1.保留几位小数

#include <iostream>

#include <iomanip> //必须包含这个头文件

using namespace std;

void main( )

{ double a =3.141596;

  cout<<fixed<<setprecision(3)<<a<<endl;  //输出小数点后3位

2. 使用了未初始化的局部变量

	Point* p ;//EORROR 使用了未初始化变量
    
    /*******************************Correct*****************************************/
    Point* p = new Point[n];//分配内存以存储变量

 Right-Code:

#include<iostream>
#include<iomanip>
#include<cmath>
using namespace std;

class Point {
	double x, y;
public:
	Point() { x = 0; y = 0; cout << "Constructor." << endl; }
	Point(double x_value, double y_value) {
		x = x_value; y = y_value;
	}
	double getx() { return x; }
	double gety() { return y; }
	void setx(double x1) { x = x1; }
	void sety(double y1) { y = y1; }
	void setxy(double x1, double y1) {
		x = x1; y = y1;
	}
	double getdistance( const Point& p) {
		return sqrt((p.x - x) * (p.x - x) + (p.y - y) * (p.y - y));
	}
	~Point() { cout << "Distructor." << endl; }
};
int main() {
	int t;
	int x, y ,n,beindex=0,foreindex=0;
	cin >> t;
	while (t--) {
		cin >> n;
		//double* dis = new double[n * (n - 1) / 2];//分配内存以存储距离
		Point* p = new Point[n];//分配内存以存储点
		int i=0,j=0,z=0;
		double maxdistance =0,h=0;
		for ( i = 0; i < n; i++) {
			cin >> x >> y;
			p[i].setxy(x, y);
		}
		
		for (z = 0; z < n; z++) {
			for (j = z + 1; j < n; j++) {
				h = p[z].getdistance(p[j]);
				
				if (h > maxdistance) {
					beindex = z;
					foreindex = j;
					maxdistance = h;
				}
				
			}
		}
		cout << "The longeset distance is " << fixed << setprecision(2) << maxdistance << ",between p[" << beindex << "] and p[" << foreindex << "]." << endl;
		//delete[]dis;//释放内存
		delete[]p;//释放内存
	}
	return 0;
}