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<complex>
#include<cmath>
using namespace std;
typedef complex<double> xy_t;

int n;
double x,y;
xy_t abcd[4];
const double eps=1e-11;

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


int main(){
    cin>>n;
    for(int i=0;i<n;i++){
        for(int j=0;j<4;j++){
            cin>>x>>y;
            abcd[j]=xy_t(x,y);
        }
        bool p=abs(cross_product(abcd[0]-abcd[1],abcd[2]-abcd[3]))<eps; //面積がepsより小さいか
        if(p){cout<<"YES"<<endl;}
        else{cout<<"NO"<<endl;}
    }
}

C++: #include <iostream>
#include<string>
#include<cmath>
#include<algorithm>
#include<cctype>
#include<queue>
#include<stack>
#include<stdio.h>
#include<vector>
#include<set>
#include<map>
#include<iomanip>
#define rep(i,n) for(int i=0;i<n;i++)
typedef int long long ll;

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

C++: #include <stdio.h>

int main(void) {
  int n, i. j;
  scanf("%d",&n);
  for(i = 0; i < n; ++i) {
    double a[8];
    for(j = 0; j < 8; ++j) scanf("%lf", &a[i]);
    a[3] -= a[1], a[2] -= a[0], a[6] -= a[4], a[7] -= a[5];
    if(a[3] / a[2] == a[7] / a[6]) printf("YES\n");
    else printf("NO\n");
  }
  return 0;
}

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

double dis(double a, double b){
	double d;
	if((0<=a && 0<=b) || (a<=0 && b<= 0)){
		d = a-b;
	}else{
		d = abs(a) + abs(b);
	}
	return d;
}
int main(){
	double x1, x2, x3, x4, y1, y2, y3, y4;
	double ab_x, ab_y, cd_x, cd_y;
	int n, i;
	cin >> n;
	for(i=0;i<n;i++){
		cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3>> x4 >> y4;
		
		ab_x = dis(x1,x2);
		ab_y = dis(y1,y2);
		cd_x = dis(x3,x4);
		cd_y = dis(y3,y4);
		if(ab_y/ab_x == cd_y/cd_x){
			cout << "YES\n";
		}else{
			cout << "NO\n";
		}
	
	}
	return 0;
}

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

struct Pos {
	double x, y;
}a, b, c, d;

int main() {
	int n;
	cin >> n;
	for(int i = 0; i < n; i++ ) {
		cin >> a.x >> a.y >> b.x >> b.y >> c.x >> c.y >> d.x >> d.y;
		Pos e;
		e.x = b.x - a.x;
		e.y = b.y - a.y;
		Pos f;
		f.x = d.x - c.x;
		f.y = d.y - c.y;
		
		double men = e.x * f.y - e.y * f.x;
		double eps = 1e-12;
		if(-eps <= men && men <= eps)
			cout << "YES" << endl;
		else
			cout << "NO" << endl;
	}
	return 0;
}

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

C++: #include <bits/stdc++.h>
#define rep(i,l,n) for(int i=l;i<n;i++)
using namespace std;

int main(){
	int n;
	cin>>n;
	rep(i,0,n){
		double x1,y1,x2,y2,x3,y3,x4,y4,m1,m2;
		cin>>x1>>y1>>x2>>y2>>x3>>y3>>x4>>y4;
		m1=(y2-y1)/(x2-x1);
		m2=(y4-y3)/(x4-x3);
		if(fabs(m1-m2)<(1e-10)) cout<<"YES"<<endl;
		else cout<<"NO"<<endl;
	}
}

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;
}

C++: 
#define _CRT_SECURE_NO_WARNINGS
#define _USE_MATH_DEFINES
#include <cfloat>
#include <climits>
#include <cmath>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <ctime>
#include <iostream>
#include <memory>
#include <string>

#include <algorithm>
#include <complex>
#include <list>
#include <map>
#include <numeric>
#include <queue>
#include <set>
#include <stack>
#include <vector>

using namespace std;

