Go: package main

import (
	"bufio"
	"os"
	"strconv"
	"fmt"
	"strings"
)

var scanner = bufio.NewScanner(os.Stdin)

func nextInt() int {
	scanner.Scan()
	i, e := strconv.Atoi(scanner.Text())
	if e != nil {
		panic(e)
	}
	return i
}

func main() {
	n := nextInt()
	for i := 0; i < n; i++ {
		scanner.Scan()
		points := strings.Split(scanner.Text(), " ")
		fpoints := make([]float64, 0)
		for _, v := range points {
			tmp, _ := strconv.ParseFloat(v, 64)
			fpoints = append(fpoints, tmp)
		}
		if fpoints[0] == fpoints[2] && fpoints[4] == fpoints[6] {
			fmt.Println("YES")
		} else {
			a := (fpoints[1] - fpoints[3]) / (fpoints[0] - fpoints[2])
			b := (fpoints[5] - fpoints[7]) / (fpoints[4] - fpoints[6])
			if a == b {
				fmt.Println("YES")
			} else {
				fmt.Println("NO")
			}
		}
	}
}

C++: #include <cstdio>
#include <cstdlib>
#include <cstdint>
#include <cstring>
#include <cctype>
#include <iostream>
#include <sstream>
#include <vector>
#include <list>
#include <deque>
#include <map>
#include <set>
#include <stack>
#include <queue>
#include <unordered_map>
#include <unordered_set>
#include <algorithm>
#include <functional>
#include <complex>
using namespace std;
typedef pair<int, int> pii;
typedef vector<int> vi;
typedef vector<vi> vvi;
typedef long long ll;
const double PI = 3.141592653589793238462643383279502884L;
const int LARGE_PRIME = 1000000007;
#define all(c) (c).begin(), (c).end()
#define rall(c) (c).rbegin(), (c).rend()
#define tr(c, i) for(auto i = (c).begin(); i != (c).end(); i++)
#define rtr(c, i) for(auto i = (c).rbegin(); i != (c).rend(); i++)
#define repfn(i,a,cont,next) \
  for(auto i = (a); [=](){return(cont);}(); i = [=](){return(next);}())
#define repby(i,a,b,s) repfn(i,a,i<b,i+s)
#define repab(i,a,b) repby(i,a,b,1)
#define rep(i,b) repab(i,0,b)
#define pb push_back
#define sz(c) int((c).size())

typedef complex<double> P;

bool EQ(double a, double b)
{
  double prec = 0.0000001;
  return abs(a - b) < prec;
}

bool parallel(P a, P b)
{
  return (EQ(arg(a), arg(b)) || EQ(arg(a), arg(-b)));
}

int main(int argc, char **argv)
{
  vector<P> v(4);
  double x, y;
  int n;
  cin >> n;
  while (n--) {
    rep (i, 4) {
      cin >> x >> y;
      v[i] = P(x, y);
    }
    P p = v[1] - v[0];
    P q = v[3] - v[2];
    if (parallel(v[1] - v[0], v[3] - v[2]))
      cout << "YES" << endl;
    else
      cout << "NO" << endl;
  }

  return 0;
}

C++: /* AOJ 0021 Parallelism */

#include <iostream>
#include <cmath>

using namespace std;

const double EPS = 1e-9;

int
main()
{
    int n;
    double x1, y1, x2, y2, x3, y3, x4, y4;
    double dx12, dy12, dx34, dy34;

    cin >> n;

    while (n--) {
        cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3 >> x4 >> y4;
        dx12 = x2 - x1;
        dy12 = y2 - y1;
        dx34 = x4 - x3;
        dy34 = y4 - y3;

        if (fabs((dx12 * dy34) - (dx34 * dy12)) < EPS)
            cout << "YES" << endl;
        else
            cout << "NO" << endl;
    }

    return 0;
}

C++: #include <cstdio>
#include <cstring>
#include <algorithm>
#include <string>
#include <cmath>
#include <stack>
#include <vector>
#include <iostream>


using namespace std;

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

		double dx1 = x[1] - x[0];
		double dy1 = y[1] - y[0];
		double dx2 = x[3] - x[2];
		double dy2 = y[3] - y[2];
		if (abs(dx1-dx2) < 0.0000001
			|| abs(dy1/dx1-dy2/dx2) < 0.00000001)
			puts("YES");
		else
			puts("NO");
	}

	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);
		if ((x2 - x1)*(y4 - y3) - (x4 - x3)*(y2 - y1) == 0)
		{
			printf("YES\n");
		}
		else
		{
			printf("NO\n");
		}
	}
	return 0;
}

