Dear expeRts,
The question is rather simple: Why does aggregate (or similarly tapply()) not
keep the order of the grouping variable(s)?
Here is an example:
x <- data.frame(group = rep(LETTERS[1:2], each=10),
year = rep(rep(2001:2005, each=2), 2),
value = rep(1:10, each=2))
## => sorted according to group, then year
aggregate(value ~ group + year, data=x, FUN=function(z) z[1])
## => sorted according to year, then group
I rather expected this to be the default:
aggregate(value ~ year + group, data=x, FUN=function(z) z[1])[,c(2,1,3)]
## => same order as input (grouping) variables
Same with tapply:
as.data.frame(as.table(tapply(x$value, list(x$group, x$year), FUN=function(z)
z[1])))
Cheers,
Marius
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