[R] aggregating using a non-summary function?

[Sherri Heck]

Dear all-

I have a data set that looks as follows (data are taken every 5 minutes):

      LST   in     mph    Deg   DegF  DegF2    %    volts   Deg    mph2   w/m2
0905010000   -999    7.5  259.3   33.0   -999   41.5   -999   -999    9.1    0.2
0905010005   -999   11.0  261.2   33.1   -999   42.6   -999   -999   18.2    0.2
0905010010   -999    8.9  252.5   33.4   -999   41.6   -999   -999   12.4    0.2
0905010015   -999    9.1  265.9   33.2   -999   41.8   -999   -999   11.7    0.2


I am able to calculate the hourly average of "mph" over a span of 3 months with the following code:


z <- read.zoo("SPL summer 2009.txt", header = TRUE, na.strings = -999,
format = "%y%m%d%H%M", FUN = as.chron,
colClasses = c("character", rep("numeric", 10)))
mph <- z[months(time(z)) %in% c("May", "Jun", "Jul"),]       
ww <- aggregate(z$mph, trunc(time(z), "hour"), mean) 


but, i need to take the hourly average (spanning the same three months) of the directional degrees (Deg) in order to use that data to create a wind rose.  I was planning to get the hourly avg step by step , i.e.  converting to radians, taking sins, etc, but "aggregate" seems to only use summary-type functions.  I have been playing around with "daply", which seems to accept other functions, but am not sure how to convert the time/date stamp within it. 



I would appreciate it if anyone could point me in the right direction-

sherri