【java】第4章数学函数&字符串测试题（作业部分）

清疚 2022-11-10 14:20 166阅读 0赞

### 文章目录 ###

*   *  4.6
     *  4.11
     *  4.16
     *  4.21
     *  4.22
     *  4.24
     *  4.25

## 4.6 ##

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzQ1ODc4Mjky_size_16_color_FFFFFF_t_70]  
**【注】**  
**1、用到的Math类中的方法**  
`Math.toRadians()` 把角度转化成弧度  
`Math.toDegrees()`把弧度转化成角度  
`Math.cos()` 求余弦  
`Math.cos()` 求正弦  
`Math.acos()` 求反余弦  
`Math.PI` 派的数值  
`Math.sqrt()` 求平方根

**2、输出浮点数保留几位小数**

System.out.format("PI is usually used as %.2f ", Math.PI);

> PI is usually used as 3.14

**3、NaN**  
是 Not a Number 的缩写，用于处理计算中出现的错误情况，比如 0.0 除以 0.0 或者求负数的平方根

package wwr;
    //4.6
    import java.util.Scanner;
    public class Demo {
        
    
    	public static void main(String[] args) {
        
    		//求3个以弧度为单位的随机角度（类似极坐标）
    //		double t1 = Math.random() * 2 * (Math.PI);
    //		double t2 = Math.random() * 2 * (Math.PI);
    //		double t3 = Math.random() * 2 * (Math.PI);
    		//测试以原点为中心的等边三角形
    		double t1 = Math.toRadians(90);
    		double t2 = Math.toRadians(210);
    		double t3 = Math.toRadians(330);
    		//求3个点的横纵坐标
    		double x1 = 40 * Math.cos(t1);
    		double x2 = 40 * Math.cos(t2);
    		double x3 = 40 * Math.cos(t3);
    		double y1 = 40 * Math.sin(t1);
    		double y2 = 40 * Math.sin(t2);
    		double y3 = 40 * Math.sin(t3);
    		//算两点距离，求3条边的长度
    		double a = Math.sqrt((x1 - x2) * (x1 - x2) + (y1 - y2) * (y1 - y2));
    		double b = Math.sqrt((x3 - x2) * (x3 - x2) + (y3 - y2) * (y3 - y2));
    		double c = Math.sqrt((x1 - x3) * (x1 - x3) + (y1 - y3) * (y1 - y3));
    		//用余弦定理，求3个角度
    		double j1 = Math.acos((a * a - b * b - c * c) / ((-2) * b * c));
    		double j2 = Math.acos((b * b - a * a - c * c) / ((-2) * a * c));
    		double j3 = Math.acos((c * c - b * b - a * a) / ((-2) * b * a));
    		System.out.format("The three pionts are: (%.2f, %.2f) (%.2f, %.2f) (%.2f, %.2f)\n"
    				, x1, y1, x2, y2, x3, y3);
    		System.out.format("The three angles are: %.2f° %.2f° %.2f°"
    				, Math.toDegrees(j1), Math.toDegrees(j2), Math.toDegrees(j3));
    	}
    
    }

## 4.11 ##

![在这里插入图片描述][20210318195715503.png]  
【注】java数组用法可以和c一样写

package wwr;
    //4.11
    import java.util.Scanner;
    public class Demo {
        
    
    	public static void main(String[] args) {
        
    		Scanner input = new Scanner(System.in);
    		char DecToHex[] = {
        '0', '1', '2', '3', '4', '5', '6', '7', '8', '9', 
    				'A', 'B', 'C', 'D', 'E', 'F'};
    		System.out.print("Enter a decimal value (0 to 15): ");
    		int num = input.nextInt();
    		if(num >= 0 && num <= 15)
    			System.out.print("The hex value is " + DecToHex[num]);
    		else
    			System.out.print(num + " is an invalid input");
    	}
    
