Vectorize this for loop: applying a function to sub-indices?

Is there some way to vectorize the following?

# x: some vector
# index: some vector of indeces
n <- length(index)
y <- rep(NA, n)
for (i in 1:n) {
  y[i] = myfunction(x[1:index[i])
}

Basically, I'd like to app开发者_C百科ly myfunction to various subsets of the vector x. It doesn't seem like the apply functions are built to handle this.

I am not sure if I understand what you are up to, but if you want to get from the x vector the first index-es number of elements, than make up some sample data:

x <- runif(10)
index <- c(2,5,4,8)

And try:

> lapply(index, function(index) return(x[1:index]))
[[1]]
[1] 0.3869757 0.4060021

[[2]]
[1] 0.3869757 0.4060021 0.4843015 0.2064443 0.4614179

[[3]]
[1] 0.3869757 0.4060021 0.4843015 0.2064443

[[4]]
[1] 0.3869757 0.4060021 0.4843015 0.2064443 0.4614179 0.9278044 0.7351291
[8] 0.9792204