How to flip the data structure of a list of lists-CodePudding

I have a list of lists like so:

a <- list(c(2, 3, 4, 5, 6, 7), c(4, 5, 6, 7, 8), c(6, 7, 8, 9, 10))

> a
[[1]]
[1] 2 3 4 5 6 7

[[2]]
[1] 4 5 6 7 8

[[3]]
[1]  6  7  8  9 10

I want to turn it inside out like so:

> a
[[1]]
NULL

[[2]]
[1] 1

[[3]]
[1] 1

[[4]]
[2] 1 2

[[5]]
[2] 1 2

[[6]]
[3] 1 2 3

[[7]]
[3] 1 2 3

[[8]]
[2] 2 3

[[9]]
[1] 3

[[10]]
[1] 3

I tried this with nested loops, but it is taking too much time and growing lists within loops is very slow. I have 60,000 sublists in my main list, so is there vectorized solution for this ?

Thanks in Advance.

CodePudding user response：

Here is a base R way.

lapply(seq.int(max(unique(unlist(a)))), \(i){
  which(sapply(a, \(x) any(i == x)))
})

CodePudding user response：

Another way:

 searchInList <- function(list2search, e){
      idx2search <- 1:length(list2search)
      list2search2 <- lapply(list2search, `length<-`, max(lengths(list2search)))
      output <- matrix(unlist(list2search2), ncol = length(list2search2[[1]]), byrow = TRUE)
      idx <- apply(output, 1, function(x){ (e %in% x) } )
      return(idx2search[idx])
    }

result <- lapply(1:max(unlist(a)), function(x) { searchInList(a, x) } )