    }

## 4.16 ##

![在这里插入图片描述][20210318195815273.png]  
【注】若求\[n, m\]之间的随机数，则  
`int a = (int)(Math.random() * (m - n + 1) + n)`

package wwr;
    //4.16
    import java.util.Scanner;
    public class Demo {
        
    
    	public static void main(String[] args) {
        
    		System.out.print((char)(Math.random() * ('Z' - 'A' + 1) + 'A'));
    	}
    
    }

## 4.21 ##

![在这里插入图片描述][20210318195821239.png]  
【注】  
1、java 的 for 循环跟C一样写  
2、java 的 bool 型是 boolean，且只能写 true、false，不能用1、0代替  
3、String 类型输入用 `String s = input.nextLine();` 读入一行

package wwr;
    //4.21
    import java.util.Scanner;
    public class Demo {
        
    
    	public static void main(String[] args) {
        
    		Scanner input = new Scanner(System.in);
    		System.out.print("Enter a SSN: ");
    		String s = input.nextLine();
    		boolean flag = true;
    		if(s.length() != 11)
    			flag = false;
    		else
    		{
        
    			for(int i = 0; i < s.length(); i++)
    			{
        
    				if(i == 3 || i == 6)
    				{
        
    					if(s.charAt(i) != '-')
    						flag = false;
    				}
    				else
    					if(s.charAt(i) > '9' || s.charAt(i) < '0')
    						flag = false;
    			}
    		}
    		if(flag)
    			System.out.print(s + " is a valid social security number");
    		else
    			System.out.print(s + " is an invalid social security number");
    	}
    
    }
    /*
    232-23-5435
    23-23-5435
    */

## 4.22 ##

![在这里插入图片描述][20210318195827285.png]  
【注】`s1.contains(s2)`判断 s2 是否为 s1 的字串

package wwr;
    //4.22
    import java.util.Scanner;
    public class Demo {
        
    
    	public static void main(String[] args) {
        
    		Scanner input = new Scanner(System.in);
    		System.out.print("Enter string s1: ");
    		String s1 = input.nextLine();
    		System.out.print("Enter string s2: ");
    		String s2 = input.nextLine();
    		if(s1.contains(s2))
    			System.out.print(s2 + " is a substring of " + s1);
    		else
    			System.out.print(s2 + " is not a substring of " + s1);
    	}
    
    }

## 4.24 ##

![在这里插入图片描述][2021031819583391.png]  
【注】`s1.compareTo(s2)`返回从左往右第一个不相同的字母的字母序之差（s1 - s2）  
(感觉方法有点儿蠢

package wwr;
    //4.24
    import java.util.Scanner;
    public class Demo {
        
    
    	public static void main(String[] args) {
        
    		Scanner input = new Scanner(System.in);
    		System.out.print("Enter the first city: ");
    		String s1 = input.nextLine();
    		System.out.print("Enter the second city: ");
    		String s2 = input.nextLine();
    		System.out.print("Enter the third city: ");
    		String s3 = input.nextLine();
    		System.out.print("The three cities in alphabetical order are " );
    		if(s1.compareTo(s2) < 0)
    		{
        
    			if(s2.compareTo(s3) < 0)
    				System.out.print(s1 + " " +  s2 + " " + s3);
    			else 
    			{
        
    				if(s1.compareTo(s3) < 0)
    					System.out.print(s1 + " " + s3 + " " + s2);
    				else
    					System.out.print(s3 + " " + s1 + " " + s2);
    			}
    		}
    		else //s2 < s1
    		{
        
    			if(s1.compareTo(s3) < 0)
    				System.out.print(s2 + " " + s1 + " " + s3);
    			else 
    			{
        
    				if(s2.compareTo(s3) < 0)
    					System.out.print(s2 + " " + s3 + " " + s1);
    				else
    					System.out.print(s3 + " " +s2 + " " + s1);
    			}
    		}
    	}
    
    }
    /*
    1 2 3*
    1 3 2*
    2 1 3
    2 3 1
    3 1 2*
    3 2 1
    */

## 4.25 ##

![在这里插入图片描述][20210318195838963.png]

package wwr;
    //4.25
    import java.util.Scanner;
    public class Demo {
        
    
    	public static void main(String[] args) {
        
    		for(int i = 0; i < 3; i++ )
    			System.out.print((char)(Math.random() * ('Z' - 'A' + 1) + 'A'));
    		for(int i = 0; i < 4; i++ )
    			System.out.print((int)(Math.random() * 10));
    	}
    
    }

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[20210318195715503.png]: https://img-blog.csdnimg.cn/20210318195715503.png
[20210318195815273.png]: https://img-blog.csdnimg.cn/20210318195815273.png
[20210318195821239.png]: https://img-blog.csdnimg.cn/20210318195821239.png
[20210318195827285.png]: https://img-blog.csdnimg.cn/20210318195827285.png
[2021031819583391.png]: https://img-blog.csdnimg.cn/2021031819583391.png
[20210318195838963.png]: https://img-blog.csdnimg.cn/20210318195838963.png