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")
			continue
		}
		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 <iostream>
#include <complex.h>
#include <math.h>
using namespace std;

const double eps = pow(10, -11);
double x[4], y[4];
int N;
typedef complex<double> xy_t;

double cross_product(xy_t a, xy_t b) { return (conj(a)*b).imag();}
int main() {
  cin >> N; //問題数
  for (int t=0; t<4; ++t) {
    for (int i=0; i<4; ++i)
	  cin >> x[i] >> y[i]; //x0,y0..x3,y3
	xy_t a[2] = {
	  xy_t(x[0], y[0]) - xy_t(x[1],y[1]),
	  xy_t(x[2], y[2]) - xy_t(x[3],y[3])
	  };
	  double s = cross_product(a[0] , a[1])/2;
	  bool p = abs(s) < eps;
	  cout << (p ? "YES" : "NO") << endl;
	  }
	 }

C++: #include<iostream>
#include<cmath>
#include<cassert>
#include<cstdlib>
using namespace std;
#define EPS 0.0000000001


main(){
  int tcase;   cin >> tcase;
  double x1, y1, x2, y2, x3, y3, x4, y4;
  for ( int i = 0; i < tcase; i++ ){
      cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3 >> x4 >> y4;

  
      if ( abs((y2 - y1)*(x4 - x3) - (y4 - y3)*(x2 - x1) ) < EPS ) cout << "YES" << endl;
      else cout << "NO" << endl;
  }
}

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

#define PRINT(STR) cout << STR << endl;
#define REP(i, n) for (int (i) = 0; (i) < (int)(n); (i)++)
#define REG(i, a, b) for (int (i) = ((int)(a)); (i) < ((int)(b)); i++)
#define ALL(V) (V).begin(), (V).end()

typedef long long ll;

using namespace std;

int nextInt() { int n = -1; cin >> n; return n; }
double nextDouble() { double d = -1; cin >> d; return d; }
ll nextll() { ll n = -1; cin >> n; return n; }
string nextString() { string str; cin >> str; return str; }

int INF = 9999999;

// BEGIN //////////////////////////////////////////////////////////////


int main() {
	int N = nextInt();
	double x1, y1, x2, y2, x3, y3, x4, y4;
	REP(i, N) {
		cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3 >> x4 >> y4;

		if (x2 - x1 == 0 || x4 - x3 == 0) {
			if (y2 - y1 == 0 && y4 - y3 == 0) {
				PRINT("YES");
			} else {
				PRINT("NO");
			}
		} else {
			if ((y2 - y1) / (x2 - x1) - (y4 - y3) / (x4 - x3) < 1e-8) {
				PRINT("YES");
			} else {
				PRINT("NO");
			}
		}
	}

	return 0;
}

// END   //////////////////////////////////////////////////////////////

C++: #include <stdio.h>
#include<cmath>
using namespace std;

int main(){
	double x1,x2,x3,x4,y1,y2,y3,y4;
	int n;
	scanf("%d",&n);
	for(int i=0; i<n; i++){
		scanf("%lf %lf %lf %lf %lf %lf %lf %lf",&x1,&y1,&x2,&y2,&x3,&y3,&x4,&y4);

		if(abs((y2 - y1)*(x4 - x3) - (y4 - y3)*(x2 - x1) ) < 0.000000001){ //I copied yutaka watabe's code here
			printf("YES\n");
		}else{
			printf("NO\n");
		}

	}
    return 0;
}

C++: #include<iostream>
#include<string>
#include<algorithm>
#include<map>
#include<vector>
#include<cmath>
#include<cstdio>
#define loop(i,a,b) for(int i=a;i<b;i++) 
#define rep(i,a) loop(i,0,a)
#define pb(in,tmp) in.push_back(tmp)
#define all(in) in.begin(),in.end()
const double PI=acos(-1);
using namespace std;
int main(){
  int n;
  cin>>n;
  rep(i,n){
    double ax,ay,bx,by,cx,cy,dx,dy;
    cin>>ax>>ay>>bx>>by>>cx>>cy>>dx>>dy;

    if((ax-bx)*(cy-dy)==(cx-dx)*(ay-by))cout<<"YES"<<endl;
    else cout<<"NO"<<endl;
  }
  return 0;
}

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

int main(){
	int t;
	double x1, y1, x2, y2, x3, y3, x4, y4;
	scanf("%d", &t);
	while(t--){
		scanf("%lf%lf%lf%lf%lf%lf%lf%lf", &x1, &y1, &x2, &y2, &x3, &y3, &x4, &y4);
		printf("%s\n", ((x2 - x1) * (y4 - y3) == (x4 - x3) * (y2 - y1))? "YES" : "NO");
	}
	return 0;
}

