Go: package main

import (
        "fmt"
        "math/big"
)

func main() {
        var x1, x2, x3, x4, y1, y2, y3, y4 big.Float
        var n int
        fmt.Scanf("%d", &n)
        for i := 0; i < n; i++ {
                fmt.Scanf("%f %f %f %f %f %f %f %f",
                        &x1, &y1, &x2, &y2, &x3, &y3, &x4, &y4)
                var lx, ly, mx, my, mul1, mul2, sub, zz big.Float
                lx.Sub(&x2, &x1)
                ly.Sub(&y2, &y1)
                mx.Sub(&x4, &x3)
                my.Sub(&y4, &y3)
                mul1.Mul(&lx, &my)
                mul2.Mul(&ly, &mx)
                sub.Sub(&mul1, &mul2)
                zz.SetFloat64(0.0)

                if sub.Cmp(&zz) == 0 {
                        fmt.Println("YES")
                } else {
                        fmt.Println("NO")
                }
        }
}

C++: #include <iostream>
using namespace std;

int main(){
    int n;
    cin >> n;
    while(n--){
        double x[4],y[4],ll,lll;
        for(int i=0;i<4;i++) cin >> x[i] >> y[i];
        ll = (y[1]-y[0])/(x[1]-x[0]);
        lll = (y[3]-y[2])/(x[3]-x[2]);
        cout << ((ll == lll)? "YES" : "NO") << endl;
    }
    return 0;
}

C++: //
//  main.cpp
//  main
//
//  Created by h3037175 on 2018/01/22.
//  Copyright © 2018年 h3037175. All rights reserved.
//

#include<iostream>
#include<stdio.h>
#include<math.h>
#include<string>
#include<algorithm>
#include<utility>
using namespace std;
double zeti(double);
int main() {
    pair<double,double> a;
    pair<double,double> b;
    pair<double,double>c;
    pair<double,double>d;
    int n;
    double s1,s2;
    cin>>n;
    for(int i=0;i<n;++i){
        cin>>a.first>>a.second>>b.first>>b.second>>c.first>>c.second>>d.first>>d.second;
        if((a.first==b.first&&c.first==d.first) || (a.second==b.second&&c.second==d.second)){
            cout<<"YES"<<endl;
            continue;
        }
        s1=(a.second-b.second)/(a.first-b.first);
        s2=(c.second-d.second)/(c.first-d.first);
        if(s1==s2){
            cout<<"YES"<<endl;
        }
        else{
            cout<<"NO"<<endl;
            
        }
    }
    
    
    return 0;
}

C++: #include<iostream>
#include<algorithm>
#include<string>
using namespace std;
int main() {
	int n;
	cin >> n;
	for (int z = 0; z < n; z++) {
		long float x1, x2, x3,y1, y2, y3, y4, x4 ;
		cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3 >> x4 >> y4;
		if (y1 > y2) {
			swap(y1, y2);
			swap(x1, x2);
		}
		if (y3 > y4) {
			swap(y3, y4);
			swap(x3, x4);
		}
		if ((x1 > x2&&x3 > x4 && ((y2 - y1) / (x1 - x2) == (y4 - y3) / (x3 - x4))) || (x2 > x1&&x4 > x3 && ((y1 - y2) / (x2 - x1) == (y3 - y4) / (x4 - x3)))) {
			cout << "YES" << endl;
		}
		else {
			cout << "NO" << endl;
		}
	}
}

C++: #include <bits/stdc++.h>

using namespace std;

typedef long long ll;
typedef vector<int> vi;
typedef vector<ll> vl;
typedef complex<double> P;
typedef pair<int,int> pii;
#define REP(i,n) for(ll i=0;i<n;++i)
#define REPR(i,n) for(ll i=1;i<n;++i)
#define FOR(i,a,b) for(ll i=a;i<b;++i)

#define DEBUG(x) cout<<#x<<": "<<x<<endl
#define DEBUG_VEC(v) cout<<#v<<":";REP(i,v.size())cout<<" "<<v[i];cout<<endl
#define ALL(a) (a).begin(),(a).end()

#define MOD (ll)(1e9+7)
#define ADD(a,b) a=((a)+(b))%MOD
#define FIX(a) ((a)%MOD+MOD)%MOD

int main(){
double x1,y1,x2,y2,x3,y3,x4,y4;
while(~scanf("%lf%lf%lf%lf%lf%lf%lf%lf",&x1,&y1,&x2,&y2,&x3,&y3,&x4,&y4)){
P a(x2-x1,y2-y1);
P b(x4-x3,y4-y3);
double cr = imag(conj(a)*b);
if(abs(cr)<1e-9){
  cout<<"YES"<<endl;
}else{
  cout<<"NO"<<endl;
}
}
  return 0;
}

