> 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/functions/common-functions.md).

# 常用函数

> StCoder @qx1063<https://stcoder.cn/discussion-detail/1991>

#### 数学函数

| 函数          | 说明                              |
| ----------- | ------------------------------- |
| `ceil()`    | 向上取整函数——将小数变为与它相邻且比它大的整数        |
| `floor()`   | 向下取整函数——将小数变为与它相邻且比它小的整数        |
| `round()`   | 四舍五入取整函数——将小数的小数点后小于4的舍去，大于5的进位 |
| `abs()`     | 绝对值函数——将负数变为正数，正数保持不变           |
| `pow(a, b)` | 指数函数——a为底数，b为指数，计算a的b次方         |
| `sqrt(a)`   | 平方根函数——为平方的逆运算（开平方）             |
| `max(a, b)` | 取最大值                            |
| `min(a, b)` | 取最小值                            |
| `rand()`    | 生成随机数                           |

#### 字符串函数

| 函数               | 说明                   |
| ---------------- | -------------------- |
| `strlen(a)`      | 长度函数——求字符串的长度        |
| `strcpy(a, b)`   | 复制函数——把 b 复制到 a      |
| `strcat(a, b)`   | 连接函数——a 连接 b         |
| `strcmp(a, b)`   | 字符串比较函数——比较 a 和 b    |
| `strstr(a, b)`   | 子串函数——判断 b 是否为 a 的子串 |
| `a.substr(始, 终)` | 在字符串中取出固定的范围         |

#### strstr 函数详解

* 如果 b 是 a 的子串，则返回第一个出现的左边的字符
* 如果 b 不是 a 的子串，则返回 `nullptr`（空指针）

**判断子串示例：**

```cpp
char a[100] = "123456789";
char b[100] = "456";
if (strstr(a, b) == nullptr) {
    cout << "b不是a的子串";
} else {
    cout << "b是a的子串";
}
```

#### 输入输出函数

| 函数                  | 说明                       |
| ------------------- | ------------------------ |
| `cin.getline(a, n)` | 带空格的输入——接收一个字符串并输入（可含空格） |
| `setw(n)`           | 设定输出宽度                   |

#### 排序函数

| 函数                               | 说明     |
| -------------------------------- | ------ |
| `sort(a, a + n)`                 | 从小到大排序 |
| `sort(a, a + n, greater<int>())` | 从大到小排序 |

#### 其他常用

| 函数                    | 说明              |
| --------------------- | --------------- |
| `swap(a, b)`          | 交换 a 和 b 变量的值   |
| `map<下标类型, 内容类型> 数组名` | 映射容器，可用任意类型作为下标 |


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