Replace sequence of identical values of length > 2

Question

I have a sensor that measures a variable and when there is no connection it returns always the last value seen instead of NA. So in my vector I would like to replace these identical values by an imptuted value (for example with na.approx).

set.seed(3)
vec <- round(runif(20)*10)
####  [1] 2 8 4 3 6 6 1 3 6 6 5 5 5 6 9 8 1 7 9 3

But I want only the sequences bigger than 2 (3 or more identical numbers) because 2 identical numbers can appear naturally. (in previous example the sequence to tag would be 5 5 5)

I tried to do it with diff to tag my identical points (c(0, diff(vec) == 0)) but I don't know how to deal with the length == 2 condition...

EDIT my expected output could be like this:

####  [1] 2 8 4 3 6 6 1 3 6 6 5 NA NA 6 9 8 1 7 9 3

(The second identical value of a sequence of 3 or more is very probably a wrong value too)

Thanks

Answer 1

you can use the lag function

set.seed(3)
> vec <- round(runif(20)*10)
> 
> vec
 [1] 2 8 4 3 6 6 1 3 6 6 5 5 5 6 9 8 1 7 9 3
> 
> vec[vec == lag(vec) & vec == lag(vec,2)] <- NA
> 
> vec
 [1]  2  8  4  3  6  6  1  3  6  6  5  5 NA  6  9  8  1  7  9  3
> 

Answer 2

you can use rle to get the indices of the positions where NA should be assigned.

vec[with(data = rle(vec),
     expr = unlist(sapply(which(lengths > 2), function(i)
         (sum(lengths[1:i]) - (lengths[i] - 2)):sum(lengths[1:i]))))] = NA
vec
#[1]  2  8  4  3  6  6  1  3  6  6  5 NA NA  6  9  8  1  7  9  3

In function

foo = function(X, length){
   replace(x = X,
           list = with(data = rle(X),
                       expr = unlist(sapply(which(lengths > length), function(i)
                           (sum(lengths[1:i]) - (lengths[i] - length)):sum(lengths[1:i])))),
           values = NA)
}
foo(X = vec, length = 2)
#[1]  2  8  4  3  6  6  1  3  6  6  5 NA NA  6  9  8  1  7  9  3