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<cstdio>
#include<cmath>

int main(){
	int n;
	double x[4], y[4];
	int xi[4], yi[4];
	scanf("%d", &n);
	for(int i = 0; i < n; i++){
		scanf("%lf %lf %lf %lf %lf %lf %lf %lf", &x[0],&y[0],&x[1],&y[1],&x[2],&y[2],&x[3],&y[3]);

		for(int i = 0; i < 4; i++){
			xi[i] = (int)(x[i] * 100000);
			yi[i] = (int)(y[i] * 100000);
		}
		if((yi[0] - yi[1])*(xi[2] - xi[3]) == (xi[0] - xi[1])*(yi[2] - yi[3])){
			printf("YES\n");
		}else{
			printf("NO\n");
		}
	}
	return 0;
}

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

using namespace std;


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

	cin>>n;

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

		if((fabs(AB-CD))<0.00001)
			cout<<"YES"<<endl;
		else
			cout<<"NO"<<endl;


	}
	

	return 0;
}

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


//二次元幾何テンプレート
//点クラス point->x,y
//直線クラス line->s,v
//
class point;
class line;
point vec_add(point vec1,point vec2);//ベクトルの足し算
point vec_sub(point vec1,point vec2);//ベクトルの引き算
double inner_product(point a,point b);//内積
double outer_product(point a,point b);//外積
bool vec_parallel(point vec1,point vec2);//ベクトルの並行判定(交差判定falseとしても使える)
bool vec_angle_right(point vec1,point vec2);//ベクトルの直行判定
bool cross_line_segment(line a,point p1,point p2);//直線と線分の交差判定
//


class point{
public:
	double x,y;
	point(){
		x=0;
		y=0;
	}
	point(double a,double b){
		x=a;
		y=b;
	}
};

//直線クラス
class line{
public:
	point s,v;
public:
	line(point a,point b){
		s=a;
		v=vec_sub(a,b);
	}
};

//多角形(頂点を保持する)
typedef vector<point> polygon;

//ベクトルの足し算
point vec_add(point vec1,point vec2){
	point res(vec1.x+vec2.x,vec1.y+vec2.y);
	return res;
}
//ベクトルの引き算
point vec_sub(point vec1,point vec2){
	point res(vec1.x-vec2.x,vec1.y-vec2.y);
	return res;
}
//内積
double inner_product(point a,point b){
	return a.x * b.x + a.y * b.y;
}
//外積
double outer_product(point a,point b){
	return a.x * b.y - a.y * b.x;
}


//ベクトルの並行判定
bool vec_parallel(point vec1,point vec2){
	if(outer_product(vec1,vec2)==0){
		return true;
	}else{
		return false;
	}
}
//ベクトルの直交判定
bool vec_angle_right(point vec1,point vec2){
	if(inner_product(vec1,vec2)==0){
		return true;
	}else{
		return false;
	}
}

bool cross_line_segment(line l,point p1,point p2){
	p1=vec_sub(p1,l.s);
	p2=vec_sub(p2,l.s);
	if(outer_product(p1,l.v)*outer_product(p2,l.v)<0){
		return true;
	}else{
		return false;
	}
}



//
int main(){
	int n;
	point p[4];
	cin>>n;
	for(;n--;){
		for(int i=0;i<4;i++){
			cin>>p[i].x;
			cin>>p[i].y;
		}
		p[0]=vec_sub(p[0],p[1]);
		p[2]=vec_sub(p[2],p[3]);
		if(vec_parallel(p[0],p[2])){
			cout<<"YES"<<endl;
		}else{
			cout<<"NO"<<endl;
		}
		
	}
	return 0;
}

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

int main(){
  double x1, y1, x2, y2, x3, y3, x4, y4;
  int n;
  cin >> n;
  for(int i = 0; i < n; i++){
    cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3 >> x4 >> y4;
    if(abs((y2 - y1) * (x4 - x3) - (y4 - y3) * (x2 - x1)) <= 1e-10){
      cout << "YES" << endl;
    }else{
      cout << "NO" << endl;
    }
  }
}

