c++ day3（一）

系统管理员 2023-06-30 02:27 14阅读 0赞

### 文章目录 ###

*  大括号初始化器
 *  整型表示范围（上溢，下溢）
 *   *  示例1
     *  另一个很好展示上图那种圆圈的示例
     *  补一个C语言的整数溢出导致无限循环示例
 *  16进制，8进制
 *  常量类型如何确定
 *  char类型
 *  转义字符
 *  bool类型
 *  const
 *  浮点数
 *   *  float和double精度差异
     *  精度造成错误
 *  小测试

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzM2NjA3ODk0_size_16_color_FFFFFF_t_70] ![在这里插入图片描述][2020011412473693.png] ![在这里插入图片描述][20200114125049130.png]  
![在这里插入图片描述][20200115151726888.png]

#include<iostream>
    #include<climits>
    
    int main()
    { 
        using namespace std;
        short n_short = SHRT_MAX;
        int n_int = INT_MAX;
        long n_long = LONG_MAX;
        long long n_llong = LONG_LONG_MAX;
    
        cout << "length in Bytes: \n";
        cout << "short: " << sizeof(n_short) << endl;
        cout << "int: " << sizeof(n_int) << endl;
        cout << "long: " << sizeof(n_long) << endl;
        cout << "long long: " << sizeof(n_llong) << endl;
    
        cout << endl << "Maximum values: \n";
        cout << "short: " << n_short << endl;
        cout << "int: " << n_int << endl;
        cout << "long: " << n_long << endl;
        cout << "long long:" << n_llong << endl;
    
        cout << endl << "Minimum values: \n";
        cout << "short: " << SHRT_MIN << endl;
        cout << "int: " << INT_MIN << endl;
        cout << "long: " << LONG_MIN << endl;
        cout << "long long:" << LONG_LONG_MIN << endl;
    
        cout << endl << "bits per bytes: " << CHAR_BIT << endl;
    
        return 0;
    }

length in Bytes:
    short: 2
    int: 4
    long: 4
    long long: 8
    
    Maximum values:
    short: 32767
    int: 2147483647
    long: 2147483647
    long long:9223372036854775807
    
    Minimum values:
    short: -32768
    int: -2147483648
    long: -2147483648
    long long:-9223372036854775808
    
    bits per bytes: 8

![在这里插入图片描述][20200114130358333.png]![在这里插入图片描述][20200114130437745.png]![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzM2NjA3ODk0_size_16_color_FFFFFF_t_70 1]![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzM2NjA3ODk0_size_16_color_FFFFFF_t_70 2]![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzM2NjA3ODk0_size_16_color_FFFFFF_t_70 3]

原来\#define并不是很受欢迎

# 大括号初始化器 #

可以用大括号初始化数组和结构，也可以用于初始化单值变量，这时候可以不加等号。虽然看起来很奇怪，但是这种大括号初始化是c++11新增的，可以用于初始化任何类型，而在此之前，c++ 中的不同类型要用不同方法初始化，非常麻烦

#include<iostream>
    #include<climits>
    
    int main()
    { 
        using namespace std;
        int num1 = { 2};
        cout << num1 << endl;
    
        int num2{ 3};
        cout << num2 << endl;
    
        int num3{ };
        cout << num3 << endl;
    
        int num4 = { };
        cout << num4 << endl;
    
        return 0;
    }

2
    3
    0
    0

# 整型表示范围（上溢，下溢） #

这图不错，编程语言的类型的表示范围只是一个圈，一个闭环，不是一个数轴，所以你如果在重置点加1减1，得到的会是错误结果，其他地方都不会错  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzM2NjA3ODk0_size_16_color_FFFFFF_t_70 4]

