计算圆的面积

• 输入圆的半径，计算圆的面积，输出结果

• 缺点是跟用户之间没有互动

package net.huawei.p02.t01;
public class task01 {
    public static void main(string[] args){
        double r,s;
        final double PI=3.1415926;
        r=4;
        s=PI*r*r;
        Syste,.out.println("s="+s);
    }
}

• 用户可以输入半径，也可以输入实数

package net.zhouwenqin.p02.t01;
import java.util.Scanner;
public class Task01_2{
    public static void main(String[] args){
        double r,s;
        final double PI = 3.1415926;
        Scanner sc = new Scanner(System.in);
        System.out.print("r = ");
        r = sc.netDouble();
        s = PI*r*r;
        System.out.println("s = " +s);
    }
}

• 输入整数，用sc.nextInt()

• 要求结果保留两位小数
• 回想一下在Python里怎么实现这个用户要求的，利用round（）函数可以搞定

•  采用遮尾法，数据没变，只是现实两位小数

package net.zhouwenqin.p02.t01;
import java.util.Scanner;
public class Task01_3{
    public static void main (String[] args){
        double r,s;
        final double PI = 3.1415926;
        Scanner sc = new Scanner (System.in);
        System.out.print("r = ");
        r = sc.nextDouble();
        s = PI*r*r;
        System.out.println("s = "+String.format("%.2f",s));
    }
}

• 采用剪尾法来处理

package net.zhouwenqin.p02.t01;
import java.util.Scanner;
public class Task01_3{
    public static void main(String[] args){
        double r,s;
        final double PI = 3.1415926;
        Scanner sc = new Scanner (System.in);
        System.out.print("r = ");
        r = sc.nextDouble();
        s = PI*r*r;
        s = (int)(s*100+0.5)/100.0;
        System.out.println("s = "+s);
    }
}