Aggregate lag

hello,

Does anybody know how to aggregate a lag series ?
when I try to use aggregate I get the following message

try<-ts(1:100,start=c(1985,1),freq=12)
aggregate(try,4,mean,na.rm=T)

    Qtr1 Qtr2 Qtr3 Qtr4
1985    2    5    8   11
1986   14   17   20   23
1987   26   29   32   35
1988   38   41   44   47
1989   50   53   56   59
1990   62   65   68   71
1991   74   77   80   83
1992   86   89   92   95
1993   98

aggregate(lag(try,-1),4,mean,na.rm=T)

Error in rep.int("", start.pad) : invalid number of copies in rep()

The ts-method seems to expect full blocks of observations. Note, that
also the last observation (100 in April 1993) is dropped from the
aggregate call above. I'm not sure what is the recommended way to
circumvent this problem with "ts": probably, you have to do some padding
with NAs yourself.

Example:
R> x <- ts(1:20,start=c(1990,1),freq=12)
R> aggregate(window(x, start = c(1990, 1), end = c(1991, 9), 
     extend = TRUE), 4, mean, na.rm = TRUE)
     Qtr1 Qtr2 Qtr3 Qtr4
1990  2.0  5.0  8.0 11.0
1991 14.0 17.0 19.5     
R> aggregate(window(lag(x, k = -1), start = c(1990, 1),
     end = c(1991, 9), extend = TRUE), 4, mean, na.rm = TRUE)
     Qtr1 Qtr2 Qtr3 Qtr4
1990  1.5  4.0  7.0 10.0
1991 13.0 16.0 19.0     

In zoo this can be done a bit easier:
R> z <- zooreg(1:20, start = yearmon(1990), freq = 12)
R> aggregate(z, as.yearqtr(time(z)), mean)
1990 Q1 1990 Q2 1990 Q3 1990 Q4 1991 Q1 1991 Q2 1991 Q3 
    2.0     5.0     8.0    11.0    14.0    17.0    19.5 
R> aggregate(lag(z, k = -1), as.yearqtr(time(lag(z, -1))), mean)
1990 Q1 1990 Q2 1990 Q3 1990 Q4 1991 Q1 1991 Q2 1991 Q3 
    1.5     4.0     7.0    10.0    13.0    16.0    19.0 

hth,
Z

Matthieu

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