Is there an idiomatic way to get a vectorized conditional mean in R

Question

I want to use a vectorized conditional average (when using ddply and transform ) meaning an average of entries that pass a certain condition. I came up with this but am wondering if it can be implemented using built-in functions or a popular package?

mean.conditional <- function(x, cond, zero = 0) {
  filter <- cond(x)
  return(sum(ifelse(filter, x, zero)) / sum(filter))
}

alternatively, the following function is probably even cleaner:

mean.conditional <- function(x, filter) {
  return(sum(ifelse(filter, x, 0)) / sum(filter))

Answer 1

没有内置函数，但这可以做到：

sum(x*filter) / sum(filter)

Answer 2

mean(x[cond(x)])做你想要的吗？