How to subtract multiple columns from one another

Question

Working in Python 3.6 with Pandas, I'm trying to work out a simple way to subtract multiple columns from one another.

Imagine a simple dataframe, df, of two rows and four columns:

    A    B    C    D
0   1    2    3    4
1   5    6    7    8

I'm trying to subtract A from B and C from D to achieve:

    A    B    C    D
0   1    1    3    1
1   5    1    7    1

This can be done quite easily done column by column, e.g. for the B-A operation:

 df["B"] = df["B"] - df["A"]

or

 df["B"] = df["B"].sub(df["A"], axis=0)

I've tried:

 df[["B", "D"]] = df[["B", "D"]] - df[["A", "C"]]

and

 df[["B", "D"]] = df[["B", "D"]].sub(df[["A", "C"]], axis=0)

But both give an error about columns not matching, so I suspect I'm inadvertently telling it to do some sort of multi-axis operation or something. Regardless, neither of those work.

ValueError: Columns must be same length as key

Sooo what is the easiest way to accomplish this for both B-A and D-C in the same line ... my actual dataframe has about 30 columns, so line-by-line isn't really feasible :P

Thanks :)

Answer 1

A numpy operation is generally used in this case, as the operation is to be done elementwise. There is a very minor addition which fixes the problem. Change df[['A','C']] to df[['A','C']].values.

Code:

df[['B', 'D']] = df[['B','D']] - df[['A','C']].values

Output:

    A   B   C   D
0   1   1   3   1
1   5   1   7   1