## 示例1 ##

#include<iostream>
    #include<climits>
    #define ZERO 0
    int main()
    { 
        using namespace std;
    
        short ss = SHRT_MAX;
        unsigned short mm = ss;
        cout << "ss has " << ss << " dollars. \n";
        cout << "mm has " << mm << " dollars. \n";
        cout << "Add $1 to each account. \n";
        ss++ ;
        mm++;
        cout << "Now ss has " << ss << " dollars. \n";
        cout << "Now mm has " << mm << " dollars. \n";
        cout << "poor ss!\n";
        cout << endl;
    
        ss = ZERO;
        mm = ss;
        cout << "ss has " << ss << " dollars. \n";
        cout << "mm has " << mm << " dollars. \n";
        cout << "Take $1 from each account. \n";
        ss-- ;
        mm--;
        cout << "Now ss has " << ss << " dollars. \n";
        cout << "Now mm has " << mm << " dollars. \n";
        cout << "lucky mm!";
    }

ss has 32767 dollars.
    mm has 32767 dollars.
    Add $1 to each account.
    Now ss has -32768 dollars.
    Now mm has 32768 dollars.
    poor ss!
    
    ss has 0 dollars.
    mm has 0 dollars.
    Take $1 from each account.
    Now ss has -1 dollars.
    Now mm has 65535 dollars.
    lucky mm!

因为short占16位，那么有符号short只有15位用于表示数值大小，2的15次方是32768，所以正数部分是0 000000000000001到0 111111111111111，即1到32767，而负数是1 111111111111111 到1 000000000000001，即-1到-32768（由补码求原码：符号位不变，其他所有位取反再加1）。

现在ss是0 11111111111111111111（32767），加1变为 1 000000000000000, 转为原码还是1 000000000000000 ，表示-32768  
mm是0 11111111111111111111（32767），加1变为 1 000000000000000，由于第一位不是符号位，所以数值就是32768（ 2 1 5 2^15 215）

ss是0， 则为0 000000000000000， 减去1， 即加上-1（1 111111111111111），得1 111111111111111， 转为原码是1 000000000000001，即-1；  
mm是0， 则为0 000000000000000， 减去1， 即加上-1（1 111111111111111），得1 111111111111111，但是最高位不是符号位，所以这就是 2 1 6 − 1 = 65535 2^16-1=65535 216−1=65535。

## 另一个很好展示上图那种圆圈的示例 ##

// 整数溢出的示例
    #include <stdio.h>
    int main()
    { 
        int x = 2147483647; // 有符号int可以表示的最大值（32位）
        unsigned int y = 4294967295; // 无符号int可以表示的最大值（32位）
        printf("%d, %d, %d\n", x, x+1, x+2);
        // 无符号int用%d显示则被当做是有符号的，unsigned int y = 4294967295本来是32个1，结果最高位被当做符号位，成为-1
        printf("%d, %d, %d\n", y, y+1, y+2);
        printf("%u, %u, %u\n", y, y+1, y+2);
        return 0;
    }

2147483647, -2147483648, -2147483647
    -1, 0, 1
    4294967295, 0, 1

## 补一个C语言的整数溢出导致无限循环示例 ##

uint8\_t无符号8位整型，i=0,i-1=255,所以永远不会退出循环

#include <inttypes.h>
    #include <stdio.h>
    #include <inttypes.h>
    uint8_t i = 5;
    uint32_t count = 0;
    
    int main()
    { 
        while(i-->=0)
        { 
            count ++;
            printf("the number of while count is : %u\r\n",count);
        }
        return 0;
    }

# 16进制，8进制 #

![在这里插入图片描述][20200114154611776.png]

#include<iostream>
    #include<climits>
    
    
    int main()
    { 
        using namespace std;
    
        int chest = 20; //十进制
        int waist = 0x30; // 十六进制
        int leg = 042; // 八进制
        cout << "chest: " << chest << " (20 in decimal.)\n";
        cout << "waist: " << waist << " (0x30 in hex.) \n";
        cout << "leg: " << leg << " (042 in octal.)\n";
        return 0;
    }

![在这里插入图片描述][20200114155145584.png]

chest: 20 (20 in decimal.)
    waist: 48 (0x30 in hex.)
    leg: 34 (042 in octal.)

