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++: #define _CRT_SECURE_NO_WARNINGS
#include <iostream>
#include <cstdio>
#include <cstring>
#include <ctime>
#include <string>
#include <vector>
#include <algorithm>
#include <cmath>
#include <sstream>
#include <utility>
using namespace std;
typedef long long ll;
typedef vector<int> vi;
typedef vector<vi> vvi;
typedef pair<int,int> pii;
#define all(c) (c).begin(), (c).end()
#define loop(i,a,b) for(ll i=a; i<ll(b); i++)
#define rep(i,b) loop(i,0,b)
#define each(e,c) for(auto&e:c)
#define pb push_back
#define eb emplace_back
#define mp make_pair
#define mt make_tuple
#define lb lower_bound
#define ub upper_bound
#ifdef DEBUG
#define dump(...) (cerr<<#__VA_ARGS__<<" = "<<(DUMP(),__VA_ARGS__).str()<<" ["<<__LINE__<<"]"<<endl)
struct DUMP:ostringstream{template<class T>DUMP &operator,(const T&t){if(this->tellp())*this<<", ";*this<<t;return *this;}};
#else
#define dump(...)
#endif

int main(){
    int n; cin>>n;
    rep(_,n){
        ll x[2][2], y[2][2];
        rep(i,2){
            rep(j,2){
                double x_,y_;
                cin >> x_ >> y_;
                x[i][j] = (x_ + 1e-10)*100000;
                y[i][j] = (y_ + 1e-10)*100000;
            }
        }
        ll dx1=x[0][0]-x[0][1], dy1=y[0][0]-y[0][1];
        ll dx2=x[1][0]-x[1][1], dy2=y[1][0]-y[1][1];
        dump(dx1,dx2,dy1,dy2);
        puts(dx1*dy2==dx2*dy1 ? "YES" : "NO");
    }
}

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

int main(){
    int n;
    for (int i = 0; i < n; i++){
	double x[5];
	double y[5];
	
	for (int i = 1; i <= 4; i++) cin >> x[i] >> y[i];
	
	if (x[1] > x[2]){
	    swap(x[1], x[2]);
	    swap(y[1], y[2]);
	}
	if (x[3] > x[4]){
	    swap(x[3], x[4]);
	    swap(y[3], y[4]);
	}
	if ((x[2] - x[1]) / (y[2] - y[1]) == (x[4] - x[3]) / (y[4] - y[3]))
	    cout << "YES" << endl;
	else cout << "NO" << endl;
    }
}

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

using namespace std;

int main() {
	int k;
	double x1,x2,x3,x4,y1,y2,y3,y4;
	double katamuki1;
	double katamuki2;
	cin >> k;

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

		katamuki1 = (y2 - y1) / (x2 - x1);
		katamuki2 = (y4 - y3) / (x4 - x3);

		if (katamuki1 == katamuki2) {
			/*if(((y4 - y1) / (x4 - x1)) == ((y3 - y2) / (x3 - x2)) && ((y3 - y1) / (x3 - x1)) == ((y4 - y2) / (x4 - x2))) {
				cout << "NO" << endl;
			} else {*/
				cout << "YES" << endl;
			//}
		} else {
			cout << "NO" << endl;
		}
	}
	return 0;
}

C++: 
#include <iostream>
#include <stdio.h>
#include <fstream>
#include <algorithm>
#include <string>
#include <list>
#include <map>
#include <set>
#include <queue>
#include <stack>
#include <vector>
#include <limits.h>
#include <math.h>
#define rep(i,n) for (int i = 0; (i) < (n); ++ (i))
using namespace std;

int main(void){
    int n;
    double x[4],y[4],a,b,c,d,det;
    cin >> n;
    rep(j,n) {

    
    //入力を多少簡略化
    for(int i=0;i<4;i++) {
        cin >> x[i];
        cin >> y[i];
    }
    //(x1,y1)(x2,y2)を結ぶax+by=p の係数
    a = y[0]-y[1];  b = x[1]-x[0];
    //(x3,y3)(x4,y4)を結ぶcx+dy=q の係数
    c = y[2]-y[3];  d = x[3]-x[2];
    //係数についての行列式
    det = a*d-c*b;
    if(fabs(det) < 0.00001) cout << "YES" << endl;
    else cout << "NO" << endl;
    }
}
#define rep(i,n) for (int i = 0; (i) < (n); ++ (i))
using namespace std;

int main(void){
    int n;
    double x[4],y[4],a,b,c,d,det;
    cin >> n;
    rep(j,n) {

    
    //入力を多少簡略化
    for(int i=0;i<4;i++) {
        cin >> x[i];
        cin >> y[i];
    }
    //(x1,y1)(x2,y2)を結ぶax+by=p の係数
    a = y[0]-y[1];  b = x[1]-x[0];
    //(x3,y3)(x4,y4)を結ぶcx+dy=q の係数
    c = y[2]-y[3];  d = x[3]-x[2];
    //係数についての行列式
    det = a*d-c*b;
    if(absf(det) < 0.00001) cout << "YES" << endl;
    else cout << "NO" << endl;
    }
}

