数据类型:  int  a;  数据类型  变量a; 在定义变量时需要指定变量的类型.

数据类型的分类:                             

A:本章介绍基本类型

1.整型分配字节的大小顺序：sizeof(short)<或=sizeof(int)<或=sizeof(long)<或=sizeof(long long)                      sizeof是测量类型或变量长度的运算符。

2.整型变量的符号属性：    类型                               字节数                        取值范围

                                  int(有符号基本整型)              2                             -32768~32767

                                                                          4                              -2147483648~2147483647      

                   unsigned int(无符号基本整型)               2                              0~65535

                                                                          4                              0~4294967295

                              short(有符号短整型)                 2                              -32768~32767

                unsigned short(无符号短整型)                 2                               0~65535

                               long(有符号长整型)                 4                               -2147483648~2147483647

                 unsigned long(无符号长整型)                 4                                0~4294967295

                     long long(有符号双长整型)                 8                                -9223372036854775808~9223372036854775807

       unsigned long long(无符号双长整型)                 8                                 0~18446744073709551615

                         unsigned修饰符的意思是无符号属性   signed修饰符为有符号属性 可以省略：如 signed short int a; 就是 short a;

3.字符型数据                                 

 1.字符与字符代码：只能使用系统的字符集中的字符（大多采用ASCII字符集) 都是以‘a‘，‘A‘，‘2‘为形式。

2.字符变量：用char定义的变量  char c=‘?‘；

                                           printf("%d   %c\n",c);

                 输出结果              63  ?     

                  原因是‘?‘在ASCII代码里是63  %d是以十进制格式输出  %c是以字符格式输出（原样输出）。

                       类型                           字节数                          取值范围

   singned char(有符号字符型)                   1                             -128~127

 unsigned char(无符号字符型)                   1                              0~255

（字符代码不可能为负数，所有实际上只用到0~127这一部分）

4.浮点型数据：浮点数据是用来表示具有小数点的实数的。由于小数点位置可以浮动，所有实数的指数形式称为 浮点数。

   3.14159 规范化的指数形式为  0.314159e001

   4.24515 规范化的指数形式为  0.424515e001

                                                           类型                                字节数                        有效数字                取值范围

根据取值范围的不同 浮点型数据又分为：float型（单精度浮点型）               4                               6                      0以及1.2e-038~3.4e038

                                                  double型(双精度浮点型)                 8                              15                     0以及2.3e-308~1.7e308

                                           long double型(长双精度型)                    8                              15                     0以及2.3e-308~1.7e308

                                                                                                   16                            19                      0以及3.4e-4932~1.1e4932