也可以用控制符hex, oct改变基数

#include<iostream>
    #include<climits>
    
    
    int main()
    { 
        using namespace std;
    
        int chest = 20; //十进制
        int waist = 0x30; // 十六进制
        int leg = 042; // 八进制
        cout << "chest: " << chest << " (20 in decimal.)\n";
        cout << hex << "waist: " << waist << " (0x30 in hex.) \n";
        cout << oct << "leg: " << leg << " (042 in octal.)\n";
        return 0;
    }

chest: 20 (20 in decimal.)
    waist: 30 (0x30 in hex.)
    leg: 42 (042 in octal.)

![在这里插入图片描述][20200114155539573.png]

# 常量类型如何确定 #

只要不是超出范围或者有后缀说明，否则就存为int  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzM2NjA3ODk0_size_16_color_FFFFFF_t_70 5]

# char类型 #

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzM2NjA3ODk0_size_16_color_FFFFFF_t_70 6]

#include<iostream>
    #include<climits>
    int main()
    { 
        using namespace std;
    
        char ch;
        cout << "enter a character. \n";
        cin >> ch; // cin把a转化为97存入内存
        cout << "thanks for the " << ch << " character. \n"; //cout把97读为a
        return 0;
    }

![在这里插入图片描述][20200114161013218.png]

enter a character.
    a
    thanks for the a character.

cin cout为我们完成了转换工作

c++对字符用单引号，对字符串用双引号

#include<iostream>
    #include<climits>
    
    
    int main()
    { 
        using namespace std;
    
        char ch = 'm';
        int in = ch;
        cout << "The ASCII code for " << ch << " is " << in << endl;
        ch ++;
        in = ch;
        cout << "The ASCII code for " << ch << " is " << in << endl;
        cout.put(ch);
        cout << '!';
        cout.put('!');
        return 0;
    }

The ASCII code for m is 109
    The ASCII code for n is 110
    n!!

cout.put()是OOP第一个示例。点号是成员运算符，put()是成员函数，用它是历史原因，总之现在已经可以不通过它了，cout已经可以一人完成所有任务了。

牛逼！char用作数值类型竟然可能不确定有无符号，必须明确指定  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzM2NjA3ODk0_size_16_color_FFFFFF_t_70 7]

# 转义字符 #

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzM2NjA3ODk0_size_16_color_FFFFFF_t_70 8]

#include<iostream>
    #include<climits>
    int main()
    { 
        using namespace std;
        cout << '\a';
        cout << "I love \"you\".";
        return 0;
    }

\\a会响诶，Windows启动的声音

I love "you".

# bool类型 #

![在这里插入图片描述][20200115120600621.png]  
c语言以前是没有bool类型的，后来C99标准引入了，引入头文件<stdbool.h>即可，详见[这篇博文][Link 1]

#include<iostream>
    int main()
    { 
        using namespace std;
        bool is_ready = true;
        int a = true;
        int b = false;
        bool c = -10;
        bool d = 0;
        cout << is_ready << endl;
        cout << a << endl;
        cout << b << endl;
        cout << c << endl;
        cout << d << endl;
        return 0;
    }

1
    1
    0
    1
    0

可以看到，给bool类型的变量赋0以外的值都会被存为1，不会存储原值  
用true和false给int等数值类型赋值，赋的是1和0

# const #

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzM2NjA3ODk0_size_16_color_FFFFFF_t_70 9]![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzM2NjA3ODk0_size_16_color_FFFFFF_t_70 10]![在这里插入图片描述][2020011512203537.png]

#include<iostream>
    int main()
    { 
        using namespace std;
        const int MONTHS = 12;
        cout << MONTHS << endl;
        cout << sizeof MONTHS << endl;
        return 0;
    }

12
    4

# 浮点数 #

整数类型共有11种:  
char  
signed char, short, int , long, long long,  
unsigned char, unsigned short, unsigned int, unsigned long, unsigned long long  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzM2NjA3ODk0_size_16_color_FFFFFF_t_70 11]

