how to take the nth value from a column based on the value of another column (Python)

Question

any help here?! let's suppose I have a dataframe with two columns:

A | B

1 | b

1 | b

1 | a

2 | a

2 | b

3 | b

3 | c

3 | d

I want to get the first occurrence for each value of the colA it would be something like

A | B

1 | b

2 | a

3 | b

then catch the second occurrence something like that:

A | B

1 | b

2 | b

3 | c

after 3 occurrence

A | B

1 | a

2 | NULL

3 | d

any tips on how to do this??

Answer 1

IIUC, here's one way:

df1 = df.pivot_table(index = 'A', columns = df.groupby('A').cumcount(), values = 'B', aggfunc = sum)
result = [df1[col].reset_index(name='B')  for col in df1.columns] #this will give you the list of df's

OUTPUT:

[   A   B
 0  1   b
 1  2   a
 2  3   b,
    A   B
 0  1   b
 1  2   b
 2  3   c,
    A    B
 0  1    a
 1  2  NaN
 2  3    d]