#ifdef _MSC_VER
#define __typeof__ decltype
#endif

#define ITER(c)         __typeof__((c).begin())
#define FOREACH(it, c)  for (ITER(c) it=(c).begin(); it != (c).end(); ++it)
#define RITER(c)        __typeof__((c).rbegin())
#define RFOREACH(it, c) for (RITER(c) it=(c).rbegin(); it != (c).rend(); ++it)
#define REP(i, n)       REPEAT(i, 0, n)
#define REPEAT(i, k, n) for (int i = (k); i < (n); ++i)
#define REVREP(i, n)    for (int i = ((n)-1); i >= 0; --i)
#define FIND(c, n)      std::find((c).begin(), (c).end(), (n));
#define BSEARCH(c, n)   std::binary_search((c).begin(), (c).end(), (n))
#define LLPOW(p1,p2)    ((ll)pow((double)(p1), (int)p2))
#define ESIZEOF(A)      (sizeof(A)/sizeof((A)[0]))
#define MIN(p1,p2)      (((p1)<(p2))?(p1):(p2))
#define MAX(p1,p2)      (((p1)>(p2))?(p1):(p2))
#define CIN_NO_SYNC     do { cin.tie(0); ios_base::sync_with_stdio(false); } while(0)
#define GETSTR(p)       fgets((p), sizeof(p), stdin)
#define CHOMP(p)        do{ char *_q = (p) + strlen(p)-1; if(*_q == '\n') *_q = 0; } while(0)

#define X real()
#define Y imag()
typedef double D;
typedef complex<D> P;
#define EPS (1e-9)
#define DEQ(a,b) (abs((a)-(b)) < EPS)

inline D outprd(const P &a, const P &b){ return (conj(a) * b).Y; }
bool is_parallel(P &p1, P &p2, P &p3, P &p4){
    return DEQ(outprd(p1-p2, p3-p4), 0.0);
}

int main(){
    int n;
    cin >> n;
    REP(i,n){
        double x1, y1, x2, y2, x3, y3, x4, y4;
        cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3 >> x4 >> y4;
        P p1(x1,y1), p2(x2,y2), p3(x3,y3), p4(x4,y4);
        if(is_parallel(p1,p2,p3,p4)){
            cout << "YES" << endl;
        } else {
            cout << "NO" << endl;
        }
    }
    return 0;
}

C++: #define _USE_MATH_DEFINES
#include <iostream>
#include <sstream>
#include <cmath>
#include <algorithm>
#include <queue>
#include <stack>

typedef long long ll;
using namespace std;

int main()
{
	int T;
	scanf("%d",&T);

	for(int t=0;t<T;t++){
		double pos[8];
		double eps = 10e-9;
		for(int i=0;i<8;i++){
			scanf("%lf",pos+i);
		}

		if(abs((pos[1]-pos[3])/(pos[0]-pos[2])-(pos[1+4]-pos[3+4])/(pos[0+4]-pos[2+4]))<eps
			&& !(pos[0] == pos[0+4] && pos[1] == pos[1+4] && pos[2] == pos[2+4] && pos[3] == pos[3+4])){
			cout << "YES" << endl;
		}else
		{
			cout << "NO" << endl;	
		}
	}
}

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

int main(){
	int n;
	double t[100][8];
	cin >> n;
	for(int i = 0; i < n; i++){
		for(int j = 0; j < 8; j++){
			cin >> t[i][j];
		}
	}
	for(int i = 0; i < n; i++){
		if(t[i][0] == t[i][2]){
			if(t[i][4] == t[i][6]){
				cout << "YES" << endl;
			}
			else{
				cout << "NO" << endl;
			}
		}
		else if(t[i][1] == t[i][3]){
			if(t[i][5] == t[i][7]){
				cout << "YES" << endl;
			}
			else{
				cout << "NO" << endl;
			}
		}
		else if((t[i][1]-t[i][3])*(t[i][4]-t[i][6]) == (t[i][5]-t[i][7])*(t[i][0]-t[i][2])){
			cout << "YES" << endl;
		}
		else{
			cout << "NO" << endl;
		}
	}
}