CODEWITHSUNDEEP
rlistpopulate
DaniCee
DaniCee

Reputation: 3207

How to properly populate a nested list and access its subelements in R

I just need to populate a nested list inside two for loops (for chain ALPHA and BETA, and for id), but I am getting a subscript out of bounds error.

This reproduces the problem:

mylist <- list()
mylist[['ALPHA']][[1]][['id1']]=46
mylist[['ALPHA']][[2]][['id1']]=28
mylist[['ALPHA']][[1]][['id2']]=87
mylist[['ALPHA']][[2]][['id2']]=12

However, if I do the following, I do not get the error, why is that?

mylist <- list()
mylist[['ALPHA']][['id1']][[1]]=46
mylist[['ALPHA']][['id1']][[2]]=28
mylist[['ALPHA']][['id2']][[1]]=87
mylist[['ALPHA']][['id2']][[2]]=12

I would rather prefer the structure above, since down the line I need the list with all ids for 1 and the list for all ids for 2.

These are easy to obtain with the structure above:

mylist1=mylist[['ALPHA']][[1]]
mylist2=mylist[['ALPHA']][[2]]

But how would I obtain such lists if the alternative structure is the only way to solve the error?

I tried this, but does not get me what I want (there is one more nested level):

sapply(mylist, "[", 2)

Any help? Thanks!

Upvotes: 0

Views: 52

Answers (1)

D.J
D.J

Reputation: 1304

You can "prebuild" your list and then fill it.

mylist <- list("ALPHA" = vector("list", 2))

> mylist
$ALPHA
$ALPHA[[1]]
NULL

$ALPHA[[2]]
NULL

Then your code works:

> mylist[['ALPHA']][[1]][['id1']]=46
> mylist[['ALPHA']][[2]][['id1']]=28
> mylist[['ALPHA']][[1]][['id2']]=87
> mylist[['ALPHA']][[2]][['id2']]=12

> mylist

$ALPHA
$ALPHA[[1]]
$ALPHA[[1]]$id1
[1] 46

$ALPHA[[1]]$id2
[1] 87


$ALPHA[[2]]
$ALPHA[[2]]$id1
[1] 28

$ALPHA[[2]]$id2
[1] 12

As to why it works when starting from the "back"... Afaik when starting from [[2]] R is creating a list of length 2 so [[1]] has to be added at runtime. This is not the case for [[2]] when starting from [[1]], which causes the subscript out of bounds error. The second list entry needs to be added explicitly with a list() command.

Upvotes: 2

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