Q:

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

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 !

A:

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.

A:

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.

A:

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. 

