mean(mtcars)
methods(mean)
mean

mean.data.frame <- function (df) {
  stopifnot(is.data.frame(df))
  m <- vector("numeric", length(df))
  m <- sapply(df, mean)
  return(m)
}

mean.data.frame(mtcars)
mean.data.frame(iris)

mean(mtcars)
mean(iris)

mean.data.frame <- function (df) {
  stopifnot(is.data.frame(df))
  # m <- vector("numeric", length(df))
  m <- rep(NA, length(df))
  names(m) <- names(df)
  # m <- sapply(df, mean)
  for (i in 1:length(df)){
    if (is.vector(df[[i]])) {
      m[i] <- mean(df[[i]])
    }
  }
  return(m)
}

mean(dm)
mean.data.frame(dm)