Find index position in nested lists for match

This can also be done with rrapply in the rrapply-package (extended version of base rapply). The condition argument decides to which elements f should be applied, and the .xpos argument in the f function evaluates to the position of the element in the nested list. how = "flatten" discards all list elements that do not satisfy condition and returns a flattened list:

library(rrapply)

rrapply(f, condition = function(x) x == 11, f = function(x, .xpos) .xpos, how = "flatten")
#> [[1]]
#> [1] 2 3 2 1

foo = function(x, sep = ".") {
    names(x) = paste0(seq_along(x))
    while(any(sapply(x, class) == "list")) {
        ind = sapply(x, class) == "list"
        temp = unlist(x[ind], recursive = FALSE)
        names(temp) = paste0(rep(names(x)[ind], lengths(x[ind])),
                             sep,
                             sequence(lengths(x[ind])))
        x = c(x[!ind], temp)
    }
    return(x)
}
f2 = foo(f)
names(which(unlist(f2) == 11))
#[1] "2.3.2.1"

You can try melt() from reshape2:

melted_lst <- reshape2::melt(f) 
melted_lst[sort(colnames(melted_lst))][melted_lst$value == 11, ]

  L1 L2 L3 L4 value
7  2  3  2  1    11

Or with melt() from data.table (as mentioned by @IceCreamToucan):

melted_lst <- data.table::melt(f)
melted_lst[sort(colnames(melted_lst))][melted_lst$value == 11, ]