R: Extrapolating x no. of values beyond known values - r

Question

I'm looking for a function/method to extrapolate (linearly) for an x number of values beyond the original values.
Let's say I start with:
a <- c(NA, NA, NA, NA, NA, NA, 1, 2, 3, NA, NA, NA, NA, NA, NA)
And I want to extrapolate two values beyond, I would end up with:
[1] NA NA NA NA -1 0 1 2 3 4 5 NA NA NA NA
What I found so far is the approxExtrap function from Hmisc (https://rdrr.io/cran/Hmisc/man/approxExtrap.html). But since you have to define 'xout', I feel that I have to write a loop and every time select pieces I want to extrapolate on. This is possible of course, but ultimately I expect to have sequences of millions of datapoints with a lot of gaps, so I feel this may be too time consuming. So I hope I'm overlooking a simpler solution.
Added: There are no small gaps in the data, but typically ~ 100 NA's and then ~ 40 datapoints. I would like to extrapolate/extend the 40 datapoints with 5 new datapoints before the start and after the end of the 40 datapoints and replace 5 NA's at both locations. It is not possible to interpolate between two sequences of 40 datapoints.

Answer 1

I managed to solve the problem by:
Determining the ranges of the different series of data
Define the range I want to extrapolate to
Do the actual extrapolation through the Hmisc package
Initially, I thought I could only manage this by some loops that had to go through the raw data row by row, and was hoping for an existing function.
I'm sure many of you would have coded this way more efficient and nicer. But wanted to post my script anyway for people with a similar problem.
require(Hmisc)
extrapol.length <- 5
test <- data.frame('Time' = c(1:100), # I didn't use this as my data was equally spread in time, if you want to use it, see the first argument in the approxExtrap-function in the secondlast line
'x' = c(rep(NA, 10), 1:30, rep(NA, 30), 1:10, rep(NA, 20)))
## Determine start and end of the continuous (non-NA) data streams
length.values <- diff(c(0, which(is.na(test[,2]))))-2 # length non-NA's
length.values <- length.values[length.values > -1]
length.nas <- diff(c(0, which(!is.na(test[,2])))) # length NA's
length.nas <- length.nas[length.nas > 1]
if(is.na(test[1,2])){
# data starts with NA
length.nas <- data.frame('Order' = seq(1, length(length.nas)*2, by = 2),
'Length' = length.nas, 'Type' = 'na')
length.values <- data.frame('Order' = seq(2, length(length.values)*2, by = 2),
'Length' = length.values, 'Type' = 'value')
start.end <- rbind(length.nas, length.values)
start.end <- start.end[order(start.end$Order),]
value.seqs <- data.frame('no' = c(1:length(start.end$Type[start.end$Type == 'na'])),
'start' = NA, 'end' = NA)
for(a in value.seqs$no){
value.seqs$start[a] <- sum(start.end$Length[1:((a*2)-1)])
value.seqs$end[a] <- sum(start.end$Length[1:(a*2)])
}
}else{
# Data starts with actual values
length.nas <- data.frame('Order' = seq(2, length(length.nas)*2, by = 2),
'Length' = length.nas, 'Type' = 'na')
length.values <- data.frame('Order' = seq(1, length(length.values)*2, by = 2),
'Length' = length.values, 'Type' = 'value')
start.end <- rbind(length.nas, length.values)
start.end <- start.end[order(start.end$Order),]
value.seqs <- data.frame('no' = c(1:length(start.end$Type[start.end$Type == 'value'])),
'start' = c(1,rep(NA, (length(start.end$Type[start.end$Type == 'value'])-1))), 'end' = NA)
for(a in value.seqs$no){
value.seqs$end[a] <- sum(start.end$Length[1:((a*2)-1)])+1
if(a < max(value.seqs$no))
value.seqs$start[a+1] <- sum(start.end$Length[1:(a*2)])+1
}
}
## Do not extrapolate outside of the time-range of the original dataframe
value.seqs$start.extr <- value.seqs$start - extrapol.length
value.seqs$start.extr[value.seqs$start.extr < 1] <- 1 # do not extrapolate below time < 1
value.seqs$end.extr <- value.seqs$end + extrapol.length
value.seqs$end.extr[value.seqs$end.extr > nrow(test) | is.na(value.seqs$end.extr)] <- nrow(test)
value.seqs$end[is.na(value.seqs$end)] <- max(which(!is.na(test[,2])))
## Extrapolate
for(b in value.seqs$no){
test[c(value.seqs$start.extr[b]:value.seqs$end.extr[b]),3] <- approxExtrap(value.seqs$start[b]:value.seqs$end[b],test[c(value.seqs$start[b]:value.seqs$end[b]),2],xout=c(value.seqs$start.extr[b]:value.seqs$end.extr[b]))[2]
}
Thanks for thinking along!