Rephrased example from Advanced R of method dispatch (http://adv-r.had.co.nz/S3.html)

method-dispatch.r

## example from http://adv-r.had.co.nz/S3.html
 newmean <- function (x, ...) {
   UseMethod("newmean", x)
 }
newmean.numeric <- function(x, ...) sum(x) / length(x)
newmean.data.frame <- function(x, ...) sapply(x, mean, ...)
newmean.matrix <- function(x, ...) apply(x, 2, mean)

#numeric a
a <- c(1, 2, 6)
newmean(a)

#data.frame a
a_df <- expand.grid(1:3, 100:103)
newmean(a_df)

#matrix a
a_m <- matrix(1:30, nrow=5)
newmean(a_m)