How to make an infinitely recursive list in R: confuse [ and [[ -

edit: question silly, confused [ , [[ (thanks @josilber), can't delete it.

how can 1 make infinitely recursive list, l == l[1], in r ?
terrible way (for r novice) @ dataset fmri in package astsa:

data( fmri, package="astsa" )  # list fmri[1] == fmri[1][1] ... ??  print_recursive = function( x ){  # x x[1] x[1][1] ... while list     for( j in 1:5 ){         cat( class(x), len(x), names(x), "\n" )         str(x)         cat( "\n" )          x = x[1]  # <-- wrong, should x[[1]]         if( class(x) != "list" )  break     }     x }  x = print_recursive( fmri ) 

the answer "how can 1 make infinitely recursive list, l == l[1], in r" is impossible, because need infinite amount of memory store list of infinite recursive depth.

that being said, can build recursive list of specified depth simple loop, @ each iteration creating new list stores old list 1 of elements:

l <- list() depth <- 50 (k in seq(depth-1)) {   l <- list(l) } 

you write recursive function check depth of recursive list:

recursive.depth <- function(l) {   if (!is.list(l)) 0   else if (length(l) == 0) 1   else 1+max(sapply(l, recursive.depth)) } recursive.depth(l) # [1] 50 recursive.depth(fmri) # [1] 1 

getting example question, list have not recursive @ (it's list of matrices). reason think recursive indexing l[1] subsets list (aka returns list first element). instance, consider following simple list:

(l <- list(2)) # [[1]] # [1] 2 

no matter how many times subset [ notation, exact same list:

l[1] # [[1]] # [1] 2 l[1][1] # [[1]] # [1] 2 

running list l through print_recursive function result in infinite loop. if wanted extract first element of list instead of subsetting list itself, should use [[ notation (e.g. l[[1]]).