How do I multiply each element of a given column of my dataframe with a scalar? (I have tried looking on SO, but cannot seem to find the right solution)
Doing something like:
df['quantity'] *= -1 # trying to multiply each row's quantity column with -1
gives me a warning:
A value is trying to be set on a copy of a slice from a DataFrame. Try using .loc[row_indexer,col_indexer] = value instead
Note: If possible, I do not want to be iterating over the dataframe and do something like this…as I think any standard math operation on an entire column should be possible w/o having to write a loop:
for idx, row in df.iterrows(): df.loc[idx, 'quantity'] *= -1
EDIT:
I am running 0.16.2
of Pandas
full trace:
SettingWithCopyWarning: A value is trying to be set on a copy of a slice from a DataFrame. Try using .loc[row_indexer,col_indexer] = value instead See the the caveats in the documentation: http://pandas.pydata.org/pandas-docs/stable/indexing.html#indexing-view-versus-copy self.obj[item] = s
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Answer
Here’s the answer after a bit of research:
df.loc[:,'quantity'] *= -1 #seems to prevent SettingWithCopyWarning