using complete.cases() with nested factors

Andrew Barr <wabarr <at> gmail.com> writes:

This maybe a newbie question.  I have a dataframe

that looks like the sample

at the bottom of the email.  I have monthly

precipitation data from several

sites over several years.  For each site,

I need to extract years that have

a complete series of 12 monthly precipitation

values, while excluding that

year for sites with incomplete data.

I can't figure out how to do this

gracefully (i.e. without a silly for loop).

Any help will be appreciate,

thanks!
SiteID    year    month    precip(mm)
670090    1941    jan    2998
670090    1941    feb    1299
670090    1941    mar    1007
670090    1941    apr    354
670090    1941    may    88
670090    1941    jun    156
670090    1941    jul    8
670090    1941    aug    4
670090    1941    sep    8
670090    1941    oct    58
670090    1941    nov    397
670090    1941    dec    248
670090    1942    jan    NA
670090    1942    feb    380
670090    1942    mar    797
670090    1942    apr    142
670090    1942    may    43
670090    1942    jun    14
670090    1942    jul    70
670090    1942    aug    51
670090    1942    sep    0
670090    1942    oct    10
670090    1942    nov    235
670090    1942    dec    405

There are likely more elegant solutions but this seems to work.
If the data frame is in a variable named dd

lapply(unique(dd$year), function(x) {s <- subset(dd, year == x)
 if (nrow(s) == 12) s})