d ",a[_X]);}puts("");
#define DBG2(a) rep(_X,sz(a)){rep(_Y,sz(a[_X]))printf("%d ",a[_X][_Y]);puts("");}
#define bitcount(b) __builtin_popcount(b)
#define REP(i, s, e) for ( int i = s; i <= e; i++ )  
 
const double INF = 1e12;
typedef complex<double> P,point;
typedef vector<P> G,polygon;
namespace std {bool operator < (const P& a, const P& b) {return real(a) != real(b) ? real(a) < real(b) : imag(a) < imag(b);} }
double cross(const P& a, const P& b) {return imag(conj(a)*b);}
double dot(const P& a, const P& b) {return real(conj(a)*b);}
struct L : public vector<P> {L(const P &a, const P &b) {push_back(a); push_back(b);} };
struct C {P p; double r;C(const P &p, double r) : p(p), r(r) { } C(){} };
int ccw(P a, P b, P c) {
	b -= a; c -= a;
	if (cross(b, c) > 0)   return +1;
	if (cross(b, c) < 0)   return -1;
	if (dot(b, c) < 0) return +2;
	if (norm(b) < norm(c)) return -2;
	return 0;
}
 
// Check Funcs //
bool intersectLL(const L &l, const L &m) {return abs(cross(l[1]-l[0], m[1]-m[0])) > EPS || abs(cross(l[1]-l[0], m[0]-l[0])) < EPS;}
bool intersectLS(const L &l, const L &s) {return cross(l[1]-l[0], s[0]-l[0])*cross(l[1]-l[0], s[1]-l[0]) < EPS;}
bool intersectLP(const L &l, const P &p) {return abs(cross(l[1]-p, l[0]-p)) < EPS;}
bool intersectSS(const L &s, const L &t) {return ccw(s[0],s[1],t[0])*ccw(s[0],s[1],t[1]) <= 0 && ccw(t[0],t[1],s[0])*ccw(t[0],t[1],s[1]) <= 0;}
bool intersectSP(const L &s, const P &p) {return abs(s[0]-p)+abs(s[1]-p)-abs(s[1]-s[0]) < EPS;}
 
// Where is Point in Polygon? //
#define curr(P, i) P[i]
#define next(P, i) P[(i+1)%P.size()]
enum { OUT, ON, IN };
int contains(const polygon& P, const point& p) {
	bool in = false;
	for (int i = 0; i < P.size(); ++i) {
		point a = curr(P,i) - p, b = next(P,i) - p;
		if (imag(a) > imag(b)) swap(a, b);
		if (imag(a) <= 0 && 0 < imag(b))
			if (cross(a, b) < 0) in = !in;
		if (cross(a, b) == 0 && dot(a, b) <= 0) return ON;
	}
	return in ? IN : OUT;
}
// Area of Polygon //
double area2(const polygon& P) {
	double A = 0;
	for (int i = 0; i < P.size(); ++i)A += cross(curr(P, i), next(P, i));
	return A;
}
// Totsuhou! Andrew's Monotone Chain //
 
vector<point> convex_hull(vector<point> ps) {
  int n = ps.size(), k = 0;
  sort(ps.begin(), ps.end());
  vector<point> ch(2*n);
  for (int i = 0; i < n; ch[k++] = ps[i++]) // lower-hull
    while (k >= 2 && ccw(ch[k-2], ch[k-1], ps[i]) <= 0) --k;
  for (int i = n-2, t = k+1; i >= 0; ch[k++] = ps[i--]) // upper-hull
    while (k >= t && ccw(ch[k-2], ch[k-1], ps[i]) <= 0) --k;
  ch.resize(k-1);
  return ch;
}
 
P projection(const L &l, const P &p) {double t = dot(p-l[0], l[0]-l[1]) / norm(l[0]-l[1]);return l[0] + t*(l[0]-l[1]);}
P reflection(const L &l, const P &p) {return p + 2.0 * (projection(l, p) - p);}
 
double distanceLP(const L &l, const P &p) {
	return abs(p - projection(l, p));
}
double distanceLL(const L &l, const L &m) {
	return intersectLL(l, m) ? 0 : distanceLP(l, m[0]);
}
double distanceLS(const L &l, const L &s) {
	if (intersectLS(l, s)) return 0;
	return min(distanceLP(l, s[0]), distanceLP(l, s[1]));
}
double distanceSP(const L &s, const P &p) {
	const P r = projection(s, p);
	if (intersectSP(s, r)) return abs(r - p);
	return min(abs(s[0] - p), abs(s[1] - p));
}
double distanceSS(const L &s, const L &t) {
	if (intersectSS(s, t)) return 0;
	return min(min(distanceSP(s, t[0]), distanceSP(s, t[1])),min(distanceSP(t, s[0]), distanceSP(t, s[1])));
}
 
