How to replace NAs with the linear interpolation between known observations? [duplicate]

Answer 1

Or you can create your own vectorized version of na.approx without any complicated loops and solve it without any external packages

myna.approx <- function(x){
  len <- length(x) 
  cumsum(c(x[1L], rep((x[len] - x[1L])/(len - 1L), len - 1L)))
}

with(df, ave(value, id, FUN = myna.approx))
## [1] 100 250 400 550 300 500 700 900

Answer 2

You could use na.approx from zoo

library(zoo)
df$value <- na.approx(df$value)
df
#  id time value
#1  a    1   100
#2  a    2   250
#3  a    3   400
#4  a    4   550
#5  b    1   300
#6  b    2   500
#7  b    3   700
#8  b    4   900