Syntax to select previous row in pandas after filtering

Question

I have a pd.DataFrame df with 5 lines, say:

Row    Value
 1      32
 2      25
 3      10
 4      18
 5      21

Lets say I use the filter df[df['Value'] < 15] and this should return

Row    Value
 3      10

My question is, I want to access the value for the row before this filter returns True, in this example I want to function to return the value 25 (from Row 2).

My end goal is to apply the condition, get the row above, and test this row for an additional condition.

What pandas functions can I use?

Thanks!

Answer 1

Shift the mask up by 1.

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

   Row  Value
1    2     25

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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

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