Python：CSV 文件中基于另一列值的平均值

Question

I am a noob and I have a large CSV file with data structured like this (with a lot more columns):

State  daydiff
CT     5.5
CT     6.5
CT     6.25
NY     3.2
NY     3.225
PA     7.522
PA     4.25

I want to output a new CSV where the daydiff is averaged for each State like this:

State  daydiff
CT     6.083
NY     3.2125
PA     5.886

I have tried numerous ways and the cleanest seemed to leverage pandas groupby but when i run the code below:

import pandas as pd

df = pd.read_csv('C:...input.csv')
df.groupby('State')['daydiff'].mean()

df.to_csv('C:...AverageOutput.csv')

I get a file that is identical to the original file but with a counter added in the first column with no header:

,State,daydiff
0,CT,5.5
1,CT,6.5
2,CT,6.25
3,NY,3.2
4,NY,3.225
5,PA,7.522
6,PA,4.25

I was also hoping to control the new average in datediff to a decimal going out only to the hundredths. Thanks

Answer 1

The "problem" with the counter is because the default behaviour for to_csv is to write the index. You should do df.to_csv('C:...AverageOutput.csv', index=False) .

You can control the output format of daydiff by converting it to string. df.daydiff = df.daydiff.apply(lambda x: '{:.2f}'.format(x))

Your complete code should be:

df = pd.read_csv('C:...input.csv')
df2 = df.groupby('State')['daydiff'].mean().apply(lambda x: '{:.2f}'.format(x))
df2.to_csv('C:...AverageOutput.csv')