How do I sum values in a column that match a given condition using pandas

Question

Suppose I have a column like so   a   b   1   5    1   7 2   3 1   3 2   5   I want to sum up the values for b where a   1  for example  This would give me 5   7   3   15   How do I do this in pandas

User · Answer

You can also do this without using groupby or loc. By simply including the condition in code. Let the name of dataframe be df. Then you can try :

df[df['a']==1]['b'].sum()

or you can also try :

sum(df[df['a']==1]['b'])

Another way could be to use the numpy library of python :

import numpy as np
print(np.where(df['a']==1, df['b'],0).sum())

User · Answer

The essential idea here is to select the data you want to sum  and then sum them  This selection of data can be done in several different ways  a few of which are shown below   Boolean indexing  Arguably the most common way to select the values is to use Boolean indexing    With this method  you find out where column  a  is equal to 1 and then sum the corresponding rows of column  b   You can use loc to handle the indexing of rows and columns    gt  gt  gt  df loc df  a      1   b   sum   15   The Boolean indexing can be extended to other columns  For example if df also contained a column  c  and we wanted to sum the rows in  b  where  a  was 1 and  c  was 2  we d write   df loc  df  a      1   amp   df  c      2    b   sum     Query  Another way to select the data is to use query to filter the rows you re interested in  select column  b  and then sum    gt  gt  gt  df query  a    1    b   sum   15   Again  the method can be extended to make more complicated selections of the data   df query  a    1 and c    2    b   sum     Note this is a little more concise than the Boolean indexing approach   Groupby  The alternative approach is to use groupby to split the DataFrame into parts according to the value in column  a   You can then sum each part and pull out the value that the 1s added up to    gt  gt  gt  df groupby  a    b   sum   1  15   This approach is likely to be slower than using Boolean indexing  but it is useful if you want check the sums for other values in column a    gt  gt  gt  df groupby  a    b   sum   a 1    15 2     8

[python] How do I sum values in a column that match a given condition using pandas?

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