补：变量与常量

一.区别

两者都是用来储存数据的容器，但变量的值可以改变，而常量的值一但设定无法再次修改

二.变量（可改值）

1.类型

date_type name
//date_type 数据类型
//name 数据名

2.命名

（1）最好有意义
（2）只能由字母（区分大小写），数字，下划线（“_”）组成
（3）不能以数字开头
（4）长度不能超过63个字符
（5）变量名不能使用关键字
（6）区分大小写（int abc != int Abc）
（7）变量最好初始化

3.初始化与赋值的区分

第一次为初始化，第二次为赋值

date_type name = 1;//初始化
name = 2;//赋值

4.变量的分类

全局变量与局部变量
（1）全局变量（放在函数外，范围广）
作用域：从定义位置开始，到整个源文件末尾都可使用（整个程序）
【若其他程序想用，可通过extern声明】

生命周期：程序启动到程序结束
【存放在静态存储区/数据段】

初始值：若未初始化，系统会自动初始化为0
【或\\0，NULL】

注：过多的使用会使模块间耦合度过高，调试困难，谨慎使用
【耦合度指代码不够独立，代码需高内聚，低耦合】

（2）局部变量（放在函数内，精度高）
作用域：仅限于定义它的函数或代码块内部，外部无法使用

生命周期：进入函数/代码时创建到退出时销毁
【存在内存的栈区】

初始值：若未初始化，值默认为垃圾值（不确定的随机数），建议初始化

优先级：局部优先于全局

5.变量本质

创建变量的本质：在内存上申请空间
变量类型的意义：申请空间大小

6.存储

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内存（c/c++需关注的三区）

栈区

局部变量，函数参数

默认不给初始值

堆区

动态内存管理，maloc，calloc，realloc，free

静态口

全局变量，静态变量

默认初始值为0

初始值给不给取决于效率

三.常量

1.分类

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常量

字面常量

符号常量

const常量

枚举常量

2.字面常量

（1）字符常量：
‘a’，‘b’，‘c’
（2）整型常量：
十进制：123，-123,0
八进制：037（以0开头）【=31】
十六进制：0x1F（以0x/0X开头）【=31】
（3）后缀：
u/U：unsigned 无符号整型
l/L:long 长整型（可与u/U组合）
ll/LL:long long 长长整型（可与u/U组合）
（4）浮点型：
无后缀
3.14【double】
有后缀
3.14f【float】
3.14L【long double】
1.2e3【科学计数法，1.2*10^3】

3.符号常量

定义出来的常量（用#define）

#define 符号 常量
//#define A 100,此时A为符号常量，值为100
//不能再给A赋值（常量不能改）

注：1.符号常量不能再次赋值
2.符号常量不分全局还是局部，定义后整个程序以下都可使用

4.const 修饰的常量

在创建变量前在变量类型前加const（后续不能再此修饰该变量）

const date_type name = 常量
//const int a = 100

本质上是变量，但是const修饰后不能再改了