Mutate pandas dataframe cell values

Question

I want to mutate column c at a specific row by adding that row and another row.

df = pd.DataFrame({
    'A': [0,1,2,3],
    'B': [0,1,2,3],
    'C': [10,10,10,10]
})

mask1 = df['A']==1
mask2 = df['B']==2
df.loc[mask1, 'C'] = df.loc[mask1, 'C'] + df.loc[mask2, 'C']

In the last line, because we are adding two pd.Series together, it tries to match the index and therefore would return NaN instead of the expected 10 + 10=20.

How do I do this properly?

Answer 1

IIUC use select first value from Series by df.loc[mask2, 'C']

df.loc[mask1, 'C'] = df.loc[mask1, 'C'] + df.loc[mask2, 'C'].iat[0]

df.loc[mask1, 'C'] = df.loc[mask1, 'C'] + df.loc[mask2, 'C'].to_numpy()[0]

If possible mask2 return all Falses use:

df.loc[mask1, 'C'] = df.loc[mask1, 'C'] + next(iter(df.loc[mask2, 'C']), 0)
print (df)
   A  B   C
0  0  0  10
1  1  1  20
2  2  2  10
3  3  3  10