熊猫：从列中减去值（以从中查找变化）

Question

I have a dataframe with this type of column:

magnetic_mag
34.451282394
44.81445845
24.833269553
25.032404588
22.027024464

I want to find the variation of every value from 30 (If the value is above 30 so I want to subtract 30 from it and if it's below I want to subtract the value itself from 30), so the output column in the data frame should look like this:

magnetic_mag
    4.451282394
    14.81445845
    5.166730447
    4.967595412
    7.972975536

Any easy way to do it with pandas ?

Edit: If I want the values below 30 to be with (-) sign (negative) is it possible? Desired output:

magnetic_mag
        4.451282394
        14.81445845
        -5.166730447
        -4.967595412
        -7.972975536

Thank you !

Answer 1

You can just subtract 30 and call abs :

(df["magnetic_mag"] - 30).abs()
Out[23]: 
0     4.451282
1    14.814458
2     5.166730
3     4.967595
4     7.972976
Name: magnetic_mag, dtype: float64

Answer 2

A variation:

import numpy as np

np.where(df['magnetic_mag']>30, df['magnetic_mag']-30, 30-df['magnetic_mag'])
#Out[100]: array([  4.45128239,  14.81445845,   5.16673045,   4.96759541,   7.97297554])