Pandas dataframe column calculation

Question

I want to replace the values in a column by a calulation (which is a multiplcation of comlun values). like this:

    df.loc[:, 'cfit'] = df['cfit'] * df['risk_nr']

I get the following message: 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

Is this a correct or false warning of pandas ? Thnx

What version of pandas are you running and are you able to post sample data to reproduce your problem as I cannot — EdChum
– EdChum, Commented Nov 30, 2014 at 16:18

elyase · Accepted Answer · 2014-11-29 16:42:07Z

1

Just do it like this:

df['cfit'] = df['cfit'] * df['risk_nr']

or even:

df['cfit'] *= df['risk_nr']

Regarding your question: I think this is a false warning in this particular case, as df.loc[:, 'cfit'] should return a view and not a copy. You can turn off the warning with:

pd.set_option('chained_assignment', None)

edited Nov 29, 2014 at 16:42

answered Nov 29, 2014 at 16:15

elyase

41.2k12 gold badges121 silver badges123 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

Bob Haffner Over a year ago

I'm inclined to agree with you that it's a false warning. I've seen that warning before, but it's generally associated with a conditional update not using .loc/row index.

elyase Over a year ago

@BobHaffner, Yep, I think something else is going on there as I don't get the warning for a simple df.loc[:, 'colname'] = something. May be the code shown is not the actual one he is using.

Collectives™ on Stack Overflow

Pandas dataframe column calculation

1 Answer 1

2 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

2 Comments

Your Answer

Sign up or log in

Post as a guest

Related