where pandas python code example

Example 1: pandas where

In [20]: df
Out[20]:
   A  B  C
0  1  2  1
1  2  3  0
2  3  4  0
3  4  5  1

# Note that df.C is a mask
# Note that this is np.where, not df.where. That is different.
In [21]: df['D'] = np.where(df.C, df.A, df.B)

In [22]: df
Out[22]:
   A  B  C  D
0  1  2  1  1
1  2  3  0  3
2  3  4  0  4
3  4  5  1  4

Example 2: pandas where

# replaces values with other where condition is False
DataFrame.where(cond, other=nan, inplace=False, axis=None, level=None, errors='raise', try_cast=False)

import pandas as pd 
df = pd.DataFrame({'values':[1,2,3,4]})

df.where(df['values'] % 2 == 0, -1) # output : [-1, 2, -3, 4]