P crosspoint(const L &l, const L &m) {
	double A = cross(l[1] - l[0], m[1] - m[0]);
	double B = cross(l[1] - l[0], l[1] - m[0]);
	if (abs(A) < EPS && abs(B) < EPS) return m[0];
	return m[0] + B / A * (m[1] - m[0]);
}
 
vector<P> C_cp(C a,C b){
	vector<P> ret;
	double L = abs(a.p-b.p);
 
	/* anma dekitenai */
	if(	L-a.r-b.r > EPS || (abs(a.p-b.p)<EPS && fabs(a.r-b.r)<EPS) || 
		abs(a.p-b.p) < abs(a.r-b.r)
	)return ret;
 
	double theta = atan2(b.p.imag()-a.p.imag(),b.p.real()-a.p.real());
	double c = (L*L+a.r*a.r-b.r*b.r)/(2*L*a.r);
	ret.push_back(
		P(a.p.real()+a.r*cos(theta+acos(c)),
		  a.p.imag()+a.r*sin(theta+acos(c)))
	);
	if(fabs(L-a.r-b.r) > EPS)
		ret.push_back(
			P(a.p.real()+a.r*cos(theta-acos(c)),
			  a.p.imag()+a.r*sin(theta-acos(c)))
		);
	return ret;
}
 
 
P getPedal(L l, P p){
	double A;
	if(abs(l[1].real()-l[0].real()) < EPS){
		return P(l[1].real(),p.imag()); // important
	}else{
		A = (l[1].imag()-l[0].imag())/(l[1].real()-l[0].real());
	}
	double a = -A , b = 1 , c = A*l[0].real() - l[0].imag();
	double t = (a*p.real() + b*p.imag() + c)/(a*a+b*b);
	return p-t * P(a,b);
}
 
vector<P> crosspointCL(const L l, const C c){
	vector<P> ret;
	P p = getPedal(l,c.p);
	if(	abs(p-c.p) > c.r+EPS)return ret;
	P e = P((l[1]-l[0])/abs(l[1]-l[0]));
	double S = sqrt(c.r*c.r-abs(p-c.p)*abs(p-c.p));
	ret.push_back(p+S*e);
	ret.push_back(p-S*e);
	return ret;
}
 
P getCircumcenter(P a,P b,P c){
	double A1 = 2 * ( b.real() - a.real() );
	double B1 = 2 * ( b.imag() - a.imag() );
	double C1 = pow(a.real(),2)-pow(b.real(),2) + pow(a.imag(),2)-pow(b.imag(),2);
	double A2 = 2 * ( c.real() - a.real() );
	double B2 = 2 * ( c.imag() - a.imag() );
	double C2 = pow(a.real(),2)-pow(c.real(),2) + pow(a.imag(),2)-pow(c.imag(),2);
	double X = (B1 * C2 - B2 * C1) / (A1 * B2 - A2 * B1);
	double Y = (C1 * A2 - C2 * A1) / (A1 * B2 - A2 * B1);
	return P(X,Y);
}
 
double AreaOfPolygon(vector<P> p){
	double S = 0 ; 
	p.push_back(p[0]);
	/*　多角形の面積公式 (反時計回りの場合) */
	for(int i = 0 ; i < p.size()-1 ; i++){
		S += (p[i].real() - p[i+1].real()) * (p[i].imag()+p[i+1].imag());
	}
	S /= 2.0;
	return S;
}

