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 <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 (x1 > x2) {
			double t = x2;
			x2 = x1; x1 = t;
			t = y2; y2 = y1; y1 = t;
		}
		if (x3 > x4) {
			double t = x4;
			x4 = x3; x3 = t;
			t = y4; y4 = y3; y3 = t;
		}
		if ((y2 - y1) / (x2 - x1) == (y4 - y3) / (x4 - x3)) {
			PRINT("YES");
		} else {
			PRINT("NO");
		}
	}

	return 0;
}

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

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,ll,lll;
        cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3 >> x4 >> y4;
        ll = (y2-y1)/(x2-x1);
        lll = (y4-y3)/(x4-x3);
        cout << ((ll == lll)? "YES" : "NO") << endl;
    }
}

C++: #include <cstdio>

int main(int argc, char *argv[]) {
	int dataN;
	scanf("%d", &dataN);
	for (int i = 0; i < dataN; i++) {
		float x1, y1, x2, y2, x3, y3, x4, y4;
		scanf("%f %f %f %f %f %f %f %f", &x1, &y1, &x2, &y2, &x3, &y3, &x4, &y4);
		
		float res1 = 1.0f, res2 = 2.0f;
		bool flg1 = false, flg2 = false;
		
		if (x2 > x1) res1 = (y2-y1)/(x2-x1);
		else if (x2 == x1) flg1 = true;
		else res1 = (y1-y2)/(x1-x2);
		if (res1 == 0.0f) flg1 = true;
		
		if (x4 > x3) res2 = (y4-y3)/(x4-x3);
		else if (x4 == x3) flg2 = true;
		else res2 = (y3-y4)/(x3-x4);
		if (res2 == 0.0f) flg2 = true;
		
		if (res1 == res2) {
			puts("YES");
		}
		else {
			if (flg1 && flg2) puts("YES");
			else puts("NO");
		}
	}
	return 0;
}

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

int main(void)
{
    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;
        if ((x2-x1)==0 || (x4-x3)==0)
            cout << (((x2-x1)==0&&(x4-x3)==0)? "YES" : "NO") << endl;
        else
            cout << (((y2-y1)*(x4-x3) == (y4-y3)*(x2-x1))? "YES" : "NO") << endl;
    }
    return 0;
}

C++: #include <iostream>
#include <complex>
#include <algorithm>
#define EPS 1e-10
#define EQ(a,b) (abs(a - b) < EPS)
#define EQV(a,b) ( EQ((a).real(), (b).real()) && EQ((a).imag(), (b).imag))
using namespace std;

typedef complex<double> P;

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

bool is_parallel(P a1, P a2, P b1, P b2) {
    return EQ( cross(a1 - a2, b1 - b2), 0.0);
}

int main(void)
{
    int n;
    cin >> n;
    while (n--) {
	P a1, a2, b1, b2;
	double x1, x2, x3, x4, y1, y2, y3, y4;
	cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3 >> x4 >> y4;
	a1 = P(x1, y1);
	b1 = P(x2, y2);
	a2 = P(x3, y3);
	b2 = P(x4, y4);

	if (is_parallel(a1, b1, a2, b2)) cout << "YES" << endl;
	else cout << "NO" << endl;
    }

    return 0;
}

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

int main()
{
    int n;
    cin>>n;
    while(n--){
        pair<double,double>a[4];
        for(int i=0;i<4;i++){
            cin>>a[i].first>>a[i].second;
        }
        if((a[1].first-a[0].first)*(a[3].second-a[2].second)-(a[3].first-a[2].first)*(a[1].second-a[0].second)>=(1e-10))
            cout<<"NO"<<endl;
        else
            cout<<"YES"<<endl;
    }
    return 0;
}

C++: #include <iostream>

using namespace std;

int main(){
  double a,b,c,d,e,f,g,h;
  int t;

  cin >> t;
  while(t--){
    cin >> a >> b >> c >> d >> e >> f >> g >> h;
    
    c -= a,d -= b,g -= e,h -= f;
    
    cout << (c*g-d*h ? "No" : "Yes") << endl;
  }
  
  return 0;
}

C++: #include <cstdio>

using namespace std;

double x1,y1,x2,y2,x3,y3,x4,y4;
    

int main()
{
    int n; scanf( "%d\n", &n );
    while( n > 0 )
    {
        scanf("%lf %lf %lf %lf %lf %lf %lf %lf\n", &x1, &y1, &y2, &y2, &y3, &y3, &y4, &y4 );
        double dx1 = x2 - x1, dx2 = x4 - x3, dy1 = y2 - y1, dy2 = y4 - y3;
        if( (dx1 == 0 && dx2 == 0) || (dy1 == 0 && dy2 == 0 ) )
        {
            printf( "YES\n" );
        }
        else if( dx1 != 0 && dx2 != 0 )
        {
            dy1 /= dx1;
            dy2 /= dx2;
            if( dy1 == dy2 || dy1 == -dy2 )
            {
                printf( "YES\n" );
            }
        }
        else
        {
            printf( "NO\n" );
        }
        
        n--;
    }
    return 0;
}

C++: #define	_USE_MATH_DEFINES
#include <iostream>
#include <algorithm>
#include <functional>
#include <vector>
#include <cstdio>
#include <string>
#include <cmath>
#include <cfloat>
#include <map>
#include <queue>
#include <stack>
#include <list>
using namespace std;

int main(){

	int n;
	cin>>n;
	double x1,y1,x2,y2,x3,y3,x4,y4;
	
	while(n>0){
		double a=0,b=0;
	cin>>x1>>y1>>x2>>y2>>x3>>y3>>x4>>y4;
		
		if(x2!=x1){
		a=(y2-y1)/(x2-x1);
		}
		if(x4!=x3){
		b=(y4-y3)/(x4-x3);
		}
		if(a==b)
			cout<<"YES"<<endl;
		else
			cout<<"NO"<<endl;
		
		n--;
	}
}

C++: #include<iostream>
#include<cstdio>
using namespace std;
int main()
{
	int n;
	double x[4],y[4];

	cin >> n;

	for(int i = 0;i < n;i++){
		for(int j = 0;j < 4;j++){
			scanf("%lf",&x[j]);
			scanf("%lf",&y[j]);
		}

		double a1 = (y[2]-y[1])/(x[2]-x[1]);
		double a2 = (y[4]-y[3])/(x[4]-x[3]);

		if(a1 == a2){
			cout << "YES" << endl;
		}
		else cout << "NO" << endl;
	}
}

C++: #include <cstdio>
#include <algorithm>
#include <complex>
#include <utility>

using namespace std;

static const double EPS=1e-12;
static const double INF=1e24;
static const double PI=3.141592653589793;

using Point=complex<double>;

namespace std {
    bool operator<(const Point &a, const Point &b) {
        return real(a)!=real(b)? real(a)<real(b) : imag(a)<imag(b);
    }
}

bool testcase_ends() {
    double x, y;
    if (scanf("%lf %lf", &x, &y) == EOF)
        return 1;

    Point A(x, y);
    scanf("%lf %lf", &x, &y);
    Point B(x, y);
    scanf("%lf %lf", &x, &y);
    Point C(x, y);
    scanf("%lf %lf", &x, &y);
    Point D(x, y);

    printf("%s\n", abs(arg(B-A)-arg(D-C))<EPS? "YES":"NO");
    return 0;
}

int main() {
    size_t n;
    scanf("%zu", &n);

    for (size_t i=0; i<n; ++i)
        if (testcase_ends())
            return 1;

    return 0;
}