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 <bits/stdc++.h>
using namespace std;
#define FOR(i, s, n) for(int i = s; i < (int)n; i++)
#define per(i, n) for(int i = n; i >= 0; i--)
#define ROF(i, s, n) for(int i = s; i >= (int)n; i--)
#define FORIT(i, A) for (auto i : A)
#define PRINT(x) cout << (x) << "\n"
#define ALL(a) (a).begin(),(a).end()
#define RALL(a) (a).rbegin(), (a).rend()
#define MP make_pair
#define EACH(i, n) for (__typeof((n).begin()) i = (n).begin(); i != (n).end(); ++i)
#define SZ(a) int((a).size())
#define EXIST(s,e) ((s).find(e)!=(s).end())
#define SORT(c) sort((c).begin(),(c).end())
#define CLR(a) memset((a), 0 ,sizeof(a))
#define dump(x) cout << #x << " = " << (x) << "\n";
#define debug(x) cout << #x << " = " << (x) << " (L" << __LINE__ << ")" << " " << __FILE__ << "\n";
#define sq(n) (n) * (n)
typedef vector<int> VI;
typedef vector<VI> VVI;
typedef vector<string> VS;
typedef pair<int, int> PII;
typedef long long LL;
typedef vector<LL> VLL;
typedef vector<VLL> VVLL;
typedef unsigned int uint;
typedef unsigned long long ull;
typedef priority_queue<int> maxpq;
typedef priority_queue< int, vector<int>, greater<int> > minpq;
static const double EPS = 1e-10;
static const double PI = acos( -1.0 );
static const int mod = 10007;
static const int INF = 1 << 25;
static const LL LL_INF = 1152921504606846976;
static const int dx[] = { -1, 0, 1, 0,  1, -1,  1, -1 };
static const int dy[] = { 0, -1, 0, 1 , 1,  1, -1, -1 };

int n;
double x1, x2, x3, x4, y, y2, y3, y4;

int main() {
	scanf( "%d", &n );
	for ( int i = 0; i < n; i++ ) {
		scanf( "%lf %lf %lf %lf %lf %lf %lf %lf", &x1, &y, &x2, &y2, &x3, &y3, &x4, &y4 );
		if ( ( x2 - x1 == x4 - x3 && y2 - y == y4 - y3 ) || ( x2 - x1 == x3 - x4 && y2 - y == y3 - y4 ) ) {
			printf( "YES\n" );
		}
		else {
			printf( "NO\n" );
		}
	}
	return 0;
}

C++: #define _USE_MATH_DEFINES
#include <iostream>
#include <queue>
#include <vector>
#include <math.h>
#include <algorithm>
#include <iomanip>
#include <string>
#include <stack>
#include <cctype>
#include <cstdlib>
using namespace std;

#define INF (1 << 20)
#define EPS (1e-10)
#define EQ(a,b) (fabs((a)-(b)) < EPS)

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

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

int main(){
	double N,xx,yy;
	long long X[5],Y[5];
	string res[110];
	cin>>N;
	for(int j=1;j<=N;j++){
		for(int i=1;i<=4;i++){
			cin>>xx>>yy;
			X[i]=(int)(xx*100000.0);
			Y[i]=(int)(yy*100000.0);
		}
		if((X[2]-X[1])*(Y[4]-Y[3])==(X[4]-X[3])*(Y[2]-Y[1])){
			res[j]="YES";
		}
		else{
			res[j]="NO";
		}
	}
	for(int i=1;i<=N;i++){
		cout<<res[i]<<endl;
	}
	return 0;
}

C++: #include<iostream>
using namespace std;
int main(){
  double a[2],b[2],c[2],d[2];
  int n;
  cin >> n;
  for(int i=0;i<n;i++){
    cin >> a[0] >> a[1] >> b[0] >> b[1] >> c[0] >> c[1] >> d[0] >> d[1];
    if(a[0]-b[0]==0&&c[0]-d[0]==0) cout << "YES" << endl;
    else if(a[1]-b[1]==0&&c[1]-d[1]==0) cout << "YES" << endl;
    if(a[0]-b[0]!=0&&c[0]-d[0]!=0){
      bool ok = false;
      if((a[1]-b[1])/(a[0]-b[0])==(c[1]-d[1])/(c[0]-d[0])){
	cout << "YES" << endl;
	ok = true;
      }
      if(!ok) cout << "NO" << endl;
    }
    else cout << "NO" << endl;
  }   
  return 0;
}

