[R] Retrieving data from nested lists

[David Winsemius]

On May 26, 2015, at 3:59 PM, blue honour via R-help wrote:

> Dear R users,
> I have a question regarding retrieving data from nested lists. To illustrate, here is an example input:
> d1<-data.table(v1=c(3,4),v2=c(2,5))
> d2<-data.table(v1=c(1,2),v2=c(1,1))l1<-list(d1,name="test")l2<-list(d2,name="test2")motherlist<-list()motherlist$l1<-l1motherlist$l2<-l2
> Let's say we are given motherlist as above. We would like to get the union of the contents of v1 vector from d1 data.table and the v1 vector from d2 data.table. How can we achieve this in a compact one line command using motherlist as input please? 
> Thank you. 
> 
> 
> 
> 	[[alternative HTML version deleted]]

Your posting is a good example of the reason we ask people _not_ to post in HTNL. Linefeeds (among many other useful bits of information) get lost:

d1<-data.table(v1=c(3,4),v2=c(2,5))
d2<-data.table(v1=c(1,2),v2=c(1,1))
l1<-list(d1,name="test")
l2<-list(d2,name="test2")
motherlist<-list()
motherlist$l1<-l1
motherlist$l2<-l2



> with( motherlist, c( l1[1][[1]]$v1, l2[1][[1]]$v1) )
[1] 3 4 1 2

OR:
> with( motherlist, c( l1[[1]]$v1, l2[[1]]$v1) )
[1] 3 4 1 2

That used the inheritance of data.tables from data.frames. This is data.table extraction syntax:

> with( motherlist, c( l1[[1]][ ,v1], l2[[1]][ ,v1]) )
[1] 3 4 1 2

Another approach with the display of intermediate values:

> sapply(motherlist, "[", 1)
$l1
   v1 v2
1:  3  2
2:  4  5

$l2
   v1 v2
1:  1  1
2:  2  1

> sapply( sapply(motherlist, "[", 1) , "[[", 1)
     l1 l2
[1,]  3  1
[2,]  4  2

> c( sapply( sapply(motherlist, "[", 1) , "[[", 1) )
[1] 3 4 1 2

Please read the posting guide.

> 
> ______________________________________________
> R-help at r-project.org mailing list -- To UNSUBSCRIBE and more, see
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.

David Winsemius
Alameda, CA, USA