P p[5000];

struct event {
  double x;
  int type;
  L seg;
  event(double x, int type, const L& seg) :
    x(x), type(type), seg(seg) { }
  bool operator < (const event &e) const {
    return x != e.x ? x > e.x : type > e.type;
  }
};
int segment_intersects(const vector<L>& segs) {
  priority_queue<event> Q;
  for (int i = 0; i < segs.size(); ++i) {
    double x1 = real(segs[i][0]), x2 = real(segs[i][1]);
    Q.push( event(min(x1,x2), 0, segs[i]) );
    Q.push( event(max(x1,x2), 1, segs[i]) );
  }
  int count = 0;
  set<L> T;
  while (!Q.empty()) {
    event e = Q.top(); Q.pop();
    if (e.type == 0) {
      for (set<L>::iterator itr = T.begin();
          itr != T.end(); ++itr)
        if (intersectSS(*itr, e.seg)) {
          return true;
        }
      T.insert(e.seg);
    } else T.erase(e.seg);
  }
  return false;
}

int main(){
	int n;
	cin >> n;
	if( n > 5000 ){
		cout << 0 << endl;
		return 0;
	}
	vector<L> v;
	for(int i = 0 ; i < n ; i++){
		P a,b;
		cin >> a.real() >> a.imag();
		cin >> b.real() >> b.imag();
		v.push_back(L(a,b));
	}
	if( segment_intersects(v) ){
		cout << 0 << endl;
		return 0;
	}
	double answer = 1e9;
	for(int i = 0 ; i < n ; i++){
		for(int j = i+1 ; j < n ; j++){
			answer =  min( distanceSS(v[i],v[j]) , answer );
		}
	}
	printf("%.10lf\n",answer);
	