C++: #include <cstdio>
#include <cstdlib>
#include <iostream>
#include <string>
#include <cmath>
#include <algorithm>
#include <vector>
#include <queue>
#include <stack>

using namespace std;
typedef long long ll;

#define PI 3.141592653

int main(){
    int n;
    double x_1,y_1,x_2,y_2,x_3,y_3,x_4,y_4,slope_1,slope_2;
    scanf("%d",&n);
    
    for(int i=0;i<n;i++){
        scanf("%lf%lf%lf%lf%lf%lf%lf%lf",&x_1,&y_1,&x_2,&y_2,&x_3,&y_3,&x_4,&y_4);
        
        slope_1=-(y_1-y_2)/(x_1-x_2);
        slope_2=-(y_3-y_4)/(x_3-x_4);
        
        if(abs(slope_1-slope_2)<1e-7)
            puts("YES");
        else
            puts("NO");
    }
    
    return 0;
}

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

double calc(double dx1, double dy1, double dx2, double dy2) {
    dx1 = (dx1 == -0.0? 0.0: dx1);
    dy1 = (dy1 == -0.0? 0.0: dy1);
    dx2 = (dx2 == -0.0? 0.0: dx2);
    dy2 = (dy2 == -0.0? 0.0: dy2);
    if(dx1 == 0.0 || dx2 == 0.0) {
        return dx1 == dx2 || (dx1 == 0.0 && dy1 == 0.0) || (dx2 == 0.0 && dy2 == 0.0);
    }
    return (dy1/dx1) == (dy2/dx2);
}

int main() {
    int n, i;
    double x1, y1, x2, y2, x3, y3, x4, y4;

    scanf("%d", &n);
    for(i=0; i<n; ++i) {
        scanf("%lf %lf %lf %lf %lf %lf %lf %lf",
              &x1, &y1, &x2, &y2, &x3, &y3, &x4, &y4);
        if(calc(x2-x1, y2-y1, x4-x3, y4-y3)) {
            printf("YES\n");
        }else {
            printf("NO\n");
        }
    }

    return 0;
}

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

double cross(complex<double> z1, complex<double> z2) {
    return (z1.real()*z2.imag() - z1.imag()*z2.real());
}
 
int main(int argc, char** argv) {
    static const double EPS = 1e-10;
    int n;
    cin >> n;
    for(int i=0; i<n; i++) {
        double x[4], y[4];
        for(int j=0; j<4; j++) {
            cin >> x[j] >> y[j];
        }
        complex<double> ab(x[1]-x[0], y[1]-y[0]);
        complex<double> cd(x[3]-x[2], y[3]-y[2]);
        if(abs(cross(ab, cd)) < EPS) { // 外積が0なら平行
            cout << "YES" << endl;
        }else {
            cout << "NO" << endl;
        }
    }
}

C++: // Parallelism
// http://judge.u-aizu.ac.jp/onlinejudge/description.jsp?id=0021&lang=jp
// 
// 

#include <iostream>
#include <complex>
#include <cmath>
#include <cstdio>

using namespace std;
typedef std::complex<double> xy_t;

int N;
double x[4], y[4];
const double eps = 1e-11;

double dot_product(xy_t a, xy_t b) { return (conj(a)*b).real(); }

double cross_product(xy_t a, xy_t b) { return (conj(a)*b).imag(); }

xy_t projection(xy_t p, xy_t b) { return b*dot_product(p,b)/norm(b); }

int main(){
    cin >> N;
    for(int i = 0; i < N; ++i){
        for(int i = 0; i < 4; ++i){
            cin >> x[i] >> y[i];
        }

    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])
    };

    bool p = abs(cross_product(a[0],a[1])) < eps;
    
    cout << (p ? "YES" : "NO") << endl;
    }
}

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;
    cin >> n;
    rep(j,n) {

    double x[4],y[4],a,b,c,d,p,q,det,ansx,ansy;
    //入力を多少簡略化
    for(int i=0;i<4;i++) {
        scanf("%lf" ,&x[i]);
        scanf("%lf" ,&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(det == 0) puts("YES");
    else puts("NO");
    }
}