如何根据其他某些列的条件用另一列的值填充一列？ [英] how to fill a column with the value of another column based on a condition on some other columns?

问题描述

假设我们在熊猫中有一个数据框，如下所示：

Suppose that we have a dataframe in pandas as follows:

col1 | col2 | col3 | col4 
22   | Nan  | 23   |  56
12   |  54  | 22   |  36
48   | Nan  | 2    |  45
76   | 32   | 13   |  6
23   | Nan  | 43   |  8
67   | 54   | 56   |  64
16   | 32   | 32   |  6
3    | 54   | 64   |  8
67   | 4    | 23   |  64

我要替换 col4 如果 col1 和 col2 不是 col1 NaN

I want to replace the value of col4 with col1 if col4<col1 and col2 is not NaN

所以结果应该是

col1 | col2 | col3 | col4 
22  | Nan   | 23   |  56
12  |  54   | 22   |  36
48  | Nan   | 2    |  45
76  | 32    | 13   |  76
23  | Nan   | 43   |  8
67  | 54    | 56   |  67
16  | 32    | 32   |  16
3   | 54    | 64   |  8
67  | NaN   | 23   |  64

我尝试了以下代码

df.loc[((df['col4'] < df['col1']) & (pd.notnull(df['col2']))), ['col4']] = df.loc['col1']

问题出在等号之后。有人知道如何解决该问题吗？

the problem is from after the equal sign. Does anyone know how to fix the problem?

推荐答案

这是一种经过验证的解决方案：

Here's a verified solution:

idx_ = df[(df['col4'] < df['col1']) & (pd.notnull(df['col2']))].index
df.loc[idx_,'col4'] = df['col1']
df


+---+------+------+------+------+
|   | col1 | col2 | col3 | col4 |
+---+------+------+------+------+
| 0 |   22 | NaN  |   23 |   56 |
| 1 |   12 | 54.0 |   22 |   36 |
| 2 |   48 | NaN  |    2 |   45 |
| 3 |   76 | 32.0 |   13 |   76 |
| 4 |   23 | NaN  |   43 |    8 |
| 5 |   67 | 54.0 |   56 |   67 |
| 6 |   16 | 32.0 |   32 |   16 |
| 7 |    3 | 54.0 |   64 |    8 |
| 8 |   67 | 4.0  |   23 |   67 |
+---+------+------+------+------+

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