Begginer/ numpy where and copy

user_17703336 0
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I am trying to copy values from one Field2 into Field1 if Field1 is null or NaN. I have tried below where statement as per documentation, but it cuts outliners instead of copyting the value.

dataframe=np.where(dataframe['field1'].isnull(),np.copy(dataframe['field2']),1)

I have interpreted it as if statement, but apparently its wrong interpretation, as results are not correct. Has anyone of you had similar issues?

np.where source np.copy source

pandas numpy python
2 Answers

You don't need np.copy, nor np.where. Use pandas' Series.mask instead

dataframe['field1'] = dataframe['field1'].where(dataframe['field1'].isnull(),
                                                dataframe['field2'])

If you really want to use np.where the syntax is:

dataframe['field1'] = np.where(dataframe['field1'].isnull(), # condition
                               dataframe['field2'],          # value if True
                               dataframe['field1'])          # value if False
user_163434640

Use fillna or combine_first:

dataframe['field1'] = dataframe['field1'].fillna(dataframe['field2'])

# OR

dataframe['field2'] = dataframe['fields'].combine_first(dataframe['field2'])

Demo:

dataframe = pd.DataFrame({'field1': [1, np.NaN, 3], 'field2': [4, 5, 6]})
print(dataframe)

# Output
   field1  field2
0     1.0       4
1     NaN       5
2     3.0       6


###
dataframe['field1'] = dataframe['field1'].fillna(df['field2'])
print(dataframe)

# Output
   field1  field2
0     1.0       4
1     5.0       5
2     3.0       6
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