R expand and process data with nested for loops tail

Question

I need to expand some data and then restrict which data remains through tail.

Example of data:

list_1 <- list(1:15)
list_2 <- list(16:30)
list_3 <- list(31:45)
short_lists[[1]] <- list_1
short_lists[[2]] <- list_2
short_lists[[3]] <- list_3
str(short_lists)
List of 3
 $ :List of 1
  ..$ : int [1:15] 1 2 3 4 5 6 7 8 9 10 ...
 $ :List of 1
  ..$ : int [1:15] 16 17 18 19 20 21 22 23 24 25 ...
 $ :List of 1
  ..$ : int [1:15] 31 32 33 34 35 36 37 38 39 40 ...

And how long I want my tail of a given list to be from list_1, list_2, list_3

how_long <- 
c(4,2,5,3,6,4,7,5,8,6,9,7,10,8,2,4,6,8,10,12,14,10,9,7,11)

And I expand through nested for loops and try to get the tail of the expanded lists, but just get the expanded lists.

for (i in 1:length(how_long)) {
for (j in 1:length(short_lists)) { 
tail_temp[[j]][i] <- tail(short_lists2[[j]], n = how_long[i])
}
}

And this yields:

str(tail_temp)
List of 3
 $ :List of 25
  ..$ : int [1:15] 1 2 3 4 5 6 7 8 9 10 ...
  ..$ : int [1:15] 1 2 3 4 5 6 7 8 9 10 ...
  ..$ : int [1:15] 1 2 3 4 5 6 7 8 9 10 ...
  [snip]
  ..$ : int [1:15] 1 2 3 4 5 6 7 8 9 10 ...
 $ :List of 25
  ..$ : int [1:15] 16 17 18 19 20 21 22 23 24 25 ...
  ..$ : int [1:15] 16 17 18 19 20 21 22 23 24 25 ...
  ..$ : int [1:15] 16 17 18 19 20 21 22 23 24 25 ...
  [snip]
  ..$ : int [1:15] 16 17 18 19 20 21 22 23 24 25 ...
 $ :List of 25
  ..$ : int [1:15] 31 32 33 34 35 36 37 38 39 40 ...
  ..$ : int [1:15] 31 32 33 34 35 36 37 38 39 40 ...
  ..$ : int [1:15] 31 32 33 34 35 36 37 38 39 40 ...
  [snip]
  ..$ : int [1:15] 31 32 33 34 35 36 37 38 39 40 ...

And I'm happy the j's were expanded, but I never get to the tail call and what I'm seeking:

str(tail_temp)
List of 3
 $ :List of 25
  ..$ : int [1:4] 12 13 14 15
  ..$ : int [1:2] 14 15
  ..$ : int [1:5] 11 12 13 14 15
[snip]

so what simple thing am I missing. Any help appreciated. Thanks.

Answer 1

Very close indeed.

I prefer vectors in lists over R.
If you're familiar with python, vectors behave like 'lists' in python.
Where as lists in R behave like dictionaries.

Therefore, you just needed to unlist first (into a vector),
to then assign to an item of a list, hence it should be assigned to:
tail_temp[[i]][[j]] instead of tail_temp[i][[j]]

list_1 <- list(1:15)
list_2 <- list(16:30)
list_3 <- list(31:45)
short_lists = list()
short_lists[[1]] <- list_1
short_lists[[2]] <- list_2
short_lists[[3]] <- list_3

how_long <- c(4,2,5,3,6,4,7,5,8,6,9,7,10,
              8,2,4,6,8,10,12,14,10,9,7,11)

tail_temp = list()

for (i in 1:length(short_lists)){
    tail_temp[[i]] = list()
    for (j in 1:length(how_long)){
       tail_temp[[i]][[j]] <- tail(unlist(short_lists[[i]]), n = how_long[j])
    }
}

Output

[[1]]
[[1]][[1]]
[1] 12 13 14 15

[[1]][[2]]
[1] 14 15

[[1]][[3]]
[1] 11 12 13 14 15
…
[[3]][[23]]
[1] 37 38 39 40 41 42 43 44 45

[[3]][[24]]
[1] 39 40 41 42 43 44 45

[[3]][[25]]
 [1] 35 36 37 38 39 40 41 42 43 44 45