> For the complete documentation index, see [llms.txt](https://docs.1l.lol/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.1l.lol/basics/identifiers.md).

# 标识符、关键字与变量

### 标识符

标识符是程序员给变量、函数等起的名字，命名规则：

* 只能由**字母、数字、下划线**组成
* **不能以数字开头**
* **不能是关键字**
* **区分大小写**（`sum` 与 `Sum` 不同）

推荐：见名知义，如 `maxScore`；常量用全大写如 `MAXN`。

### 关键字

关键字是 C++ 语言保留、有特殊含义的单词，不能当作标识符，如： `int`、`double`、`char`、`bool`、`if`、`else`、`for`、`while`、`return`、`const`、`break`、`continue` 等。

### 常量与变量

* **变量**：值可以改变，使用前要先定义类型，如 `int a;`
* **常量**：值不能改变，用 `const` 定义，如 `const double PI = 3.14;`

```cpp
#include <iostream>
using namespace std;

int main() {
    int age = 12;          // 定义并初始化
    age = 13;              // 修改
    const int MAXN = 100;  // 常量，之后不可修改

    int a, b = 5;          // a 未初始化（值不确定），b = 5
    a = b + 1;
    cout << a << " " << age << " " << MAXN << endl;
    return 0;
}
```

### 表达式

由运算符和操作数组成的有意义式子，如 `a + b * 2`、`x > 0 && y < 10`。


---

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