Go: package main

import "fmt"

func abs(x float64) float64 {
	if x < 0 {
		return -x
	}
	return x
}

func main() {
	var n int
	var x1, y1, x2, y2, x3, y3, x4, y4 float64
	fmt.Scan(&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)
		if abs((x2-x1)*(y4-y3) - (y2-y1)*(x4-x3)) < 1e-10 {
			fmt.Println("YES")
		} else {
			fmt.Println("NO")
		}
	}
}

C++: //include
//------------------------------------------
#include <vector>
#include <list>
#include <map>
#include <set>
#include <queue>
#include <stack>
#include <bitset>
#include <algorithm>
#include <functional>
#include <numeric>
#include <utility>
#include <sstream>
#include <iostream>
#include <iomanip>
#include <cstdio>
#include <cmath>
#include <cstdlib>
#include <cctype>
#include <string>
#include <cstring>
#include <ctime>
#include <fstream>
#include <complex>
using namespace std;

//conversion
//------------------------------------------
inline int toInt(string s) {int v; istringstream sin(s);sin>>v;return v;}
template<class T> inline string toString(T x) {ostringstream sout;sout<<x;return sout.str();}

//math
//-------------------------------------------
template<class T> inline T sqr(T x) {return x*x;}

//typedef
//------------------------------------------
typedef vector<int> VI;
typedef vector<VI> VVI;
typedef vector<string> VS;
typedef pair<int, int> PII;
typedef pair<long, long> PLL;
typedef long long LL;
//container util
//------------------------------------------
#define ALL(a)  (a).begin(),(a).end()
#define RALL(a) (a).rbegin(), (a).rend()
#define PB push_back
#define MP make_pair
#define SZ(a) int((a).size())
#define EACH(i,c) for(typeof((c).begin()) i=(c).begin(); i!=(c).end(); ++i)
#define EXIST(s,e) ((s).find(e)!=(s).end())
#define SORT(c) sort((c).begin(),(c).end())

//repetition
//------------------------------------------
#define FOR(i,a,b) for(int i=(a);i<(b);++i)
#define REP(i,n)  FOR(i,0,n)

//constant
//--------------------------------------------

//clear memory
#define CLR(a) memset((a), 0 ,sizeof(a))

const double EPS = 1e-12;
const double INF = 1e12;

typedef complex<double> P;
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);
}
struct L : public vector<P> {
  L(const P &a, const P &b) {
    push_back(a); push_back(b);
  }
};

bool intersectLL(const L &l, const L &m) {
  return abs(cross(l[1]-l[0], m[1]-m[0])) > EPS || // non-parallel
         abs(cross(l[1]-l[0], m[0]-l[0])) < EPS;   // same line
}

int main(){
	int n;
	cin>>n;
	REP(i,n){
		P p1,p2,p3,p4;
		cin>>p1.real()>>p1.imag()>>p2.real()>>p2.imag()>>p3.real()>>p3.imag()>>p4.real()>>p4.imag();
		L l1(p1,p2),l2(p3,p4);
		if(!intersectLL(l1,l2))cout<<"YES"<<endl;
		else cout<<"NO"<<endl;
	}
	return 0;
}

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

    }
    return 0;
}

C++: #include <iostream>
#include <queue>
#include <stack>
#include <vector>
#include <stdio.h>
#include <algorithm>
#include <string.h>
#include <string>
#include <cmath>
#include <complex>
#include <map>
using namespace std;

#define rep(i,n) for(int i=0;i<int(n);++i)
#define ALL(v) (v).begin(),(v).end()
#define PB push_back
#define EPS 1e-10
#define F first
#define S second

static const double PI=6*asin(0.5);
typedef long long ll;
typedef complex<double> CP;
typedef pair<double,double> P;
static const int INF=1<<24;

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(abs((x[0]-x[1])*(y[2]-y[3])-(x[2]-x[3])*(y[0]-y[1]))<=EPS){
            cout<<"YES"<<endl;
        }else{
            cout<<"NO"<<endl;
        }
    }
    return 0;
}

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

int judge(double a1,double a2,double b1,double b2,double c1,double c2,double d1,double d2);

int main(){
	int n;
	double a1,a2,b1,b2;
	double c1,c2,d1,d2;
	
	while(cin>>n){
	for(int i=0;i<n;i++){
		cin>>a1>>a2>>b1>>b2>>c1>>c2>>d1>>d2;
		if(judge(a1,a2,b1,b2,c1,c2,d1,d2))
			cout<<"YES"<<endl; 
		else
			cout<<"NO"<<endl;
	}
	}