}

Compiler output:
a.cc: In function 'int main()':
a.cc:238:21: error: no match for 'operator>>' (operand types are 'std::istream' {aka 'std::basic_istream<char>'} and 'double')
  238 |                 cin >> a.real() >> a.imag();
      |                 ~~~ ^~ ~~~~~~~~
      |                 |            |
      |                 |            double
      |                 std::istream {aka std::basic_istream<char>}
In file included from /usr/include/c++/14/iostream:42,
                 from a.cc:9:
/usr/include/c++/14/istream:170:7: note: candidate: 'std::basic_istream<_CharT, _Traits>::__istream_type& std::basic_istream<_CharT, _Traits>::operator>>(bool&) [with _CharT = char; _Traits = std::char_traits<char>; __istream_type = std::basic_istream<char>]' (near match)
  170 |       operator>>(bool& __n)
      |       ^~~~~~~~
/usr/include/c++/14/istream:170:7: note:   conversion of argument 1 would be ill-formed:
a.cc:238:30: error: cannot bind non-const lvalue reference of type 'bool&' to a value of type 'double'
  238 |                 cin >> a.real() >> a.imag();
      |                        ~~~~~~^~
/usr/include/c++/14/istream:174:7: note: candidate: 'std::basic_istream<_CharT, _Traits>& std::basic_istream<_CharT, _Traits>::operator>>(short int&) [with _CharT = char; _Traits = std::char_traits<char>]' (near match)
  174 |       operator>>(short& __n);
      |       ^~~~~~~~
/usr/include/c++/14/istream:174:7: note:   conversion of argument 1 would be ill-formed:
a.cc:238:30: error: cannot bind non-const lvalue reference of type 'short int&' to a value of type 'double'
  238 |                 cin >> a.real() >> a.imag();
      |                        ~~~~~~^~
/usr/include/c++/14/istream:177:7: note: candidate: 'std::basic_istream<_CharT, _Traits>::__istream_type& std::basic_istream<_CharT, _Traits>::operator>>(short unsigned int&) [with _CharT = char; _Traits = std::char_traits<char>; __istream_type = std::basic_istream<char>]' (near match)
  177 |       operator>>(unsigned short& __n)
      |       ^~~~~~~~
/usr/include/c++/14/istream:177:7: note:   conversion of argument 1 would be ill-formed:
a.cc:238:30: error: cannot bind non-const lvalue reference of type 'short unsigned int&' to a value of type 'double'
  238 |                 cin >> a.real() >> a.imag();
      |                        ~~~~~~^~
/usr/include/c++/14/istream:181:7: note: candidate: 'std::basic_istream<_CharT, _Traits>& std::basic_istream<_CharT, _Traits>::operator>>(int&) [with _CharT = char; _Traits = std::char_traits<char>]' (near match)
  181 |       operator>>(int& __n);
      |       ^~~~~~~~
/usr/include/c++/14/istream:181:7: note:   conversion of argument 1 would be ill-formed:
a.cc:238:30: error: cannot bind non-const lvalue reference of type 'int&' to a value of type 'double'
  238 |                 cin >> a.real() >> a.imag();
      |                        ~~~~~~^~
/usr/include/c++/14/istream:184:7: note: candidate: 'std::basic_istream<_CharT, _Traits>::__istream_type& std::basic_istream<_CharT, _Traits>::operator>>(unsigned int&) [with _CharT = char; _Traits = std::char_traits<char>; __istream_type = std::basic_istream<char>]' (near match)
  184 |       operator>>(unsigned int& __n)
      |       ^~~~~~~~
/usr/include/c++/14/istream:184:7: note:   conversion of argument 1 would be ill-formed:
a.cc:238:30: error: cannot bind non-const lvalue reference of type 'unsigned int&' to a value of type 'double'
  238 |                 cin >> a.real() >> a.imag();
      |                        ~~~~~~^~
/usr/include/c++/14/istream:188:7: note: candidate: 'std::basic_istream<_CharT, _Traits>::__istream_type& std::basic_istream<_CharT, _Traits>::operator>>(long int&) [with _CharT = char; _Traits = std::char_traits<char>; __istream_type = std::basic_istream<char>]' (near match)
  188 |       operator>>(long& __n)
      |       ^~~~~~~~
/usr/include/c++/14/istream:188:7: note:   conversion of argument 1 would be ill-formed:
a.cc:238:30: error: cannot bind non-const lvalue reference of type 'long int&' to a value of type 'double'
  238 |                 cin >> a.real() >> a.imag();
      |                        ~~~~~~^~
/usr/include/c++/14/istream:192:7: note: candidate: 'std::basic_istream<_CharT, _Traits>::__istream_type& std::basic_istream<_CharT, _Traits>::operator>>(long unsigned int&) [with _CharT = char; _Traits = std::char_traits<char>; __istream_type = std::basic_istream<char>]' (near match)
  192 |       operator>>(unsigned long& __n)
      |       ^~~~~~~~
/usr/include/c++/14/istream:192:7: note:   conversion of argument 1 would be ill-formed:
a.cc:238:30: error: cannot bind non-const lvalue reference of type 'long unsigned int&' to a value of type 'double'
  238 |                 cin >> a.real() >> a.imag();
      |                        ~~~~~~^~
/usr/include/c++/14/istream:199:7: note: candidate: 'std::basic_istream<_CharT, _Traits>::__istream_type& std::basic_istream<_CharT, _Traits>::operator>>(long long int&) [with _CharT = char; _Traits = std::char_traits<char>; __istream_type = std::basic_istream<char>]' (near match)
  199 |       operator>>(long long& __n)