C++: #include <iostream>
#include <vector>
#include <queue>
#include <stack>
#include <map>
#include <set>
#include <cmath>
#include <cstdio>
#include <cstring>
#include <cstdlib>
#include <functional>
#include <string>
#include <algorithm>
#include <climits>
#include <utility>

#define PRINT(STR) cout << STR << endl;
#define REP(i, n) for (int (i) = 0; (i) < (int)(n); (i)++)
#define REG(i, a, b) for (int (i) = ((int)(a)); (i) < ((int)(b)); i++)
#define ALL(V) (V).begin(), (V).end()

typedef long long ll;

using namespace std;

int nextInt() { int n = -1; cin >> n; return n; }
double nextDouble() { double d = -1; cin >> d; return d; }
ll nextll() { ll n = -1; cin >> n; return n; }
string nextString() { string str; cin >> str; return str; }

int INF = 9999999;

// BEGIN //////////////////////////////////////////////////////////////


int main() {
	int N = nextInt();
	REP(i, N) {
		double x1 = nextDouble();
		double y1 = nextDouble();
		double x2 = nextDouble();
		double y2 = nextDouble();
		double x3 = nextDouble();
		double y3 = nextDouble();
		double x4 = nextDouble();
		double y4 = nextDouble();

		if (x2 - x1 == 0 && x4 - x3 == 0) {
			if (y2 - y1 == 0 || y4 - y3 == 0) {
				PRINT("NO");
			} else {
				PRINT("YES");
			}
		} else if (x2 - x1 == 0 || x4 - x3 == 0) {
			PRINT("NO");
		} else {
			if ((y2 - y1) / (x2 - x1) - (y4 - y3) / (x4 - x3) < 0.000006) {
				PRINT("YES");
			} else {
				PRINT("NO");
			}
		}
	}

	return 0;
}

// END   //////////////////////////////////////////////////////////////

C++: #include <stdio.h>

int main(){
    int n,i,j;
    double x[5],y[5];
    long long X[5],Y[5];
    scanf("%d",&n);
    for(i=0;i<n;i++){
        for(j=1;j<=4;j++){
            scanf("%lf %lf",&x[i],&y[i]);
            X[i] = (int)(x[i]*1000000);
            Y[i] = (int)(y[i]*1000000);
        }
        if((X[2]-X[1])*(Y[4]-Y[3])-(X[4]-X[3])*(Y[2]-Y[1])){
            printf("NO\n");
        }else{
            printf("YES\n");
        }
    }
}

C++: #include <iostream>
#include <cmath>
using namespace std;

struct Point2D {
    double x, y;

    Point2D() {}
    Point2D(double ax, double ay) {
        x = ax;
        y = ay;
    }
    Point2D operator -(Point2D &p) {
        return Point2D(x - p.x, y - p.y);
    }
    double slope(Point2D &p) {
        return (p.y - y) / (p.x - x);
    }
    double cross(Point2D &p) { // only z
        return x * p.y - y * p.x;
    }
};

namespace std {
    istream &operator>>(istream &is, Point2D &p) {
        is >> p.x >> p.y;
        return is;
    }

    ostream &operator<<(ostream &os, Point2D &p) {
        os << fixed << p.x << " " << p.y;
        return os;
    }
}

int main()
{
    int n;
    cin >> n;
    for (Point2D a, b, c, d; n-- && cin >> a >> b >> c >> d;)
        cout << ((a.slope(b) == c.slope(d)) ? "YES" : "NO") << endl;

    return 0;
}

C++: #include <iostream>
using namespace std;

int main() {
	int n;
	cin >> n;
	for(int i = 0; i < n; i++) {
		double x1, y1, x2, y2, x3, y3, x4, y4;
		cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3 >> x4 >> y4;
		//cout << (x1 - x2) * (y3 - y4) << endl;
		//cout << (y1 - y2) * (x3 - x4) << endl;
		if((x1 - x2) * (y3 - y4) == (y1 - y2) * (x3 - x4)) cout << "YES" << endl;
		else cout << "NO" << endl;
	}
	return 0;
}

C++: #include <iostream>
#include <complex>
#include <cmath>
#include <vector>
using namespace std;

typedef complex<double> P;
const double eps = 1e-8;

bool cross(P a, P b) {
  return a.real()*b.imag() - a.imag()*b.real();
}

int main() {
  int Tc;
  cin >> Tc;
  while(Tc--) {
    vector<P> ps(4);
    for(int i = 0; i < 4; ++i) {
      cin >> ps[i].real() >> ps[i].imag();
    }
    if(abs(cross(ps[1]-ps[0], ps[3]-ps[2])) < eps) cout << "YES" << endl;
    else cout << "NO" << endl;
  }
  return 0;
}