C++:  
 #include<iostream>
 #include<stdio.h>
 #include<string>
 #include<math.h>
 #include<iomanip>
 #include<algorithm>
 #include<string.h>
 #include<cctype>
 #include<map>
 #include<set>
 #include<vector>
 #include<sstream>
 #include<stack>
 
 using namespace std;
 
 double abs(double x)
 {
   if(x<0) return -x;
   else return x;       
 }
  
 int main()
 {
   double x1,y1,x2,y2,x3,y3,x4,y4;
   int n;
   cin>>n;
   while(n--)
   {
     cin>>x1>>y1>>x2>>y2>>x3>>y3>>x4>>y4;
     if(x2==x1&&x3==x4) cout<<"YES"<<endl;
     else 
     {
       double k1=(y2-y1)/(x2-x1);
       double k2=(y4-y3)/(x4-x3);
       if(abs(k1-k2)<0.000001) cout<<"YES"<<endl;
       else cout<<"NO"<<endl;     
     }          
   }
   //while(1);      
   return 0;    
 }

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

int main(){
    int n;
    double x1,y1,x2,y2,x3,y3,x4,y4,ll,lll;
    cin >> n;
    for(int i=0;i<n;i++){
        cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3 >> x4 >> y4;
        ll = (y2-y1)/(x2-x1);
        lll = (y4-y3)/(x4-x3);
        cout << ((ll == lll)? "YES" : "NO") << endl;
    }
}

C++: #include<iostream>
using namespace std;
int main(){
  int n;
  double x[4],y[4];
  cin>>n;
  for(int i=0;i<n;i++){
    for(int j=0;j<4;j++)
      cin>>x[j]>>y[j];
    if((x[3]-x[2])==0&&(x[1]-x[0])==0) cout<<"YES"<<endl;
    else if((x[3]-x[2])==0||(x[1]-x[0])==0) cout<<"NO"<<endl;
    else if((y[3]-y[2])/(x[3]-x[2])==(y[1]-y[0])/(x[1]-x[0])) cout<<"YES"<<endl;
    else cout<<"NO"<<endl;
  }
  return 0;
}

C++: #include<iostream>
using namespace std;
int main(){
    int n;
    double x1,x2,x3,x4,y1,y2,y3,y4;
    cin>>n;
    for(int i=0;i<n;i++){
        cin>>x1>>y1>>x2>>y2>>x3>>y3>>x4>>y4;

        double line1,line2;
        line1=(y2-y1)/(x2-x1);
        line2=(y4-y3)/(x4-x3);
        (line1==line2)? cout<<"YES"<<endl : cout<<"NO"<<endl;
    }
    return 0;
}

C++: #include<bits/stdc++.h>
#define loop(i,a,b) for(int i=a;i<b;i++)
#define rep(i,a) loop(i,0,a)
using namespace std;

struct vec{
	double x;
	double y;
};

int main(){
	int n;
	cin>>n;
	double x[4],y[4];//0:A 1:B 2:C 3:D
	rep(i,n){
		rep(j,4)cin>>x[j]>>y[j];
		vec ABvec;
		vec CDvec;
		ABvec.x=x[1]-x[0];
		ABvec.y=y[1]-y[0];
		CDvec.x=x[3]-x[2];
		CDvec.y=y[3]-y[2];
		double elmeet=abs(ABvec.x*CDvec.x+ABvec.y*CDvec.y);
		double ABnolme=sqrt(pow(ABvec.x,2)+pow(ABvec.y,2));
		double CDnolme=sqrt(pow(CDvec.x,2)+pow(CDvec.y,2));
		if(abs(ABnolme*CDnolme-elmeet)<10e-7)cout<<"YES"<<endl;
		else cout<<"NO"<<endl;
	}
	return 0;
}

C++: #include<iostream>
#include <string>
#include<stdio.h>

using namespace std;

int _tmain(int argc, _TCHAR* argv[])
{
	double x1[100], y1[100], x2[100], y2[100], x3[100], y3[100], x4[100], y4[100];
	double tendAB, tendCD;
	int num;
	int pos = 0;

	cin >> num;
	while (num--) {
		cin >> x1[pos] >> y1[pos] >> x2[pos] >> y2[pos] >> x3[pos] >> y3[pos] >> x4[pos] >> y4[pos];
		pos++;
	}
	for (int i = 0; i < pos; i++) {
		tendAB = (y1[i] == y2[1]) ? 0 : (x2[i] - x1[i]) / (y2[i] - y1[i]);
		tendCD = (y3[i] == y4[1]) ? 0 : (x4[i] - x3[i]) / (y4[i] - y3[i]);
		if (tendAB == tendCD) cout << "YES" << endl; else cout << "NO" << endl;
	}

	return 0;
}