Apply a function to dataframe subsetted by all possible combinations of categorical variables

Question

An example dataframe with categorical variables catA, catB, and catC. Obs is some observed value.

catA <- rep(factor(c(\"a\",\"b\",\"c\")), length.out=10         


        

Answer 1

ans <- with(dat, tapply(obs, list(catA, catB, catC), mean))
ans <- data.frame(expand.grid(dimnames(ans)), results=c(ans))
names(ans)[1:3] <- names(dat)[1:3]

str(ans)
# 'data.frame':  36 obs. of  4 variables:
#  $ catA   : Factor w/ 3 levels "a","b","c": 1 2 3 1 2 3 1 2 3 1 ...
#  $ catB   : Factor w/ 4 levels "1","2","3","4": 1 1 1 2 2 2 3 3 3 4 ...
#  $ catC   : Factor w/ 3 levels "d","e","f": 1 1 1 1 1 1 1 1 1 1 ...
#  $ results: num  69.7 NA NA 55.3 NA ...