CODEWITHSUNDEEP
rlist
jay.sf
jay.sf

Reputation: 72673

How to apply functions on deep nested lists in a clean way?

In nested lists like this one

l <- list(list(list(111, 112, 113), list(121, 122, 123)),
          list(list(211, 212, 213), list(221, 222, 223)))

> str(l )
List of 2
 $ :List of 2
  ..$ :List of 3
  .. ..$ : num 111
  .. ..$ : num 112
  .. ..$ : num 113
  ..$ :List of 3
  .. ..$ : num 121
  .. ..$ : num 122
  .. ..$ : num 123
 $ :List of 2
  ..$ :List of 3
  .. ..$ : num 211
  .. ..$ : num 212
  .. ..$ : num 213
  ..$ :List of 3
  .. ..$ : num 221
  .. ..$ : num 222
  .. ..$ : num 223

we can apply a function, e.g. length() on the first level by this

lapply(l, length)

But when applying functions on deeper nested levels the size of my code increases exponentially...

# apply length() on second level
lapply(l, function(x)
  lapply(x, function(y) length(y)))

# square every list element of second level
lapply(l, function(x) 
  lapply(x, function(y) 
    lapply(y, function(z) z^2)))

Or, to change elements that appear at certain positions in lists of lower levels (i.e. modifications of LHS), I have no better idea than do a for loop.

# subtract 1 from first element of each second level list
for(i in sequence(length(l))) {
  l[[i]][[1]][[1]] <- l[[i]][[1]][[1]] - 1
  l[[i]][[2]][[1]] <- l[[i]][[2]][[1]] - 1
}

Have I missed something? Are there any cleaner base R solutions?

Upvotes: 2

Views: 495

Answers (1)

s_baldur
s_baldur

Reputation: 33488

In my experience lists are not easy to master (hope to get there one day). I don't know of a magic bullet but here are some marginal steps you can do simplify your current code:

lengths(l)
# instead of
lapply(l, length)


lapply(l, lengths)
# instead of
lapply(l, function(x)
  lapply(x, function(y) length(y)))


rapply(l, function(x) x^2, how="list") # credit to Cath
# instead of
lapply(l, function(x) 
  lapply(x, function(y) 
    lapply(y, function(z) z^2)))


for (i in seq_along(l)) {
  l[[i]] <- lapply(l[[i]], function(x) {x[[1]] <- x[[1]] - 1; x})
}
# Instead of 
for(i in sequence(length(l))) {
  l[[i]][[1]][[1]] <- l[[i]][[1]][[1]] - 1
  l[[i]][[2]][[1]] <- l[[i]][[2]][[1]] - 1
}

Upvotes: 3

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