pandas apply custom function on subset of the columns

I have a dataframe:

A  B  C  v1  v2  v3
q  2  3  4   9   1
8  f  2  7   4   7

I want to calc a new columns, that will have the RMS (sqrt(sum(x^2)) of all the v columns. So the new df will be:

A  B  C  v1  v2  v3 v_rms
q  2  3  4   9   1   9.9
8  f  2  7   2   4   8.3

since sqrt(4^2 + 9^2 + 1^2) = 9.9, sqrt(7^2 + 2^2 + 4^2) = 8.3

What is the best way to do so?

Solution 1:

Use DataFrame.filter for v columns, then DataFrame.pow with sum and for sqrt is used pow with 1/2:

df['v_rms'] = df.filter(like='v').pow(2).sum(axis=1).pow(1/2)
print (df)
   A  B  C  v1  v2  v3     v_rms
0  q  2  3   4   9   1  9.899495
1  8  f  2   7   2   4  8.306624