R: ddply function applied to certain months obtained from date field -

can functions applied month/year combinations of date field in ddply? want figure mean (among other functions) specifying month/year.

monthlysummary <- ddply(tempdata, .(date, siteid, substrateid), summarize, monthlymean=mean(temp_c)) 

not sure mean "only month/year combinations" perhaps subset want, thought might asking summarization month. assuming date field of class date:

monthlysummary <- ddply(tempdata, .(format(date, "%m" ),                         summarize, monthlymean=mean(temp_c)) 

if it's not date class variable, maybe should make one:

tempdata$date2 <- as.date(tempdata$date, "%d/%m/%y") # or format 

and if wanted site , substrate month then:

monthlysummary <- ddply(tempdata,                   .( format(date, "%m" ), siteid, substrateid),                     summarize, monthlymean=mean(temp_c)) 

other date-aggregation options besides format.posixt include functions in package:lubridate , 'yearmon' class supported in package:zoo. exaple offered above lump event occurring in january of year. if wanted year-month distinction maintained need include in format-string: format(date, "%m-%y").