python - How to extract the n-th maximum/minimum value in a column of a DataFrame in pandas?

Question

I would like to obtain the n-th minimum or the n-th maximum value from numerical columns in the DataFrame in pandas.

Example:

df = pd.DataFrame({'a': [3.0, 2.0, 4.0, 1.0],'b': [1.0, 4.0 , 2.0, 3.0]})

     a    b
0  3.0  1.0
1  2.0  4.0
2  4.0  2.0
3  1.0  3.0

The third largest value in column a is 2 and the second smallest value in column b is also 2.

Answer 1

You can use nlargest/nsmallest -

df    
     a    b
0  3.0  1.0
1  2.0  4.0
2  4.0  2.0
3  1.0  3.0

df.a.nlargest(3).iloc[-1]
2.0

Or,

df.a.nlargest(3).iloc[[-1]]

1    2.0
Name: a, dtype: float64

And, as for b -

df.b.nsmallest(2).iloc[-1]
2.0

Or,

df.b.nsmallest(2).iloc[[-1]]

2    2.0
Name: b, dtype: float64

Quick observation here - this sort of operation cannot be vectorised. You are essentially performing two completely different operations here.