Java学习第二章练习题

编写程序如下：

import java.util.Scanner;

public class Exe21 {

	public static void main(String[] args) {
		double c,t;//c摄氏温度
		
		System.out.println("Enter a degree in Celsius:");
		Scanner in = new Scanner(System.in);
	
		c = in.nextDouble();
		t = (9.0 / 5) * c + 32;
		System.out.println((int)c + "Celsius is " + t + "Fahrenheit");

	}

}

其中要注意的是9 / 5 == 1

2.2

编写程序如下：

import java.util.Scanner;

public class Day21 {

	public static void main(String[] args) {
		double r,h,s,v;//定义半径为r，高为h
		System.out.print("Enter the radius and length of a cylinder:");
		Scanner in = new Scanner(System.in);
		r=in.nextDouble();
		double p = 3.1415926;
		s = p*r*r;
		System.out.printf("The area is %.4f" , s);
		System.out.println ();
		
		h = in.nextDouble();
		v = s * h;
		System.out.printf("The volume is %.1f" , v);
		
	}

}

其中println是换行输出，print不换行，%.4f是保留小数点后四位。

编程如下：

import java.util.Scanner;

public class Day23 {

	public static void main(String[] args) {
		
		int num;//num 为这个数字
		int sum =  0;
		
		System.out.println("Enter a number between 0 and 1000:");
		Scanner in = new Scanner(System.in);
		num = in.nextInt();
		do {
			sum = sum + num % 10;
			num = num / 10;
			
		}while(num != 0);
		System.out.println("The  sum of digits is"+ sum);
		
			
	}

}

定义sum为0然后每次取其个位数字 也就是%10，然后再除掉这个个位数字，直到这个数为零时结束循环

编写程序如下：

import java.util.*;
		public class Day24 { 	public static void main(String[] args) {
			Scanner in = new Scanner(System.in);
			long total =	(int) System.currentTimeMillis();
			int change = in.nextInt();//读取时区
			long totalseconds = total/1000;
			//总秒数
			long totalminutes = totalseconds/60;
			//总分钟数	
			long seconds = totalseconds%60;
			//当前秒数
			long minutes = totalminutes%60;//当前分钟数
			long totalhours = totalminutes/60;//总小时数
			long hours = totalhours%24;//当前小时数	
			System.out.println(hours + ":"+minutes +":"+seconds);//输出			
		}
		
	}

编写程序如下：

public class Day25 {

	public static void main(String[] args) {
		int amount;//定义amount为每月存入的钱数
		System.out.print("Enter the monthly saving amount : ");//输入存入的钱数
		Scanner in = new Scanner(System.in);
		amount = in.nextInt();
		double one  = amount * (1 + 0.00417);
		double two =  (amount + one) * (1 + 0.00417);
		double three =  (amount + two) * (1 + 0.00417);
		double four =  (amount + three) * (1 + 0.00417);
		double five =  (amount + four) * (1 + 0.00417);
		double six =  (amount + five) * (1 + 0.00417);
		System.out.printf("After the sixth month,the account value is $%.2f", six);
	}

}

import java.util.Scanner;

public class Day26 {

	public static void main(String[] args) {
		double x1,y1,x2,y2;
		System.out.print("Enter x1 and y1 :");
		System.out.print("Enter x2 and y2 :");
		//输入x1 y1 x2 y2
		Scanner in = new Scanner(System.in);
		x1 = in.nextDouble();
		y1 = in.nextDouble();
		x2 = in.nextDouble();
		y2 = in.nextDouble();
		double a = Math.pow(x2 - x1, 2);//求x2 - x1 的平方
		double b = Math.pow(y2 - y1,2);//求y2 - y1的平方
		double dist = Math.pow(a - b, 0.5);
		System.out.print("The distance between the two points is : " + dist);
		
	}

}

2.7

其代码为：

import java.util.Scanner;

public class Day27 {

	public static void main(String[] args) {
		Scanner in = new Scanner(System.in);
		System.out.print("Enter three points for a tringle :");
		double x1,y1,x2,y2,x3,y3;
		x1 = in.nextDouble();
		y1 = in.nextDouble();
		x2 = in.nextDouble();
		y2 = in.nextDouble();
		x3 = in.nextDouble();
		y3 = in.nextDouble();
		double a = Math.pow((Math.pow((x1 - x2),2)) + (Math.pow((y1 - y2 ), 2)), 0.5);
		double b = Math.pow((Math.pow((x1 - x3),2)) + (Math.pow((y1 - y3 ), 2)), 0.5);
		double c = Math.pow((Math.pow((x3 - x2),2)) + (Math.pow((y3 - y2 ), 2)), 0.5);
		double s = (a + b + c) / 2;
		double k = s * (s - a) * (s - b)*(s - c);
		double area = Math.pow(k, 0.5);
		System.out.printf("The area of the triangle is %.1f", area);
	}

}