}


int judge(double a1,double a2,double b1,double b2,double c1,double c2,double d1,double d2){
	double aa,bb;
	aa=(b2-a2)/(b1-a1);
	bb=(d2-c2)/(d1-c1);
//	cout<<aa<<" "<<bb<<endl;
	if(aa==bb)return 1; else return 0;
}

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

int main() {
    double n,xx[4],yy[4],x,y;
    cin >> n;
    for(int i=0;i<n;i++){
        for(int j=0;j<4;j++) cin>>xx[j]>>yy[j];
        x=(yy[1]-yy[0])/(xx[1]-xx[0]),y=(yy[3]-yy[2])/(xx[3]-xx[2]);
        if(x==y)cout << "YES";
        else cout << "NO";
        cout << endl;
    }
}

C++: #include <stdio.h>

int
main(void)
{
	char line[100];
	int n;

	if (fgets(line, sizeof line, stdin) == NULL) {
		return 1;
	}
	if (sscanf(line, "%d", &n) != 1) {
		return 1;
	}

	double x1, y1, x2, y2, x3, y3, x4, y4;
	for (int i = 0; i < n; i++) {
		if (fgets(line, sizeof line, stdin) == NULL) {
			return 1;
		}
		
		sscanf(line, "%lf %lf %lf %lf %lf %lf %lf %lf",
				   &x1, &y1, &x2, &y2, &x3, &y3, &x4, &y4);

		double ab = (y2 - y1)/(x2 - x1);
		double cd = (y4 - y3)/(x4 - x3);
		
		printf("%s\n", ((ab == cd) ? "YES" : "NO"));
	}

	return 0;
}

C++: #include <cstdio>
#include <iostream>
#include <algorithm>
#include <cstdlib>
#include <cmath>
#include <vector>
#include <ctime>
#include <string>
#include <map>
#include <stack>
#include <queue>
#include <complex>

using namespace std;

typedef complex<double> xy_t;
double x[4], y[4];
int n;
xy_t P[4];
const double eps = 1e-11;

double cross_product(xy_t a, xy_t b) { return (conj(a)*b).imag(); }

int main(void){

	cin >> n;

	for(int i = 0; i < n; i++){
		for (int j = 0; j < 4; ++j){
			cin >> x[j] >> y[j];
			P[i] = xy_t(x[j], y[j]);
		}
		

		xy_t a[2] = {
			xy_t(x[0],y[0]) - xy_t(x[1],y[1]),
			xy_t(x[2],y[2]) - xy_t(x[3],y[3])
		};

		bool p = abs(cross_product(a[0], a[1])) < eps;
		cout << (p ? "YES" : "NO") << endl;
	}
	
	
    return 0;
}

C++: 
#include <iostream>
#include <complex>
#include <cmath>

using namespace std;

typedef complex<double> Point;

bool parallel(const Point &v1, const Point &v2) {
    double dot = v1.real()*v2.real() + v1.imag()*v2.imag();
    double n = abs(v1) * abs(v2);
    return abs(dot - n) < 1e-4;
}

int main() {
    int N;
    cin >> N;
    while(N--) {
        Point ps[4];
        for(int i = 0; i < 4; ++i) cin >> ps[i].real() >> ps[i].imag();
        Point v1 = ps[1]-ps[0], v2 = ps[3]-ps[2];

        if(parallel(v1, v2) && !parallel(v1, ps[2]-ps[1])) cout << "YES" << endl;
        else cout << "NO" << endl;
    }
    return 0;
}

C++: #include <iostream>

using namespace std;

int main() {
  int n, i;
  double x1, y1, x2, y2, x3, y3, x4, y4;

  cin >> n;

  for (i = 0; i < n; i++) {
    cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3 >> x4 >> y4;

    if (((y2 - y1) / (x2 - x1)) == ((y3 - y4) / (x3 - x4))) {
      cout << "YES" << endl;
    } else {
      cout << "NO" << endl;
    }
  }

  return 0;
}

C++: #include<iostream>
#include<cstdio>
#include<string>
#include<algorithm>

using namespace std;

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