当时间序列中的一个值出现多次时替换它

Question

I am trying to replace a value in a python Series, the clue here is that I want to replace it only once, for instance:

S = pd.Series([0, 1, 2, 3, 2, 5, 2, 6])

The number 2 is repeated 3 times and I want to replace only the "second" 2 for 4, I don't want to replace all the '2' numbers within the series.

Is there any way to do it?

Thank you in advance

Answer 1

You can do the following:

In my code min_pos gets you the first position where 2 is in the Series. We then look for the index of the next position where 2 appears in the Series using iloc to filter the first occurrence.

import pandas as pd

S = pd.Series([0, 1, 2, 3, 2, 5, 2, 6])
min_pos = S[S==2].idxmin(2)
replace_idx = S.iloc[min_pos+1:].idxmin(2)
S.iloc[replace_idx] = 'new'

print(S)

Output

 0      0
1      1
2      2
3      3
4    new
5      5
6      2
7      6

Answer 2

You can try this way:

S.loc[((S == 2).cumsum() == 2).idxmax()] = 4
print(S)

Output:

0    0
1    1
2    2
3    3
4    4
5    5
6    2
7    6
dtype: int64

Details, find all the places in the Series where equal to 2, then use cumsum to count those 2's, now, select the first occurrence of the count of 2 and use .loc to set the value.