Efficiently replace values from a column to another column Pandas DataFrame

Question

I have a Pandas DataFrame like this:

   col1 col2 col3
1   0.2  0.3  0.3
2   0.2  0.3  0.3
3     0  0.4  0.4
4     0    0  0.3
5     0    0    0
6   0.1  0.         


        

Answer 1

I'm not sure if it's faster, but you're right that you can slice the dataframe to get your desired result.

df.col1[df.col1 == 0] = df.col2
df.col1[df.col1 == 0] = df.col3
print(df)

Output:

   col1  col2  col3
0   0.2   0.3   0.3
1   0.2   0.3   0.3
2   0.4   0.4   0.4
3   0.3   0.0   0.3
4   0.0   0.0   0.0
5   0.1   0.4   0.4

Alternatively if you want it to be more terse (though I don't know if it's faster) you can combine what you did with what I did.

df.col1[df.col1 == 0] = df.col2.replace(0, df.col3)
print(df)

Output:

   col1  col2  col3
0   0.2   0.3   0.3
1   0.2   0.3   0.3
2   0.4   0.4   0.4
3   0.3   0.0   0.3
4   0.0   0.0   0.0
5   0.1   0.4   0.4