[R] means by year, month and day

[Tom Mosca]

Hello Good Folk,

My dataframe looks like this:
> mydata
     X.YY MM DD hh WDI R.WSP D.GST   PRES  ATMP  DEWP
1    2015  1  1  0 328   3.6   4.5 1028.0   3.8  -3.5
2    2015  1  1  1 300   2.1   2.7 1027.9   3.7  -4.4
3    2015  1  1  2 264   2.4   2.9 1027.7   3.6  -4.5
4    2015  1  1  3 230   4.1   4.5 1027.4   4.2  -3.8
5    2015  1  1  4 242   8.1   9.2 1026.6   4.4  -3.1
6    2015  1  1  5 262   9.3  10.1 1026.6   4.1  -3.8
7    2015  1  1  6 267   8.6   9.6 1026.3   4.2  -3.8
8    2015  1  1  7 264   9.3   9.9 1026.1   3.9  -2.8
9    2015  1  1  8 268   8.2   9.1 1026.1   3.5  -3.0
10   2015  1  1  9 272   8.8   9.6 1025.4   3.2  -3.3 …

The first four columns are year, month, day, hour (0 – 23).  I wish to take the means of the next six columns (WDIR, WSPD, GST, PRES, ATMP and DEWP) by year, month and day.  That is, I want daily averages.

Please help.  Thank you.

Tom

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