[R] aggregate in zoo

[Alfonso Sammassimo]

Hi R-experts,

Thanks very much to Jim Holtman and Gabor on my previous question.

I am having another problem with data manipulation in zoo. The following is 
data (Z) for first business day of every month in zoo format. I am trying to 
get mean of "open" for each year. I subset Z <- Z[,2] then

> sapply(split(Z, format(index(Z), "%Y")),mean)

I get error message:

>2000 2001 2002 2003 2004 2005 2006 2007
  NA   NA   NA   NA   NA   NA   NA   NA
Warning messages:
1: argument is not numeric or logical: returning NA in: mean.default(X[[1]], 
...)
2: argument is not numeric or logical: returning NA in: mean.default(X[[2]], 
...)
etc...................

Any help on what I'm missing would be appreciated. I am particularly 
confused by the fact that the command used works fine on the original data 
file (i.e. before subsetting by first day of month). Sorry if I have 
overlooked something very simple.

<Z
                 dayofmonth    open
2000-02-01 01        1636.10
2000-03-01 01        1596.75
2000-04-03 03        1737.70
2000-05-01 01        1695.65
2000-06-01 01        1651.90
2000-07-03 03        1669.20
2000-08-01 01        1628.35
2000-09-01 01        1717.35
2000-10-02 02        1614.55
2000-11-01 01        1587.10
2000-12-01 01        1475.60
2001-01-02 02        1450.65
2001-02-01 01        1503.60
2001-03-01 01        1351.95
2001-04-02 02        1268.10
2001-05-01 01        1369.20
2001-06-01 01        1362.75
2001-07-02 02        1331.55
2001-08-01 01        1309.70
2001-09-04 04        1235.55
2001-10-01 01        1109.20
2001-11-01 01        1155.55
2001-12-03 03        1207.30

Thank you,
Alfonso Sammassimo
Melbourne, Australia