Hi,

I am using aggregate() to add up groups of data according to year and month.
It seems that the function aggregate() automatically sorts the levels of
factors of the grouping elements, even if the order of the levels of factors
is supplied. I am wondering if this is a bug, or if I missed something
important. Below is an example that shows what I mean. Does anyone know if
this is just the way the aggregate function works, or are there ways
to force aggregate() to keep the order of levels of factors supplied by the
grouping elements? Thanks!

library(chron)
dts=seq.dates("1/1/01","12/31/03")
rnum=rnorm(1:length(dts))
df=data.frame(date=dts,obs=rnum)
agg=aggregate(df[,2],list(year=years(df[,1]),month=months(df[,1])),sum)
levels(agg$month) # aggregate() automatically generates levels sorted by
alphabet.

[1] "Apr" "Aug" "Dec" "Feb" "Jan" "Jul" "Jun" "Mar" "May" "Nov" "Oct" "Sep"

fmonth=factor(months(df[,1]))
levels(fmonth) # factor() automatically generates the correct order of
levels.

[1] "Jan" "Feb" "Mar" "Apr" "May" "Jun" "Jul" "Aug" "Sep" "Oct" "Nov" "Dec"


agg2=aggregate(df[,2],list(year=years(df[,1]),month=fmonth),sum)
levels(agg2$month) # even if a factor with levels in the correct order is
supplied, aggregate(), sortsthe levels by alphabet regardless.

[1] "Apr" "Aug" "Dec" "Feb" "Jan" "Jul" "Jun" "Mar" "May" "Nov" "Oct" "Sep"


-- 
Tom

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