How to na.locf in R without using additional packages [duplicate]

Question

This question already has answers here:

propagating data within a vector (5 answers)

Closed 8 years ago.

Given a vector such as (say) c(2,NA,5,NA,NA,1,NA) the problem is to "last observation carry forward" resulting in vector c(2,2,5,5,5,1,1).

As answered here, na.locf from the zoo package can do this. However, given the simplicity of the problem, and the fact that this is to be performed many times from a "blank" R environment, I would like to do this without loading packages. Is there a way to do it simply and quickly using just basic R? (The vector may be long and may contain many consecutive NAs.)

Answer 1

Extracted from zoo::na.locf.default

fillInTheBlanks <- function(S) {
  L <- !is.na(S)
  c(S[L][1], S[L])[cumsum(L)+1]
}

See also here.

Answer 2

This is one way using rle:

x <- c(2,NA,5,NA,NA,1,NA) 
x[is.na(x)] <- Inf
x[is.infinite(x)] <- with(rle(x), 
    rep(values[which(is.infinite(values)) - 1], lengths[is.infinite(values)])
)
# [1] 2 2 5 5 5 1 1