Add a numpy array with one value less to a pandas column

I have a pandas dataframe df:

A  B  C
1  2  NaN
2  3  NaN
4  5  NaN

How can I add a numpy array with the shape of 1 element less then the length of the dataframe, in this case, the numpy array:

[10 20]

And I want to start at the second row and fill the column C:

A  B  C
1  2  NaN
2  3  10
4  5  20

>Solution :

You can use:

l = [10, 20]

df.loc[df.index[1:1+len(l)], 'C'] = l

Or with iloc:

df.iloc[1:1+len(l), df.columns.get_loc('C')] = l

Output:

   A  B     C
0  1  2   NaN
1  2  3  10.0
2  4  5  20.0