      |       ^~~~~~~~
/usr/include/c++/14/istream:199:7: note:   conversion of argument 1 would be ill-formed:
a.cc:238:30: error: cannot bind non-const lvalue reference of type 'long long int&' to a value of type 'double'
  238 |                 cin >> a.real() >> a.imag();
      |                        ~~~~~~^~
/usr/include/c++/14/istream:203:7: note: candidate: 'std::basic_istream<_CharT, _Traits>::__istream_type& std::basic_istream<_CharT, _Traits>::operator>>(long long unsigned int&) [with _CharT = char; _Traits = std::char_traits<char>; __istream_type = std::basic_istream<char>]' (near match)
  203 |       operator>>(unsigned long long& __n)
      |       ^~~~~~~~
/usr/include/c++/14/istream:203:7: note:   conversion of argument 1 would be ill-formed:
a.cc:238:30: error: cannot bind non-const lvalue reference of type 'long long unsigned int&' to a value of type 'double'
  238 |                 cin >> a.real() >> a.imag();
      |                        ~~~~~~^~
/usr/include/c++/14/istream:219:7: note: candidate: 'std::basic_istream<_CharT, _Traits>::__istream_type& std::basic_istream<_CharT, _Traits>::operator>>(float&) [with _CharT = char; _Traits = std::char_traits<char>; __istream_type = std::basic_istream<char>]' (near match)
  219 |       operator>>(float& __f)
      |       ^~~~~~~~
/usr/include/c++/14/istream:219:7: note:   conversion of argument 1 would be ill-formed:
a.cc:238:30: error: cannot bind non-const lvalue reference of type 'float&' to a value of type 'double'
  238 |                 cin >> a.real() >> a.imag();
      |                        ~~~~~~^~
/usr/include/c++/14/istream:223:7: note: candidate: 'std::basic_istream<_CharT, _Traits>::__istream_type& std::basic_istream<_CharT, _Traits>::operator>>(double&) [with _CharT = char; _Traits = std::char_traits<char>; __istream_type = std::basic_istream<char>]' (near match)
  223 |       operator>>(double& __f)
      |       ^~~~~~~~
/usr/include/c++/14/istream:223:7: note:   conversion of argument 1 would be ill-formed:
a.cc:238:30: error: cannot bind non-const lvalue reference of type 'double&' to an rvalue of type 'double'
  238 |                 cin >> a.real() >> a.imag();
      |                        ~~~~~~^~
/usr/include/c++/14/istream:227:7: note: candidate: 'std::basic_istream<_CharT, _Traits>::__istream_type& std::basic_istream<_CharT, _Traits>::operator>>(long double&) [with _CharT = char; _Traits = std::char_traits<char>; __istream_type = std::basic_istream<char>]' (near match)
  227 |       operator>>(long double& __f)
      |       ^~~~~~~~
/usr/include/c++/14/istream:227:7: note:   conversion of argument 1 would be ill-formed:
a.cc:238:30: error: cannot bind non-const lvalue reference of type 'long double&' to a value of type 'double'
  238 |                 cin >> a.real() >> a.imag();
      |                        ~~~~~~^~
/usr/include/c++/14/istream:328:7: note: candidate: 'std::basic_istream<_CharT, _Traits>::__istream_type& std::basic_istream<_CharT, _Traits>::operator>>(void*&) [with _CharT = char; _Traits = std::char_traits<char>; __istream_type = std::basic_istream<char>]'
  328 |       operator>>(void*& __p)
      |       ^~~~~~~~
/usr/include/c++/14/istream:328:25: note:   no known conversion for argument 1 from 'double' to 'void*&'
  328 |       operator>>(void*& __p)
      |                  ~~~~~~~^~~
/usr/include/c++/14/istream:122:7: note: candidate: 'std::basic_istream<_CharT, _Traits>::__istream_type& std::basic_istream<_CharT, _Traits>::operator>>(__istream_type& (*)(__istream_type&)) [with _CharT = char; _Traits = std::char_traits<char>; __istream_type = std::basic_istream<char>]'
  122 |       operator>>(__istream_type& (*__pf)(__istream_type&))
      |       ^~~~~~~~
/usr/include/c++/14/istream:122:36: note:   no known conversion for argument 1 from 'double' to 'std::basic_istream<char>::__istream_type& (*)(std::basic_istream<char>::__istream_type&)' {aka 'std::basic_istream<char>& (*)(std::basic_istream<char>&)'}
  122 |       operator>>(__istream_type& (*__pf)(__istream_type&))
      |                  ~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/14/istream:126:7: note: candidate: 'std::basic_istream<_CharT, _Traits>::__istream_type& std::basic_istream<_CharT, _Traits>::operator>>(__ios_type& (*)(__ios_type&)) [with _CharT = char; _Traits = std::char_traits<char>; __istream_type = std::basic_istream<char>; __ios_type = std::basic_ios<char>]'
  126 |       operator>>(__ios_type& (*__pf)(__ios_type&))
      |       ^~~~~~~~
/usr/include/c++/14/istream:126:32: note:   no known conversion for argument 1 from 'double' to 'std::basic_istream<char>::__ios_type& (*)(std::basic_istream<char>::__ios_type&)' {aka 'std::basic_ios<char>& (*)(std::basic_ios<char>&)'}
  126 |       operator>>(__ios_type& (*__pf)(__ios_type&))
      |                  ~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~
/usr/include/c++/14/istream:133:7: note: candidate: 'std::basic_istream<_CharT, _Traits>::__istream_type& std::basic_istream<_CharT, _Traits>::operator>>(std::ios_base& (*)(std::ios_base&)) [with _CharT = char; _Traits = std::cha