Multiple functions in a single tapply or aggregate statement

Question

Is it possible to include two functions within a single tapply or aggregate statement?

Below I use two tapply statements and two aggregate statements: one for mean a

Answer 1

If you'd like to use data.table, it has with and by built right into it:

library(data.table)
myDT <- data.table(my.Data, key="animal")


myDT[, c("mean", "sd") := list(mean(weight), sd(weight)), by=list(age, sex)]


myDT[, list(mean_Aggr=sum(mean(weight)), sd_Aggr=sum(sd(weight))), by=list(age, sex)]
     age    sex mean_Aggr   sd_Aggr
1: adult female     96.0  3.6055513
2: young   male     76.5  2.1213203
3: adult   male     91.0  1.4142136
4: young female     84.5  0.7071068

I used a slightly different data set so as to not have NA values for sd