C++: #include <stdio.h>
struct point{
    double x;
    double y;
};
int main(){
    int n;
    point a[4];
    scanf("%d",&n);
    for(int i=0;i<n;i++){
        for(int j=0;j<4;j++){
            scanf("%lf%lf",&a[j].x,&a[j].y);
        }
        if(a[1].x==a[0].x){
            if(a[3].x==a[2].x)printf("YES\n");
            else printf("NO\n");
        }
        else  if(a[1].y==a[0].y){
            if(a[3].y==a[2].y)printf("YES\n");
            else printf("NO\n");
        }
        else  if(a[3].x==a[2].x){
            printf("NO\n");
        }
        else if(a[3].y==a[2].y){
            printf("NO\n");
        }
        else if(((a[1].x-a[0].x)/(a[1].y-a[0].y))==((a[3].x-a[2].x)/(a[3].y-a[2].y)))printf("YES\n");
        else printf("NO\n");
    }
}

C++: #include <iostream>
#include <cmath>
using namespace std;
#define EPS 1e-10

int main(){
  int n;
  double x1,x2,x3,x4,y1,y2,y3,y4;
  cin >> n;
  while(n--){
    cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3 >> x4 >> y4;
    if(fabs((y4-x3)*(x2-x1) - (x4-x3)*(y2-y1)) < EPS) cout << "YES" << endl;
    else cout << "NO" << endl;
  }
}

C++: #include<cstdio>
#include<algorithm>
using namespace std;
int main(void)
{
	int n,i;
	double x1,y1,x2,y2,x3,y3,x4,y4,x;
	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);
		if((x2-x1==0) || (x4-x3==0))	{
			if((x2-x1==0) && (x4-x3==0))	{
				printf("YES\n");
			}
			else {
				printf("NO\n");
			}
		}
		else {
			x=(y2-y1)/(x2-x1)-(y4-y3)/(x4-x3);
			if(x<0.0000001 && x>-0.0000001)	{
				printf("YES\n");
			}
			else {
				printf("NO\n");
			}
		}
	}
	return 0;
}

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

struct vec {
	double x, y;
};

static vec P(double x, double y){
	vec P = {x, y};
	return P;
}

static vec sub(vec a, vec b){
	return P(a.x - b.x, a.y - b.y);
}

static double cross(vec a, vec b){
	return (a.x * b.y - a.y * b.x);
}

int main(void){
    
    int n;
    cin >> n;
    double D[8];
    vec V[4];
    for(int i = 0; i < n; i++){
    	for(int j = 0; j < 4; j++){
    		cin >> D[2*j] >> D[2*j+1];
    		V[j] = P(D[2*j], D[2*j+1]);
    	}
    	vec a = sub(V[0], V[1]);
    	vec b = sub(V[2], V[3]);
    	if (cross(a, b) == 0){
    		cout << "YES" << endl;
    	}
    	else{
    		cout << "NO" << endl;
    	}
    }
    return 0;

}

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

using namespace std;

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

C++: #include <iostream>
#include <string>
#include <algorithm>
#include <cstdio>
#include <vector>
#include <cmath>
#include <stack>
#define REP(i, n) for(int i = 0;i < n;i++)
#define REPR(i, n) for(int i = n;i >= 0;i--)
using namespace std;

bool parallel(double x1, double y1, double x2, double y2){
	double vec;
	vec = x1*y2 - x2*y1;
	if (vec < 0.0000000001 && vec > -0.0000000001) return true;
	return false;
}

int main(){
    int n;
    double x1, x2, x3, x4, y1, y2, y3, y4;
    cin >> n;
    REP(i, n) {
    	cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3 >> x4 >> y4;
    	if (parallel(x1-x2, y1-y2, x3-x4, y3-y4)) cout << "YES" << endl;
    	else cout << "NO" << endl;
    }
}

C++: #include<stdio.h>
#include<math.h>
int main(void)
{
	int n;
	int i,j;
	double x[5],y[5];
	double xx,yy;
	
	scanf("%d",&n);
	for(i=0;i<n;i++){
		for(j=0;j<4;j++){
			scanf("%lf %lf",&x[j],&y[j]);
		}
		if(x[0]-x[1]==0 || x[2]-x[3]==0){
			if(x[0]-x[1]==0 && x[2]-x[3]==0){
				printf("YES\n");
			}
			else {
				printf("NO\n");
			}
		}
		else {
			xx=(y[0]-y[1])/(x[0]-x[1]);
			yy=(y[2]-y[3])/(x[2]-x[3]);
//			printf("%lf %lf\n",xx,yy);
			xx=xx-yy;		
			if(xx<0.0000001 && xx>-0.0000001){
				printf("YES\n");
			}
			else {
				printf("NO\n");
			}
		}
	}	
	return 0;
}