#include<iostream>
    int main()
    { 
        using namespace std;
        float a = 5e12;
        double b = 5e22;
        long double c =5e202;
        cout << a << endl;
        cout << sizeof a << endl;
        cout << b << endl;
        cout << sizeof b << endl;
        cout << c << endl;
        cout << sizeof c << endl;
        return 0;
    }

5e+012
    4
    5e+022
    8
    5e+202
    12

可以看到我的系统中float占4字节，double8字节，long double12字节，如果把c替换为5e602,cout会显示为inf，大概是超出了指数最大位数，但是我没有在float.h或者cfloat文件中找到描述指数最大位数的语句，于是不清楚最大指数位数

## float和double精度差异 ##

ios\_base::fixed和ios\_base::floatfield是iostream头文件提供的常量  
大概ios\_base也是个名称空间？？

#include<iostream>
    int main()
    { 
        using namespace std;
        float a = 10.0/3.0;
        double b = 10.0/3.0;
        const float million = 1e6;
        cout.setf(ios_base::fixed, ios_base::floatfield);//使用定点表示法输出，默认只打印小数点后6位
        cout << a << endl;
        cout << b << endl;
        cout << a * million << endl;
        cout << b * million << endl;
        cout << b * million * 10000 << endl;
        return 0;
    }

3.333333
    3.333333
    3333333.250000
    3333333.333333
    33333333333.333336

a, b初始化相同，刚开始cout显示6位小数，看不出区别，乘以100万后，再显示6位小数，就看出了区别，float从第8个有效位，即小数点后第一位就出错了；但是b的13位都是对的；但是第17位也开始出错了

suo以float正确了7位，double正确了16位

## 精度造成错误 ##

#include<iostream>
    int main()
    { 
        using namespace std;
        float a = 5.2e18;
        float b = a + 1.0f;
        cout.setf(ios_base::fixed, ios_base::floatfield);
        cout << a << endl;
        cout << b << endl;
        cout << a - b << endl;
        return 0;
    }

5200000055834050560.000000
    5200000055834050560.000000
    0.000000

可以看到，float显示了前8位是对的，第9位就错了（但是后面实验表明实际上第8位并没被表示，只是巧合），而且b也没加上1，而是和a相等，这是因为a加1需要在小数点后18位加1，float根本没表示出来，当然不行啦

改a为float a = 5.2e6;

5200000.000000
    5200001.000000
    -1.000000

结果才对，说明float只能正确表示数字的前7位，从第8位开始就不行啦

通过这样的方式测试发现，double可以正确表示数字的前16位

#include<iostream>
    int main()
    { 
        using namespace std;
        double a = 5.2e15;
        double b = a + 1.0f;
        cout.setf(ios_base::fixed, ios_base::floatfield);
        cout << a << endl;
        cout << b << endl;
        cout << a - b << endl;
        return 0;
    }

5200000000000000.000000
    5200000000000001.000000
    -1.000000

long double 可以正确表示数字的前19位

#include<iostream>
    int main()
    { 
        using namespace std;
        long double a = 5.2e18;
        long double b = a + 1.0f;
        cout.setf(ios_base::fixed, ios_base::floatfield);
        cout << a << endl;
        cout << b << endl;
        cout << a - b << endl;
        return 0;
    }

5200000000000000000.000000
    5200000000000000001.000000
    -1.000000

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzM2NjA3ODk0_size_16_color_FFFFFF_t_70 12]

# 小测试 #

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzM2NjA3ODk0_size_16_color_FFFFFF_t_70 13]

用多种方式输出一个字符：

#include<iostream>
    int main()
    { 
        using namespace std;
        char c = 88;
        cout << c << endl;
        cout << char(88) << endl; // c++风格强制转换，int to char
        cout << (char)88 << endl; // c语言风格
        cout.put(char(88));
        return 0;
    }

X
    X
    X
    X

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[20200115151726888.png]: https://img-blog.csdnimg.cn/20200115151726888.png
[20200114130358333.png]: https://img-blog.csdnimg.cn/20200114130358333.png
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[Link 1]: https://blog.csdn.net/syzcch/article/details/8152368
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