Create a new column based on a conditional subtraction in python-CodePudding

I'm trying to create a new column based on a conditional subtraction in python. I want to first group the dataframe by column A, then take the row value of C where B equals 2, and subtract that value from all values in column C.

import pandas as pd

data = [
["R", 1, 2],
["R", 2, 4],
["R", 3, 6],
["R", 4, 8],
["S", 0, 5],
["S", 1, 4],
["S", 2, 1],
["S", 3, 3]]

df = pd.DataFrame(data=data, columns=["a", "b", "c"])
df

Out[1]:
    a   b   c   
0   R   1   2   
1   R   2   4   
2   R   3   6   
3   R   4   8   
4   S   0   5   
5   S   1   4   
6   S   2   1   
7   S   3   3

Want it to yield the column 'd' of:

Out[2]:
    a   b   c   d   
0   R   1   2  -2
1   R   2   4   0 
2   R   3   6   2
3   R   4   8   4
4   S   0   5   4
5   S   1   4   3
6   S   2   1   0 
7   S   3   3   2

I would appreciate any tips of how to use transform or map functions to solve the problem.

CodePudding user response：

It's easiest to temporarily set the index to a. Then you can do the subtraction as usual while pandas will automatically align the index. Finally reset the index.

df1 = df.set_index('a')
df1['d'] = df1.c - df1.loc[df1.b.eq(2), 'c']
df1.reset_index()

Result:

   a  b  c  d
0  R  1  2 -2
1  R  2  4  0
2  R  3  6  2
3  R  4  8  4
4  S  0  5  4
5  S  1  4  3
6  S  2  1  0
7  S  3  3  2

(this of course only works if there is no more than one row for each value in a where b equals 2)