Using lapply and the lm function together in R - r

Question

I have a df as follows:
t r
1 0 100.00000
2 1 135.86780
3 2 149.97868
4 3 133.77316
5 4 97.08129
6 5 62.15988
7 6 50.19177
and so on...
I want to apply a rolling regression using lm(r~t).
However, I want to estimate one model for each iteration, where the iterations occur over a set time window t+k. Essentially, the first model should be estimated with t=0,t=1,...t=5, if k = 5, and the second model estimated with t=1, t=2,...,t=6, and so on.
In other words, it iterates from a starting point with a set window t+k where k is some pre-specified window length and applies the lm function over that particular window length iteratively.
I have tried using lapply like this:
mdls = lapply(df, function(x) lm(r[x,]~t))
However, I got the following error:
Error in r[x, ] : incorrect number of dimensions
If I remove the [x,], each iteration gives me the same model, in other words using all the observations.
If I use rollapply:
coefs = rollapply(df, 3, FUN = function(x) coef(lm(r~t, data =
as.data.frame(x))), by.column = FALSE, align = "right")
res = rollapply(df, 3, FUN = function(z) residuals(lm(r~t, data =
as.data.frame(z))), by.column = FALSE, align = "right")
Where:
t = seq(0,15,1)
r = (100+50*sin(0.8*t))
df = as.data.frame(t,r)
I get 15 models, but they are all estimated over the entire dataset, providing the same intercepts and coefficients. This is strange as I managed to make rollapply work just before testing it in a new script. For some reason it does not work again, so I am perplexed as to whether R is playing tricks on me, or whether there is something wrong with my code.
How can I adjust these methods to make sure they iterate according to my wishes?

Answer 1

I enclose a possible solution. The idea is to use a vector 1: nrow (df) in the function rollapply to indicate which rows we want to select.
df = data.frame(t = 0:6, r = c(100.00000, 135.86780, 149.97868, 133.77316, 97.08129, 62.15988, 50.19177))
N = nrow(df)
require(zoo)
# Coefficients
coefs <- rollapply(data = 1:N, width = 3, FUN = function(x){
r = df$r[x]
t = df$t[x]
out <- coef(lm(r~t))
return(out)
})
# Residuals
res <- rollapply(data = 1:N, width = 3, FUN = function(x){
r = df$r[x]
t = df$t[x]
out <- residuals(lm(r~t))
return(out)
})