[R] Calculate a mean for several months for several years

[saba]

Hello everyone,

I have a dataset with 3 colums (Year, Month, MeanTemp). Now I would like to
calculate the average of the mean temperature for the summer months (Juli,
August, September) for each of the 20 years.

I'm sure it's somehow possible with a loop, but all I tried so far didn't
worked and by the time I spent looking for a solution I would have even be
faster doing this in excel!!!


Any help is greatly appreciated!!!
Cheers
Sandra
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