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 <cstdio>
#include <cmath>
#include <cstring>
#include <cstdlib>
#include <climits>
#include <ctime>
#include <queue>
#include <stack>
#include <algorithm>
#include <list>
#include <vector>
#include <set>
#include <map>
#include <iostream>
#include <deque>
#include <complex>
#include <string>
#include <iomanip>
#include <sstream>
#include <bitset>
#include <valarray>
#include <iterator>
using namespace std;
typedef long long int ll;
typedef unsigned int uint;
typedef unsigned char uchar;
typedef unsigned long long ull;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
typedef vector<int> vi;

#define REP(i,x) for(int i=0;i<(int)(x);i++)
#define FOR(i,c) for(__typeof((c).begin())i=(c).begin();i!=(c).end();i++)
#define RREP(i,x) for(int i=((int)(x)-1);i>=0;i--)
#define RFOR(i,c) for(__typeof((c).rbegin())i=(c).rbegin();i!=(c).rend();i++)
#define ALL(container) container.begin(), container.end()
#define RALL(container) container.rbegin(), container.rend()
#define SZ(container) ((int)container.size())
#define mp(a,b) make_pair(a, b)
#define UNIQUE(v) v.erase( unique(v.begin(), v.end()), v.end() );

template<class T> bool chmax(T &a, const T &b) { if (a<b) { a=b; return 1; } return 0; }
template<class T> bool chmin(T &a, const T &b) { if (a>b) { a=b; return 1; } return 0; }
template<class T> ostream& operator<<(ostream &os, const vector<T> &t) {
os<<"["; FOR(it,t) {if(it!=t.begin()) os<<","; os<<*it;} os<<"]"; return os;
}
template<class T> ostream& operator<<(ostream &os, const set<T> &t) {
os<<"{"; FOR(it,t) {if(it!=t.begin()) os<<","; os<<*it;} os<<"}"; return os;
}
template<class S, class T> ostream& operator<<(ostream &os, const pair<S,T> &t) { return os<<"("<<t.first<<","<<t.second<<")";}
template<class S, class T> pair<S,T> operator+(const pair<S,T> &s, const pair<S,T> &t){ return pair<S,T>(s.first+t.first, s.second+t.second);}
template<class S, class T> pair<S,T> operator-(const pair<S,T> &s, const pair<S,T> &t){ return pair<S,T>(s.first-t.first, s.second-t.second);}

const int INF = 1<<28;
const double EPS = 1e-10;
const int MOD = 1000000007;

#define X real()
#define Y imag()

typedef complex<double> P;
struct L{
	P p1, p2;
	L(P p1, P p2):p1(p1), p2(p2){}
	L(){}
};
istream& operator>>(istream &is, P &p) {
	double x, y;
	is >> x >> y;
	p = P(x, y);
	return is;
}
inline int sig(double x) { return (abs(x) < EPS ? 0 : x > 0 ? 1 : -1); }
inline double norm(const P &p){return p.X*p.X+p.Y*p.Y;}
inline double inp(const P& a, const P& b){return (conj(a)*b).X;}
inline double outp(const P& a, const P& b){return (conj(a)*b).Y;}


int T;

int main(){
	ios::sync_with_stdio(false);
	cin >> T;
	while(T--){
		P p1, p2, p3, p4;
		cin >> p1 >> p2 >> p3 >> p4;
		puts(!sig(outp(p2-p1, (p4-p3))) ? "YES" : "NO");
	}
	return 0;
}

C++: #include <iostream>

int main() {
  int a, b;
  int i;

  double x[4];
  double y[4];

  bool isYes;

  std::cin >> a;

  for(i = 0; i < a; i++) {

    std::cin >> x[0] >> y[0];
    std::cin >> x[1] >> y[1];
    std::cin >> x[2] >> y[2];
    std::cin >> x[3] >> y[3];

    if( (x[1] - x[0]) != 0 ) {
      if( (x[3] - x[2]) != 0 ) {
	if( ((x[3] - x[2])*(y[1] - y[0])) == ((x[1] - x[0])*(y[3] - y[2])) ) {
	  isYes = true;
	}
	else {
	  isYes = false;
	}
      }
      else {
	isYes = false;
      }
    }
    else {
      if( (x[3] - x[2]) == 0 ) {
	isYes = true;
      }
      else {
	isYes = false;
      }
    }
      
    if( isYes ) {
      std::cout << "YES" << std::endl;
    }
    else {
      std::cout << "NO" << std::endl;
    }

  }
 
  return 0;
}

C++: #include<stdio.h>
#include<math.h>

int main()
{
	double x1, x2, x3, x4, y1, y2, y3, y4;
	int n, i;
	scanf("%d", &n);
	for (i = 0; i < n; i++)
	{
		scanf("%lf %lf %lf %lf %lf %lf %lf %lf", &x1, &y1, &x2, &y2, &x3, &y3, &x4, &y4);
		double a1 = ((y2 - y1) / (x2 - x1)), a2 = ((y4 - y3) / (x4 - x3));
		if (a1 == a2)
		{
			printf("YES\n");
		}
		else
		{
			printf("NO\n");
		}
	}
	return 0;
}

C++: #include <iostream>

int main(){
	double x1,y1,x2,y2,x3,y3,x4,y4;
	double a,b;
	int n;
	std::cin >> n;
	for(int i=0 ; i<n ; ++i){
		std::cin >>x1>>y1>>x2>>y2>>x3>>y3>>x4>>y4;
		bool parallel=false;
		if(x1==x2 && x3==x4) {
			parallel=true;
			goto result;
		}
		else if(x1==x2 || x3==x4) {
			goto result;
		}
		else if ((y2-y1)/(x2-x1)==(y4-y3)/(x4-x3)) {
			parallel=true;
		}
		result:
		std::cout << ( parallel ? "YES" : "NO") << std::endl;
	}
	return 0; 
}

C++: #include<iostream>
using namespace std;
int main(){
	long double ax[100],ay[100],bx[100],by[100],cx[100],cy[100],dx[100],dy[100];
	int n;
	cin>>n;
	for(int i=0;i<n;i++){
		cin>>ax[i]>>ay[i]>>bx[i]>>by[i]>>cx[i]>>cy[i]>>dx[i]>>dy[i];
	}
	for(int i=0;i<n;i++){
		long double A=0,B=0;
		A=(by[i]-ay[i])/(bx[i]-ax[i]);
		B=(dy[i]-cy[i])/(dx[i]-cx[i]);
		if(A==B){
			cout<<"YES"<<endl;
		}
		else{
			cout<<"NO"<<endl;
		}
	}
}

C++: #include <bits/stdc++.h>
using namespace std;

int main()
{
  int n;
  cin >> n;
  while (n--){
    double x1, x2, x3, x4, y1, y2, y3, y4, a, b;
    cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3 >> x4 >> y4;
    a = (y2 - y1) / (x2 - x1);
    b = (y4 - y3) / (x4 - x3);
    if (a == b) cout << "YES\n";
    else cout << "NO\n";
  }
}

C++: #include<iostream>
using namespace std;
 
int main(){
    int n;
    double x[4],y[4];
    cin>>n;
    for(;n--;){
        for(int i=0;i<4;i++){
            cin>>x[i];
            cin>>y[i];
        }
        if((x[0]-x[1])*(y[2]-y[3])==(x[2]-x[3])*(y[0]-y[1])){
            cout<<"YES"<<endl;
        }else{
            cout<<"NO"<<endl;
        }
    }
    return 0;
}