Syntax to select previous row in pandas after filtering

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Chapters
00:00 Question
00:56 Accepted answer (Score 15)
01:25 Answer 2 (Score 4)
01:42 Answer 3 (Score 2)
01:55 Answer 4 (Score 2)
02:07 Thank you

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Full question
https://stackoverflow.com/questions/5103...

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https://meta.stackexchange.com/help/lice...

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Tags
#python #pandas #dataframe #conditionalstatements

#avk47

ACCEPTED ANSWER

Score 15

Shift the mask up by 1.

df[(df['Value'] < 15).shift(-1).fillna(False)]

   Row  Value
1    2     25

More generally, if you're trying to find all rows greater than 15, whose next row is lesser than 15, you can compute two separate masks and AND them:

df[(df['Value'].shift(-1) < 15) & (df['Value'] > 15)]

   Row  Value
1    2     25

ANSWER 2

Score 4

`np.flatnonzero`

To find where the mask is True then subtract one

df.iloc[np.flatnonzero(df.Value < 15) - 1]

   Row  Value
1    2     25

ANSWER 3

Score 2

Using idxmax

>>> df.iloc[df.Value.le(15).idxmax() - 1]

Row       2
Value    25

ANSWER 4

Score 2

Using nonzero

df.iloc[(df['Value'] < 15).nonzero()[0]-1]
Out[34]: 
   Row  Value
1    2     25