docs/zh/UserGuide/Operators-Functions/Machine-Learning.md

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## 机器学习

### AR

#### 函数简介

本函数用于学习数据的自回归模型系数。

**函数名:**  AR

**输入序列:** 仅支持单个输入序列，类型为 INT32 / INT64 / FLOAT / DOUBLE。

**参数:**

- `p`：自回归模型的阶数。默认为1。

**输出序列:** 输出单个序列，类型为 DOUBLE。第一行对应模型的一阶系数，以此类推。

**提示:** 

- `p`应为正整数。

- 序列中的大部分点为等间隔采样点。
- 序列中的缺失点通过线性插值进行填补后用于学习过程。

#### 使用示例

##### 指定阶数

输入序列：

```
+-----------------------------+---------------+
|                         Time|root.test.d0.s0|
+-----------------------------+---------------+
|2020-01-01T00:00:01.000+08:00|           -4.0|
|2020-01-01T00:00:02.000+08:00|           -3.0|
|2020-01-01T00:00:03.000+08:00|           -2.0|
|2020-01-01T00:00:04.000+08:00|           -1.0|
|2020-01-01T00:00:05.000+08:00|            0.0|
|2020-01-01T00:00:06.000+08:00|            1.0|
|2020-01-01T00:00:07.000+08:00|            2.0|
|2020-01-01T00:00:08.000+08:00|            3.0|
|2020-01-01T00:00:09.000+08:00|            4.0|
+-----------------------------+---------------+
```

用于查询的 SQL 语句：

```sql
select ar(s0,"p"="2") from root.test.d0
```

输出序列：

```
+-----------------------------+---------------------------+
|                         Time|ar(root.test.d0.s0,"p"="2")|
+-----------------------------+---------------------------+
|1970-01-01T08:00:00.001+08:00|                     0.9429|
|1970-01-01T08:00:00.002+08:00|                    -0.2571|
+-----------------------------+---------------------------+
```