Go: package main

import (
        "fmt"
        "math/big"
)

func main() {
        var x1, x2, x3, x4, y1, y2, y3, y4 big.Float
        var n int
        fmt.Scanf("%d", &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)
                var lx, ly, mx, my, mul1, mul2, sub, zz big.Float
                lx.Sub(&x2, &x1)
                ly.Sub(&y2, &y1)
                mx.Sub(&x4, &x3)
                my.Sub(&y4, &y3)
                mul1.Mul(&lx, &my)
                mul2.Mul(&ly, &mx)
                sub.Sub(&mul1, &mul2)
                zz.SetFloat64(0.0)

                if sub.Cmp(&zz) == 0 {
                        fmt.Println("YES")
                } else {
                        fmt.Println("NO")
                }
        }
}

C++: #include <algorithm>
#include <cmath>
#include <climits>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <fstream>
#include <iostream>
#include <list>
#include <map>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <string>
#include <vector>
#include <cassert>
#include <functional>

using namespace std;

#define LOG(...) printf(__VA_ARGS__)
//#define LOG(...)
#define FOR(i,a,b) for(int i=(int)(a);i<(int)(b);++i)
#define REP(i,n) for(int i=0;i<(int)(n);++i)
#define ALL(a) (a).begin(),(a).end()
#define RALL(a) (a).rbegin(),(a).rend()
#define EXIST(s,e) ((s).find(e)!=(s).end())
#define SORT(c) sort((c).begin(),(c).end())
#define RSORT(c) sort((c).rbegin(),(c).rend())
#define CLR(a) memset((a), 0 ,sizeof(a))

typedef long long ll;
typedef unsigned long long ull;
typedef vector<bool> vb;
typedef vector<int> vi;
typedef vector<ll> vll;
typedef vector<vb> vvb;
typedef vector<vi> vvi;
typedef vector<vll> vvll;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;

const int dx[] = { -1, 0, 1, 0 }; const int dy[] = { 0, 1, 0, -1 };

int main() {
	int n;
	cin>>n;
	while (n--){
		vector<pair<double, double>> pos(4);
		REP(i, 4){
			cin >> pos[i].first >> pos[i].second;
		}

		if (abs((pos[1].first - pos[0].first)*(pos[3].second - pos[2].second) - (pos[1].second - pos[0].second)*(pos[3].first - pos[2].first))<=1e-10)
			cout << "YES"<<endl;
		else
			cout << "NO"<<endl;
		
	}
}

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

#define eps 1e-5

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;

        if( !((y2-y1)*(x4-x3)-(x2-x1)*(y4-y3)) ) cout << "YES\n";
        else cout << "NO\n";
    }

    return 0;
}

C++: #include <iostream>
#include <cmath>
using namespace std;
const long double EPS = 1e-9;
int main() {
	double x1, y1, x2, y2, x3, y3, x4, y4;
	int n;
	double veku1[2];
	double veku2[2];
	cin >> n;
	for (int i = 0; i < n; i++) {
		cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3 >> x4 >> y4;
		veku1[0] = x2 - x1;
		veku1[1] = y2 - y1;
		veku2[0] = x4 - x3;
		veku2[1] = y4 - y3;
		//cout<<fabs(veku1[0]*veku2[1]-veku1[1]*veku2[0])<<endl;
		if (fabs(veku1[0]*veku2[1]-veku1[1]*veku2[0]) == 0) {
			/*veku1[0] = x3 - x1;
			veku1[1] = y3 - y1;
			veku2[0] = x4 - x1;
			veku2[1] = y4 - y1;
			if (fabs(veku1[0]*veku2[1]-veku1[1]*veku2[0]) <= 0.0000001) {
				cout << "NO" << endl;
			} else {*/
				cout << "YES" << endl;
			//}
		} else {
			cout << "NO" << endl;
		}
	}
	return 0;
}

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

int main ()
{
	int n;
	cin>>n;
	for(int h=0;h<n;h++)
	{
		double x[4],y[4];
		for(int i=0;i<4;i++)cin>>x[i]>>y[i];
		if(x[0]==x[1]&&x[2]==x[3])
		{
			if(x[0]==x[1]||x[2]==x[3])
			{
				cout<<"NO"<<endl;
			}
			else
			{
				cout<<"YES"<<endl;
			}
		}
		else if(y[1]-y[0]==0&&y[3]-y[2]==0)
		{
			if(y[1]-y[0]==0||y[3]-y[2]==0)
			{
				cout<<"NO"<<endl;
			}
			else cout<<"YES"<<endl;
		}else
		{
			double Z,X;
			Z=(y[1]-y[0])/(x[1]-x[0]);
			X=(y[3]-y[2])/(x[3]-x[2]);
			if(Z==X)
			{
				cout<<"YES"<<endl;
			}
			else cout<<"NO"<<endl;
		
		}
	}
	
}

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;
}