如何根据 Pandas 中的第三列将值从一列复制到另一列？

Question

So I have a sample dataframe below:

   A  B     C D
0  1  2   Red  
1  4  5  Blue  
2  7  8   Red  

Output

   A  B     C  D
0  1  2   Red  1
1  4  5  Blue  5
2  7  8   Red  7

I am trying to fill column D using a value from either column A or B depending on what is in C?

So for example, if for the first row, since C = Red, I want to fill D with what is in A. If C = Blue, I want to fill in D with what is in B.

df.loc[df['C'] == 'Red', 'D'] =   df['A']
df.loc[df['C'] == 'Blue', 'D'] =  df['B']

I think I've got the left assignments down, but struggling what to do on the right to reference the same row. Trying to avoid looping through manually and doing this somewhat efficiently.

Answer 1

One way is to use np.where .

df['D'] = np.where(df.C == 'red', df.A, df.B)

Output

   A  B     C  D
0  1  2   red  1
1  4  5  blue  5
2  7  6   red  7

Answer 2

The way you have it set up will loop automatically. df.loc[df['C'] == 'Red', 'D'] = df['A'] will loop through every row. You have written this efficiently.