I want to multiply two columns in a pandas DataFrame and add the result into a new column

Question

I m trying to multiply two existing columns in a pandas Dataframe  orders df  - Prices  stock close price  and Amount  stock quantities  and add the calculation to a new column called  Value   For some reason when I run this code  all the rows under the  Value  column are positive numbers  while some of the rows should be negative  Under the Action column in the DataFrame there are seven rows with the  Sell  string and seven with the  Buy  string   for i in orders df Action   if i      Sell     orders df  Value     orders df Prices orders df Amount  elif i     Buy     orders df  Value     -orders df Prices orders df Amount    Please let me know what i m doing wrong

User · Answer

You can use the DataFrame apply method   order df  Value     order df apply lambda row   row  Prices   row  Amount                                                  if row  Action     Sell                                                 else -row  Prices   row  Amount                                        axis 1    It is usually faster to use these methods rather than over for loops

User · Answer

If we re willing to sacrifice the succinctness of Hayden s solution  one could also do something like this     In  22   orders df  C     orders df Action apply                 lambda x   1 if x     Sell  else -1    In  23   orders df     New column C represents the sign of the transaction Out 23      Prices  Amount Action  C 0       3      57   Sell  1 1      89      42   Sell  1 2      45      70    Buy -1 3       6      43   Sell  1 4      60      47   Sell  1 5      19      16    Buy -1 6      56      89   Sell  1 7       3      28    Buy -1 8      56      69   Sell  1 9      90      49    Buy -1   Now we have eliminated the need for the if statement  Using DataFrame apply    we also do away with the for loop  As Hayden noted  vectorized operations are always faster    In  24   orders df  Value     orders df Prices   orders df Amount   orders df C  In  25   orders df     The resulting dataframe Out 25      Prices  Amount Action  C  Value 0       3      57   Sell  1    171 1      89      42   Sell  1   3738 2      45      70    Buy -1  -3150 3       6      43   Sell  1    258 4      60      47   Sell  1   2820 5      19      16    Buy -1   -304 6      56      89   Sell  1   4984 7       3      28    Buy -1    -84 8      56      69   Sell  1   3864 9      90      49    Buy -1  -4410   This solution takes two lines of code instead of one  but is a bit easier to read  I suspect that the computational costs are similar as well

User · Answer

To make things neat  I take Hayden s solution but make a small function out of it    def create value row       if row  Action       Sell           return row  Prices     row  Amount       else          return -row  Prices   row  Amount     so that when we want to apply the function to our dataframe  we can do    df  Value     df apply lambda row  create value row   axis 1       and any modifications only need to occur in the small function itself   Concise  Readable  and Neat

User · Answer

Good solution from bmu  I think it s more readable to put the values inside the parentheses vs outside        df  Values     np where df Action     Sell                                df Prices df Amount                              -df Prices df Amount    Using some pandas built in functions       df  Values     np where df Action eq  Sell                                 df Prices mul df Amount                               -df Prices mul df Amount

User · Answer

For me  this is the clearest and most intuitive   values      for action in   Sell   Buy        amounts   orders df  Amounts   orders df  Action   action   values     if action     Sell           prices   orders df  Prices   orders df  Action   action   values     else          prices   -1 orders df  Prices   orders df  Action   action   values     values    list amounts prices    orders df  Values     values   The  values method returns a numpy array allowing you to easily multiply element-wise and then you can cumulatively generate a list by  adding  to it

User · Answer

Since this question came up again  I think a good clean approach is using assign   The code is quite expressive and self-describing   df   df assign Value   lambda x  x Prices   x Amount   x Action replace   Buy    1   Sell    -1

User · Answer

I think an elegant solution is to use the where method  also see the API docs    In  37   values   df Prices   df Amount  In  38   df  Values     values where df Action     Sell   other -values   In  39   df Out 39       Prices  Amount Action  Values 0       3      57   Sell     171 1      89      42   Sell    3738 2      45      70    Buy   -3150 3       6      43   Sell     258 4      60      47   Sell    2820 5      19      16    Buy    -304 6      56      89   Sell    4984 7       3      28    Buy     -84 8      56      69   Sell    3864 9      90      49    Buy   -4410   Further more this should be the fastest solution

[python] I want to multiply two columns in a pandas DataFrame and add the result into a new column

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