Replacing NAs in R with nearest value

Question

I'm looking for something similar to `na.locf()` in the `zoo` package, but instead of always using the previous non-`NA` value I'd like to use the nearest non-`NA` value. Some example data: ``` dat <- c(1, 3, NA, NA, 5, 7) ``` Replacing `NA` with `na.locf` (3 is carried forward): ``` library(zoo) na.locf(dat) # 1 3 3 3 5 7 ``` and `na.locf` with `fromLast` set to `TRUE` (5 is carried backwards): ``` na.locf(dat, fromLast = TRUE) # 1 3 5 5 5 7 ``` But I wish the nearest non-`NA` value to be used. In my example this means that the 3 should be carried forward to the first `NA`, and the 5 should be carried backwards to the second `NA`: ``` 1 3 3 5 5 7 ``` I have a solution coded up, but wanted to make sure that I wasn't reinventing the wheel. Is there something already floating around? FYI, my current code is as follows. Perhaps if nothing else, someone can suggest how to make it more efficient. I feel like I'm missing an obvious way to improve this: ``` na.pos <- which(is.na(dat)) if (le…

Accepted Answer

Here is a very fast one. It uses `findInterval` to find what two positions should be considered for each `NA` in your original data:

```
f1 <- function(dat) {
  N <- length(dat)
  na.pos <- which(is.na(dat))
  if (length(na.pos) %in% c(0, N)) {
    return(dat)
  }
  non.na.pos <- which(!is.na(dat))
  intervals  <- findInterval(na.pos, non.na.pos,
                             all.inside = TRUE)
  left.pos   <- non.na.pos[pmax(1, intervals)]
  right.pos  <- non.na.pos[pmin(N, intervals+1)]
  left.dist  <- na.pos - left.pos
  right.dist <- right.pos - na.pos

dat[na.pos] <- ifelse(left.dist <= right.dist,
                        dat[left.pos], dat[right.pos])
  return(dat)
}
```

And here I test it:

```
# sample data, suggested by @JeffAllen
dat <- as.integer(runif(50000, min=0, max=10))
dat[dat==0] <- NA

# computation times
system.time(r0 <- f0(dat))    # your function
# user  system elapsed 
# 5.52    0.00    5.52
system.time(r1 <- f1(dat))    # this function
# user  system elapsed 
# 0.01    0.00    0.03
identical(r0, r1)
# [1] TRUE
```

Replacing NAs in R with nearest value

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