Trick to replace NA

Dear members,

I have a series of values in a vector and some value are missing and
replaced with NA.
For example:
a <- c(27, 25, NA, NA, 24, 26, 27, NA, 26)
I would like to replace the NAs with the value taken from the previous value
that is non-NA. The output would be in this case:

27 25 25 25 24 26 27 27 26

Now I do that with a for loop, but I try to eliminate all the loops to gain
in performance

I try this and it works but again I have a while and then I do not eliminate
completely loop:

l <- length(a)
while(any(is.na(a))) {a[2:l] <- ifelse(is.na(a[2:l]), a[1:(l-1)], a[2:l])}

If someone have another solution, I will be most happy !

Sincerely

Marc Girondot

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