Python Pandas Dataframe how to multiply entire column with a scalar

Question

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

User · Answer

Also it s possible to use numerical indeces with  iloc   df iloc   0      -1

User · Answer

I got this warning using Pandas 0 22  You can avoid this by being very explicit using the assign method   df   df assign quantity   df quantity mul -1

User · Answer

A little late to the game  but for future searchers  this also should work   df quantity   df quantity    -1

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Note  for those using pandas 0 20 3 and above  and are looking for an answer  all these options will work   df   pd DataFrame np ones  5 6   columns   one   two   three                                           four   five   six    df one   5 df two   df two 5 df three   df three multiply 5  df  four     df  four   5 df loc     five     5 df iloc    5    df iloc    5  5   which results in     one  two  three  four  five  six 0  5 0  5 0    5 0   5 0   5 0  5 0 1  5 0  5 0    5 0   5 0   5 0  5 0 2  5 0  5 0    5 0   5 0   5 0  5 0 3  5 0  5 0    5 0   5 0   5 0  5 0 4  5 0  5 0    5 0   5 0   5 0  5 0

User · Answer

Try df  quantity     df  quantity     -1

User · Answer

The real problem of why you are getting the error is not that there is anything wrong with your code  you can use either iloc  loc  or apply  or     another of them could have worked    The real problem that you have is due to how you created the df DataFrame   Most likely you created your df as a slice of another DataFrame without using  copy     The correct way to create your df as a slice of another DataFrame is df   original df loc some slicing  copy       The problem is already stated in the error message you got   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  You will get the same message in the most current version of pandas too     Whenever you receive this kind of error message  you should always check how you created your DataFrame  Chances are you forgot the  copy

User · Answer

You can use the index of the column you want to apply the multiplication for   df loc   6     -1   This will multiply the column with index 6 with -1

User · Answer

More recent pandas versions have the pd DataFrame multiply function   df  quantity     df  quantity   multiply -1

User · Answer

try using apply function    df  quantity     df  quantity   apply lambda x  x -1

User · Answer

A bit old  but I was still getting the same SettingWithCopyWarning  Here was my solution   df loc     quantity     df  quantity     -1

User · Answer

Here s the answer after a bit of research   df loc    quantity      -1  seems to prevent SettingWithCopyWarning

[python] Python: Pandas Dataframe how to multiply entire column with a scalar

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