Conditional interpolation of time series data in R - r

Question

I have time series data with N/As. The data are to end up in an animated scatterplot
Week X Y
1 1 105
2 3 110
3 5 N/A
4 7 130
8 15 160
12 23 180
16 30 N/A
20 37 200
For a smooth animation, the data will be supplemented by calculated, additional values/rows. For the X values this is simply arithmetical. No problem so far.
Week X Y
1 1 105
2
2 3 110
4
3 5 N/A
6
4 7 130
8
9
10
11
12
13
14
8 15 160
16
17
18
19
20
21
22
12 23 180
24
25
26
27
28
29
16 30 N/A
31
32
33
34
35
36
20 37 200
The Y values should be interpolated and there is the additional requirement, that interpolation should only appear between two consecutive values and not between values, that have a N/A between them.
Week X Value
1 1 105
2 interpolated value
2 3 110
4
3 5 N/A
6
4 7 130
8 interpolated value
9 interpolated value
10 interpolated value
11 interpolated value
12 interpolated value
13 interpolated value
14 interpolated value
8 15 160
16 interpolated value
17 interpolated value
18 interpolated value
19 interpolated value
20 interpolated value
21 interpolated value
22 interpolated value
12 23 180
24
25
26
27
28
29
16 30 N/A
31
32
33
34
35
36
20 37 200
I have already experimented with approx, converted the "original" N/A to placeholder values and tried the zoo package with na.approx etc. but don´t get it, to express a correct condition statement for this kind of "conditional approximation" or "conditional gap filling". Any hint is welcome and very appreciated.
Thanks in advance

Answer 1

Replace the NAs with Inf, interpolate and then revert infinite values to NA.
library(zoo)
DF2 <- DF
DF2$Y[is.na(DF2$Y)] <- Inf
w <- merge(DF2, data.frame(Week = min(DF2$Week):max(DF2$Week)), by = 1, all.y = TRUE)
w$Value <- na.approx(w$Y)
w$Value[!is.finite(Value)] <- NA
giving the following where Week has been expanded to all weeks, Y is such that the original NAs are shown as Inf and the inserted NAs as NA. Value is the interpolated Y.
> w
Week X Y Value
1 1 1 105 105.0
2 2 3 110 110.0
3 3 5 Inf NA
4 4 7 130 130.0
5 5 NA NA 137.5
6 6 NA NA 145.0
7 7 NA NA 152.5
8 8 15 160 160.0
9 9 NA NA 165.0
10 10 NA NA 170.0
11 11 NA NA 175.0
12 12 23 180 180.0
13 13 NA NA NA
14 14 NA NA NA
15 15 NA NA NA
16 16 30 Inf NA
17 17 NA NA NA
18 18 NA NA NA
19 19 NA NA NA
20 20 37 200 200.0
Note: Input DF in reproducible form:
Lines <- "
Week X Y
1 1 105
2 3 110
3 5 N/A
4 7 130
8 15 160
12 23 180
16 30 N/A
20 37 200"
DF <- read.table(text = Lines, header = TRUE, na.strings = "N/A")