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 <vector>
#include <cstdio>
using namespace std;

const double EPS = 1e-8;
const double INF = 1e12;
typedef complex<double> P;

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);
}
double dot(const P& a, const P& b) {
  return real(conj(a)*b);
}
struct L : public vector<P> {
  L(const P &a, const P &b) {
    push_back(a); push_back(b);
  }
};

int ccw(P a, P b, P c) {
  b -= a; c -= a;
  if (cross(b, c) > 0)   return +1;       // counter clockwise
  if (cross(b, c) < 0)   return -1;       // clockwise
  if (dot(b, c) < 0)     return +2;       // c--a--b on line
  if (norm(b) < norm(c)) return -2;       // a--b--c on line
  return 0;
}

int main() {
  int n;
  cin >> n;
  while(n--) {
    P p[4];
    for (int i=0; i<4; i++) {
      double x,y;
      cin >> x >> y;
      p[i] = P(x,y);
    }
    if (abs(cross(p[1]-p[0], p[3]-p[2])) < EPS)
      cout << "YES" << endl;
    else
      cout << "NO" << endl;
  }
}

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 <cstdio>
#include <cmath>
#include <cstring>
#include <cstdlib>
#include <climits>
#include <ctime>
#include <queue>
#include <stack>
#include <algorithm>
#include <list>
#include <vector>
#include <set>
#include <map>
#include <iostream>
#include <deque>
#include <complex>
#include <string>
#include <iomanip>
#include <sstream>
#include <bitset>
#include <valarray>
#include <iterator>
using namespace std;
typedef long long int ll;
typedef unsigned int uint;
typedef unsigned char uchar;
typedef unsigned long long ull;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
typedef vector<int> vi;
 
#define REP(i,x) for(int i=0;i<(int)(x);i++)
#define FOR(i,c) for(__typeof((c).begin())i=(c).begin();i!=(c).end();i++)
#define RREP(i,x) for(int i=((int)(x)-1);i>=0;i--)
#define RFOR(i,c) for(__typeof((c).rbegin())i=(c).rbegin();i!=(c).rend();i++)
#define ALL(container) container.begin(), container.end()
#define RALL(container) container.rbegin(), container.rend()
#define SZ(container) ((int)container.size())
#define mp(a,b) make_pair(a, b)
#define UNIQUE(v) v.erase( unique(v.begin(), v.end()), v.end() );
 
template<class T> bool chmax(T &a, const T &b) { if (a<b) { a=b; return 1; } return 0; }
template<class T> bool chmin(T &a, const T &b) { if (a>b) { a=b; return 1; } return 0; }
template<class T> ostream& operator<<(ostream &os, const vector<T> &t) {
os<<"["; FOR(it,t) {if(it!=t.begin()) os<<","; os<<*it;} os<<"]"; return os;
}
template<class T> ostream& operator<<(ostream &os, const set<T> &t) {
os<<"{"; FOR(it,t) {if(it!=t.begin()) os<<","; os<<*it;} os<<"}"; return os;
}
template<class S, class T> ostream& operator<<(ostream &os, const pair<S,T> &t) { return os<<"("<<t.first<<","<<t.second<<")";}
template<class S, class T> pair<S,T> operator+(const pair<S,T> &s, const pair<S,T> &t){ return pair<S,T>(s.first+t.first, s.second+t.second);}
template<class S, class T> pair<S,T> operator-(const pair<S,T> &s, const pair<S,T> &t){ return pair<S,T>(s.first-t.first, s.second-t.second);}
 
const int INF = 1<<28;
const double EPS = 1e-8;
const int MOD = 1000000007;
 
#define X real()
#define Y imag()
 
typedef complex<double> P;
struct L{
    P p1, p2;
    L(P p1, P p2):p1(p1), p2(p2){}
    L(){}
};
istream& operator>>(istream &is, P &p) {
    double x, y;
    is >> x >> y;
    p = P(x, y);
    return is;
}
inline int sig(double x) { return (abs(x) < EPS ? 0 : x > 0 ? 1 : -1); }
inline double norm(const P &p){return p.X*p.X+p.Y*p.Y;}
inline double inp(const P& a, const P& b){return (conj(a)*b).X;}
inline double outp(const P& a, const P& b){return (conj(a)*b).Y;}
 
 
int T;
 
int main(){
    ios::sync_with_stdio(false);
    cin >> T;
    while(T--){
        P p1, p2, p3, p4;
        cin >> p1 >> p2 >> p3 >> p4;
        p2 -= p1; p4 -= p3;
        puts(!sig(inp(p2, p4*P(0, 1))) ? "YES" : "NO");
    }
    return 0;
}

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

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

C++: #include <iostream>
#include <cstdio>
using namespace std;
int main(){
	int n;
	cin >> n;
	for(int i=0;i<n;i++){
		double x1,x2,x3,x4,y1,y2,y3,y4;
		cin>>x1>>y1>>x2>>y2>>x3>>y3>>x4>>y4;
		double v1x = x2-x1;
		double v1y = y2-y1;
		double v2x = x4-x3;
		double v2y = y4-y3;
		double op = v1x*v2y-v1y*v2x;
		if(op<0.000000001) cout << "YES" << endl;
		else cout << "NO" << endl;
	}
}

C++: #include <iostream>
#include <cmath>
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 && x4==x3)   cout << "YES" <<endl;
        else{
            if(fabs((y2-y1)/(x2-x1) - (y4-y3)/(x4-x3))<0.00001)  cout << "YES" << endl;
            else cout << "NO" << endl;
        }

    }
    return 0;
}

C++: #include<iostream>
#include<cmath>
using namespace std;
int main()
{
	int n;cin>>n;
	for(;n--;)
	{
		double a,b,c,d,e,f,g,h;cin>>a>>b>>c>>d>>e>>f>>g>>h;
		if(c==a||g==e)cout<<(c==a&&g==e?"YES":"NO")<<endl;
		else cout<<(abs((d-b)*(g-e)-(h-f)*(c-a))<1e-8?"YES":"NO")<<endl;
	}
}

C++: #include<complex>
#include<iostream>
using namespace std;
typedef complex<double> P;

int cross(P a, P b, P c, P d);

int main(){
    int n;
    cin >>n;
    for(int i = 0; i < n; i++){
        P a, b, c, d;
        cin >>a.real() >>a.imag() >>b.real() >>b.imag() >>c.real() >>c.imag() >>d.real() >>d.imag();
        if(cross(a, b, c, d)){
            cout <<"YES" <<endl;
        } else{
            cout <<"NO" <<endl;
        }
    }
    return 0;
}
int cross(P a, P b, P c, P d){
    P x = a - b, y = c - d;
    int ret;
    if(!(x.real() * y.imag() - x.imag() * y.real())){
        ret = 1;
    } else{
        ret = 0;
    }
    return ret;
}