Dear expeRts,
I have a data.frame with certain covariate combinations ('group' and 'year')
and corresponding values:
set.seed(1)
x <- data.frame(group = c(rep("A", 4), rep("B", 3)),
year = c(2001, 2003, 2004, 2005,
2003, 2004, 2005),
value = rexp(7))
My goal is essentially to construct a data.frame which contains all (group,
year)
combinations with corresponding number of values. This can easily be done with
tapply():
as.data.frame(as.table(tapply(x$value, list(x$group, x$year), FUN=length))) #
=> 2002 missing
However, the tricky part is now that I would like to have *all* years in
between 2001 and 2005.
Although tapply() sees the missing year 2001 for group "B" (since group "A" has
a value there),
tapply() does not 'see' the missing year 2002.
How can such a data.frame be constructed [ideally without using additional R
packages]?
Here is a straightforward way (hopelessly inefficient for the application in
mind):
num <- cbind(expand.grid(group = LETTERS[1:2], year=2001:2005), num=0)
covar <- c("group", "year")
for(i in seq_len(nrow(num)))
num[i,"num"] <- sum(apply(x[,covar], 1, function(z) all(z == num[i,covar])))
num
Cheers,
Marius
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