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

int main(void){
    int n = 0;
    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(fabs((x1 - x2) * (y3 - y4) - (y1 - y2) * (x3 - x4)) < 0.0000000001){
            cout << "YES" << endl;
        }
        else{
            cout << "NO" << endl;
        }
    }
}

C++: #include <iostream>
#include <iomanip>
#include <cassert>
#include <algorithm>
#include <functional>
#include <vector>
#include <string>
#include <cstring>
#include <stack>
#include <queue>
#include <map>
#include <bitset>
#include <sstream>
#include <istream>
#include <cmath>
#include <cstdio>

using namespace std;

#define vci vector<int>
#define vcs vector<string>
#define pb push_back
#define sz size()
#define mapci map<char, int>
#define mapsi map<string, int>
#define all(x) x.begin(), x.end()

#define for_(i, a, b) for (int i=(int)a; i<(int)b; i++)
#define for_d(i, a, b) for (int i=(int)a-1; i>=b; i--)
#define for_r(i, a, b, c) for (int i=(int)a; i<(int)b; i += c)
#define for_dr(i, a, b, c) for (int i=(int)a-1; i>=b; i -= c)
#define FOR(i,c) for(__typeof((c).begin())i=(c).begin();i!=(c).end();++i)

#define vcitr vector<int>::iterator
#define vcsitr vector<string>::iterator
#define mapcitr map<char, int>::iterator
#define mapsitr map<string, int>::iterator

typedef long long ll;

const int iINF = 2147483647;
const ll lINF = 9223372036854775807;

template <class T> inline void dbg(T t) { cout << t << endl; }

struct Point2D { 
	double x, y;
	Point2D (double a=0, double b=0) { x=a; y=b; }
};

int main() {
	int n; cin >> n;
	Point2D a, b, c, d;
	
	for (int i=0; i<n; i++) {		
		cin >> a.x >> a.y >> b.x >> b.y >> c.x >> c.y >> d.x >> d.y;
		
		double p = (a.y-b.y)/(a.x-b.x), q = (c.y-d.y)/(c.x-d.x);
		
		if (p==q) cout << "YES" << endl;
		else cout << "NO" << endl;
	}
	
	return 0;
}

C++: #include <iostream>
#include <vector>
#include <algorithm>
#include <stack>
#include <string>
#include <map>
#include <functional>
#include <cmath>
#include <cstdio>
using namespace std;

int main(){
  double x1, y1, x2, y2, x3, y3, x4, y4;
  int n;
  cin >> n;
  for(int i = 0; i < n; i++){
    cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3 >> x4 >> y4;
    if(abs((y2 - y1) * (x4 - x3) - (y4 - y3) * (x2 - x1)) <= pow(10, -15)){
      cout << "YES" << endl;
    }else{
      cout << "NO" << endl;
    }
  }
}

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

int main(){
	double x1, y1, x2, y2, x3, y3, x4, y4;
	cin>>x1;
	while(cin>>x1>>y1>>x2>>y2>>x3>>y3>>x4>>y4){
		double x12 = x2-x1;
		double y12 = y2-y1;
		double x34 = x4-x3;
		double y34 = y4-y3;
		if(x12<0){
			x12 *= -1;
			y12 *= -1;
		}
		if(x34<0){
			x34 *= -1;
			y34 *= -1;
		}
		if(x12==0&&x34==0){
			cout<<"YES"<<endl;
		} else if(x12==0||x34==0){
			cout<<"NO"<<endl;
		} else {
			if(y12/x12==y34/x34){
				cout<<"YES"<<endl;
			} else {
				cout<<"NO"<<endl;
			}
		}
	}
	return 0;
}

C++: #include <stdio.h>
#include <cmath>


int main(){
	double x1,x2,x3,x4,y1,y2,y3,y4;
	int n;
	scanf("%d",&n);
	for(int i=0; i<n; i++){
		scanf("%lf %lf %lf %lf %lf %lf %lf %lf",&x1,&y1,&x2,&y2,&x3,&y3,&x4,&y4);

		if(abs((y2 - y1)*(x4 - x3) - (y4 - y3)*(x2 - x1) ) < 0.0000001){ //I copied yutaka watabe's code here
			printf("YES\n");
		}else{
			printf("NO\n");
		}

	}
    return 0;
}