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

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?

This question is related to python pandas dataframe data-analysis

The answer is


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())

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