Merging and subtracting DataFrame columns in pandas?

Question

I have a pandas DataFrame, something like:

col1  col2 col3 col5
NaN    1    2    8
2     NaN   4    8
4     NaN   4    8

I want to do two t

Answer 1

To get the new column, you could use fillna (or combine_first):

df['newcol1'] = df.col1.fillna(df.col2)

Then for the subtraction, use sub and specify axis=0 since we want to consider the row indices when matching up labels (not the column indices as is the default):

>>> df[['newcol1', 'col3']].sub(df['col5'], axis=0)
   newcol1  col3
0       -7    -6
1       -6    -4
2       -4    -4