我是Pandas的新手
我的数据框架：
**df**

**期望的输出**

我试图仅在`B`列为`True`时对`C`列应用函数。
**我使用的代码**
```python
if (df['B'] == True)):
    df['C'] = df['A'].apply(
        lambda x: TextBlob(x).detect_language())

ValueError: The truth value of a Series is ambiguous. Use a.empty, a.bool(), a.item(), a.any() or a.all().

df['B'].bool()
df['B'] is True
df['B'] == 'True'

<details>
<summary>英文:</summary>
I am new to Pandas
My dataframe:
**df**

**Desired output**

I am trying to apply a function to column `C` only when the `B` column is `True`.
**What I use**

But I get an error:

**What I've tried**

But the error persists, not sure how I would form a statement saying 'only where column B is True'.
Thank you for your suggestions.
</details>
# 答案1
**得分**: 3
如果希望在没有匹配行的情况下获取缺失值，请在“apply”之前过滤行，仅处理具有“True”的行：
```python
df['C'] = df.loc[df['B'], 'A'].apply(lambda x: TextBlob(x).detect_language())
print(df)
        A      B    C
0   first   True   en
1  second  False  NaN
2   third  False  NaN
3  fourth   True   en
4   fifth  False  NaN

df['C'] = np.where(df['B'], df['A'].apply(lambda x: TextBlob(x).detect_language()), '')
print(df)
        A      B   C
0   first   True  en
1  second  False    
2   third  False    
3  fourth   True  en
4   fifth  False