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

int main() {
double x1,x2,x3,x4,y1,y2,y3,y4;
int n;
cin>>n;
for(int jjj=1;jjj<=n;jjj++){
	cin>>x1>>y1>>x2>>y2>>x3>>y3>>x4>>y4;
	if(x1==x2||x3==x4){
		if(x3==x4&&x1==x2){
			cout<<"YES"<<endl;
		}
		else{
			cout<<"NO"<<endl;
		}}
	else{
		double k=(y2-y1)/(x2-x1),l=(y4-y3)/(x4-x3);
		if(k==l){
			cout<<"YES"<<endl;
		}
		else{
			cout<<"NO"<<endl;
		}
	}	
	}

	return 0;
}

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

using namespace std;

int main() {
	int k;
	double x1,x2,x3,x4,y1,y2,y3,y4;
	double katamuki1;
	double katamuki2;
	cin >> k;

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

		katamuki1 = (y2 - y1) / (x2 - x1);
		katamuki2 = (y4 - y3) / (x4 - x3);

		if (katamuki1 == katamuki2) {
			if(((y4 - y1) / (x4 - x1)) == ((y3 - y2) / (x3 - x2))) {
				cout << "NO" << endl;
			} else {
				cout << "YES" << endl;
			}
		} else {
			cout << "NO" << endl;
		}
	}
	return 0;
}

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

using namespace std;

typedef struct _POINT {
	double x;
	double y;
} POINT;

typedef struct _LINE {
	POINT a;
	POINT b;
} LINE;

bool isParallel (LINE l1, LINE l2) {
	double f1 = l1.b.x - l1.a.x;
	double g1 = l1.b.y - l1.a.y;
	double f2 = l2.b.x - l2.a.x;
	double g2 = l2.b.y - l2.a.y;

	return (fabs(f2*g1 - f1*g2) < 0.00001);
/*
	double det = f2*g1 - f1*g2;
	if (det == 0.0)
		return true;
	else
		return false;
*/
}

int main() {
	LINE ab, cd;
	int n;
	cin >> n;
	for (int i = 0; i < n; i++) {
		cin >> ab.a.x >> ab.a.y >> ab.b.x >> ab.b.y >> cd.a.x >> cd.a.y >> cd.b.x >> cd.b.y;
		if (isParallel(ab, cd))
			cout << "YES" << endl;
		else
			cout << "NO" << endl;
	}
	return 0;
}

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

using namespace std;

int main(void)
{
  double x,y;
  double x1,y1;
  int n;
  int i;
  long double a=0,b=0;
  int k=0;
  int j=0;

  cin>>n;

  for(i=0;i<n;i++)
    {
      cin>>x>>y>>x1>>y1;
      if(x1-x!=0.0)
	{
          a=(y1-y)/(x1-x);
          j++;
	}
      else
        k++;

      cin>>x>>y>>x1>>y1;
      if(x1-x!=0.0)
	{
          b=(y1-y)/(x1-x);
          j++;
	}
      else
        k++;

      if((a==b && j==2) || k==2)
        puts("YES");
      else
        puts("NO");
    }

}

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;
		double dx1 = x2-x1,
			   dy1 = y2-y1,
			   dx2 = x4-x3,
			   dy2 = y4-y3;
		
		if (dx2 * dy1 == dx1 * dy2) {
			if (dx1*dy1*dx2*dy2 != 0.0) {
				cout << "YES" << endl;
			} else {
				if (dx2+dy2 == 0.0 || dx1+dy1 == 0.0) {
					cout << "NO" << endl;
				} else {
					cout << "YES" << endl;
				}
			}
		} else {
			cout << "NO" << endl;
		}
	}
	return 0;
}

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

int main()
{
	int n ;
	double ax,ay,bx,by,cx,cy,dx,dy,a,b ;

	cin >> n ;
	for( int i = 0 ; i < n ; i ++ )
	{
		cin >> ax >> ay >> bx >> by >> cx >> cy >> dx >> dy ;

		a = (ay-by)/(ax-bx) ;
		b = (cy-dy)/(cx-dx) ;

		if( a == b )
		{
			cout << "YES" << endl ;
		}
		else
		{
			cout << "NO" << endl ;
		}
	}
	return 0 ;
}

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

}