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>
using namespace std;
int main(){
  
  double x1, y1, x2, y2, x3, y3, x4, y4;
  int dataSets;
  cin >> dataSets;
  for(int i=0;i<dataSets;i++){
    bool flag=false;
    cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3 >> x4 >> y4;

    
    if((x2-x1)==0 || (x4-x3)==0){
      if((x2-x1) == (x4-x3)){
	if(x1==x3){
	  flag == true;
	}
      }
    }


    // cout << (y2-y1)/(x2-x1) << endl;
    // cout << (y4-y3)/(x4-x3) << endl;
    if((y2-y1)/(x2-x1) == (y4-y3)/(x4-x3)){
      // cout << "here" << endl;
      flag = true;
    }

    if(flag){
      cout << "YES" << endl;
    }else{
      cout << "NO" << endl;
    }
    
  }

}

C++: #include <iostream>
#include <iomanip>
#include <sstream>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <cmath>
#include <climits>
#include <cfloat>
#include <vector>
#include <string>
#include <stack>
#include <queue>
#include <set>
#include <map>
#include <algorithm>
#include <functional>
#include <utility>
#include <numeric>
#include <iterator>

using namespace std;

typedef long long LL;
typedef unsigned long long ULL;
typedef vector<int> VI;
typedef vector<VI> VVI;
typedef vector<string> VS;
typedef pair<int,int> PII;
typedef vector<PII> VPII;
typedef istringstream ISS;
typedef ostringstream OSS;

#define REP( i, m, n ) for ( int i = (int)( m ); i < (int)( n ); ++i )
#define FOR( v, c ) for ( auto &v : c )
#define EACH( it, c ) for ( auto it = c.begin(); it != c.end(); ++it )
#define ALL( c ) (c).begin(), (c).end()
#define DRANGE( c, p ) (c).begin(), (c).begin() + p, (c).end()

#define PB( n ) push_back( n )
#define MP( a, b ) make_pair( ( a ), ( b ) )
#define EXIST( c, e ) ( (c).find( e ) != (c).end() )

#define fst first
#define snd second

#define DUMP( x ) cerr << #x << " = " << ( x ) << endl
#define DEBUG( x ) cerr << __FILE__ << ":" << __LINE__ << ": " << #x << " = " << ( x ) << endl

#include <complex>
typedef complex<double> Point;
const double EPS = 1e-8;

// テ・ツ?・テ・ツ環崚」ツつケテ」ツδ暗」ツδェテ」ツδシテ」ツδ?」ツ?凝」ツつ嘉・ツョツ淌ヲツ閉ーテ、ツコツ古」ツ?、テ」ツつ津」ツ?ィテ」ツ?」テ」ツ?ヲ Point テ」ツ?ク
istream& operator >> ( istream &s, Point &a )
{
	double r, i;
	s >> r >> i;
	a = Point( r, i );
	return s;
}

// a == b
bool eq( const double a, const double b )
{
	return abs( a - b ) <= EPS;
}


// テ・ツ、ツ姪ァツゥツ催ッツシツ暗」ツつッテ」ツδュテ」ツつケテァツゥツ催ッツシツ?
double cross( const Point &a, const Point &b )
{
	return a.real() * b.imag() - a.imag() * b.real();
}

int main()
{
	cin.tie( 0 );
	ios::sync_with_stdio( false );

	int n;
	cin >> n;
	REP( i, 0, n )
	{
		Point a, b, c, d;
		cin >> a >> b >> c >> d;
		cout << ( eq( cross( a - b, c - d ), 0 ) ? "YES" : "NO" ) << endl;
	}

	return 0;
}

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

int main()
{
	const double EPS = 1e-30;
	const double PI = 4.*atan(1.);

	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;
		complex<double> A(x1,y1),B(x2,y2),C(x3,y3),D(x4,y4);
		double a = arg((A-B)/(C-D));
		cout << ( abs(a)<EPS || abs(a)>PI-EPS ? "YES" : "NO" ) << endl;
		//cout << ( abs(a)==0 || abs(a)==PI ? "YES" : "NO" ) << endl;
	}
	return 0;
}

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

int main(){
	int n;
	cin >> n;
	for(int i = 0; i < n; i++){
		float x[4], y[4];
		for(int j = 0; j < 4; j++){
			cin >> x[j] >> y[j];
		}
		float ab = 999999, cd = 999999;
		if(x[0] != x[1]) ab = (y[0]-y[1]) / (x[0]-x[1]);
		if(x[2] != x[3]) cd = (y[2]-y[3]) / (x[2]-x[3]);
		if(ab == cd) cout << "YES"<< endl;
		else cout << "NO" << endl;
	}
	return 0;
}

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;
const double precision = 1e-10;

bool EQ(double d, double e)
{
  return abs(d - e) < precision;
}

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

bool parallel(P a, P b)
{
  return EQ(cross(a, b), 0.0);
}

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

int main(void) {
  int n;
  cin >> n;
  while (n--) {
    double x1, y1, x2, y2;
    double ang1, ang2;
    cin >> x1 >> y1 >> x2 >> y2;
    ang1 = (y2 - y1) / (x2 - x1);
    cin >> x1 >> y1 >> x2 >> y2;
    ang2 = (y2 - y1) / (x2 - x1);

    if (ang1 == ang2) {
      cout << "YES" << endl;
    } else {
      cout << "NO" << endl;
    }
  }

  return 0;
}

C++: #include <algorithm>
#include <iostream>
#include <cstdio>
#include <map>
#include <numeric>
#include <set>
#include <sstream>
#include <string>
#include <vector>
#include <queue>
#include <string.h>
#include <cmath>
using namespace std;

#define ISEQ(c) (c).begin(), (c).end()
typedef long long ll;

int main() {
  int n;
  scanf("%d", &n);
  double x[4], y[4];
  const double EPS = 1e-12;
  for (int z = 0; z < n; z++) {
    for (int i = 0; i < 4; i++) {
      scanf("%lf%lf", &x[i], &y[i]);
    }
    printf("%s\n", abs((y[1]-y[0])*(x[3]-x[2])-(y[3]-y[2])*(x[1]-x[0])) < EPS ? "YES" : "NO");
  }
}

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

int main()
{
    pair<double,double>a[4];
    for(int i=0;i<4;i++){
        cin>>a[i].first>>a[i].second;
    }
    if((a[1].second-a[0].second)/(a[1].first-a[0].first)!=0)
        cout<<"NO"<<endl;
    else cout<<"YES"<<endl;
    return 0;
}