pandas 根据不同列中的先前值取值 [英] Pandas taking value based on previous value in a different column

问题描述

这可能很简单，但由于某种原因，我想不出解决它的好方法.

It may be simple, but for some reason I can't think of a good way to solve it.

所以，假设这是我的 DataFrame:

So, let's say this is my DataFrame:

     a    b 
0   99    3
1   99    56
2    1    7
3    1    80  
4    1    55
5    2    601
6    2    -4
7    1    33
8    1    22

我想创建一个新列 c，其中的值将是 a 前一个值中 b 的最后一个值>，所以输出应该是:

I would like to create a new column, c, where the values will be the last value of b in the previous value of a, so the output should be:

     a    b      c
0   99    3     nan
1   99    56    nan
2    1    7     56
3    1    80    56  
4    1    55    56
5    2    601   55
6    2    -4    55
7    1    33    -4
8    1    22    -4

任何帮助将不胜感激！

推荐答案

我会将 df['a'] 与其位移进行比较以识别值的变化，以及 ffill() 和 shift():

I would compare df['a'] to its shift to identify value changes, and ffill() with shift():

df['c'] = df.loc[df['a'] != df['a'].shift(-1),'b']
df['c'] = df['c'].ffill().shift()

输出:

    a    b     c
0  99    3   NaN
1  99   56   NaN
2   1    7  56.0
3   1   80  56.0
4   1   55  56.0
5   2  601  55.0
6   2   -4  55.0
7   1   33  -4.0
8   1   22  -4.0

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