Python Value Error when creating new column using np.where

Question

Problem

I am trying to compare values in two columns using an if statement (np.where) however I keep getting an error. I can't figure out for the life of me why, I've used np.where in dataframes previously with no issue.

ValueError: The truth value of a Series is ambiguous. Use a.empty, a.bool(), a.item(), a.any() or a.all().

Example

df = pd.DataFrame(
    {'colors': ['red', 'white', 'blue'], 
     'n1': [1, 2, 3], 
     'n2': [4, 6, 7], 
     'n3': [5, 3, 2]
     }
)

df['test'] = np.where(
    df.n1 > df.n2, 
    max(0, df.n1 - df.n3), 
    df.n3
)

Error Message

Traceback (most recent call last):
  File "C:/Users/user/PycharmProjects/project/example.py", line 15, in <module>
    df['test'] = np.where(df.n1 > df.n2, max(0, df.n1 - df.n3), df.n3)
  File "C:\Users\user\Continuum\anaconda3\envs\CondaEnv\lib\site-packages\pandas\core\generic.py", line 1442, in __nonzero__
    f"The truth value of a {type(self).__name__} is ambiguous. "
ValueError: The truth value of a Series is ambiguous. Use a.empty, a.bool(), a.item(), a.any() or a.all().

Process finished with exit code 1

Any help be greatly appreciated.

jezrael · Accepted Answer · 2021-03-26 12:04:29Z

1

Use numpy.maximum:

df['test'] = np.where(
df.n1 > df.n2, 
np.maximum(0, df.n1 - df.n3), 
df.n3)
print (df)
  colors  n1  n2  n3  test
0    red   1   4   5     5
1  white   2   6   3     3
2   blue   3   7   2     2

answered Mar 26, 2021 at 12:04

jezrael

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1 Comment

Bobby Heyer Over a year ago

Brilliant! That fixed the issue. Thank you so much for your quick reply. You were too quick, I can't accept answer yet, but will do in 10 mins.

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