Pandas - explode a column and set a specific value to a column for replicated rows

Question

I would like to explode a column Col1 of a dataframe and for all the replicated rows, set a specific value z for a given column Col2.

For example if my dataframe df is:

Col1	Col2	Col3
[A,B,C]	x	y

I would like to find a way using df.explode("Col1") and achieve:

Col1	Col2	Col3
A	x	y
B	z	y
C	z	y

Thank you for any idea.

so would you supply a list of values in Col1 that you would like to change in Col2? — rhug123
– rhug123, Commented Aug 9, 2022 at 17:25
No, Col1 is the column that I want to explode. A, B and C are also dummy variables. — Mathieu
– Mathieu, Commented Aug 9, 2022 at 17:26
If you use df.explode('Col1') then Col1 will have A,B,C, Col2 will have all x's and Col3 will have all y's. What is the logic for the z's to come in? — rhug123
– rhug123, Commented Aug 9, 2022 at 17:28

Ynjxsjmh · Accepted Answer · 2022-08-09 17:42:20Z

0

You can try

out = (df.explode('Col1')
       .groupby(level=0)
       .apply(lambda g: g.assign(Col2=[g['Col2'].iloc[0]]+['z']*(len(g)-1)))  # keep first row of Col2 and replace rest with z
       .reset_index(drop=True))

print(out)

  Col1 Col2 Col3
0    A    x    y
1    B    z    y
2    C    z    y

answered Aug 9, 2022 at 17:42

Ynjxsjmh

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1 Comment

Mathieu Over a year ago

Thanks it works great. Nice use of apply/assign!

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