在 pandas 中的某一行之前设置行而不迭代

Question

我在 pandas 数据框中有一列，我想将 'large' 之前的 'normal' 的任何不间断序列更改为 'large.' 我不想替换all个实例'normal'在'large.'的列例如我想把下面的左列改成右列：

input	result
small	small
small	small
normal	large
normal	large
large	large
small	small
normal	normal
small	small
normal	large
large	large

迭代很简单：

for i, v in df['input'].iterrows():
    if v == 'large':
        index = i
        while df['input'].iloc(index-1) == 'normal':
            df['input'].iloc(index-1) = 'large'
            index -= 1

然而，这是低效的。有没有一种简洁的矢量化方法可以做到这一点？

Answer 1

这是使用 bfill 的一种方法。

See Pandas docs for fillna for further information

import pandas as pd

input_frame = pd.Series([
    'small',
    'small',
    'normal',
    'normal',
    'large',
    'small',
    'normal',
    'small',
    'normal',
    'large',
]).to_frame()
# Change 'normal' to None in order to use bfill
input_frame = input_frame.replace({'normal': None})
input_frame['bfilled'] = input_frame[0].bfill()
# Change rows that were not bfilled to 'large' back to 'normal'
input_frame.loc[
    (input_frame['bfilled'] != 'large') & input_frame[0].isna(),
    'bfilled'
] = 'normal'
# Select the result, essentially drop the original column
result = input_frame['bfilled']

示例输出：

>>> input_frame = input_frame.replace({'normal': None})
>>> input_frame['bfilled'] = input_frame[0].bfill()
>>> input_frame
       0 bfilled
0  small   small
1  small   small
2   None   large
3   None   large
4  large   large
5  small   small
6   None   small  <--- This should be changed
7  small   small
8   None   large
9  large   large
>>> # Select the row(s) that need to be changed with
>>> input_frame.loc[(input_frame['bfilled'] != 'large') & input_frame[0].isna()]
      0 bfilled
6  None   small

Answer 2

尝试将 bfill 与 where 一起使用：

df1.loc[df1['input'].eq("normal")]  = np.nan
df1['result'] = df1.fillna(method='bfill').where(df1.notnull() | (df1.shift(-1) != 'small')).fillna('normal')

df1:

    input   result
0   small   small
1   small   small
2   NaN     large
3   NaN     large
4   large   large
5   small   small
6   NaN     normal
7   small   small
8   NaN     large
9   large   large

在 pandas 中的某一行之前设置行而不迭代

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