[FIXED] Check if Value Greater than Previous Row by Group

Issue

With the dataframe below I want to compare the values in Val1 with the value in the preceding row by group. I then want to create a new column containing the word "Abv" if the value is greater than the preceding row and "Blw" if the value is lower than the predecing row. If the values are equal the new column should be blank.

Here's an extract:

Ref1	Val1
A	1
A	2
A	3
A	4
B	1
B	1
B	2
B	0

Desired result:

Ref1	Val1	AbvBlw
A	1
A	2	Abv
A	3	Abv
A	4	Abv
B	1
B	1
B	2	Abv
B	0	Blw

Solution

Code

get groupby diff & chk condition

boolean masking with np.select

import numpy as np
s = df.groupby(['Ref1'])['Val1'].diff()
df['AbwBlw'] = np.select([s > 0, s < 0], ['Abw', 'Blw'], None)

df

     Ref1  Val1 AbwBlw
0    A     1    None
1    A     2    Abw
2    A     3    Abw
3    A     4    Abw
4    B     1    None
5    B     1    None
6    B     2    Abw
7    B     0    Blw

Answered By - Panda Kim