How to update DataFrame column in Pandas?

Question

I have a dataframe

Col1 Col2
A    ...
C    ...
B    ...
X    ...
NaN  ...
D    ...

I want to update the values in Col1 to ord(Col1) - ord('A') + 1 , ignoring NaN . How to update it?

Answer 1

Try:

df['Col1'] = df['Col1'].dropna().map(ord) - 66

Output:

   Col1 Col2
0  -1.0  ...
1   1.0  ...
2   0.0  ...
3  22.0  ...
4   NaN  ...
5   2.0  ...

Note in pandas, most operations are done with intrinsic data alignment. Meaning that pandas will mostly always try to match indexes, hence dropping NaN values here will produce NaN values where the index isn't present on the left-side.