How to names lists under list when using lapply?

Question

I have a two nested loop which I want to do it with lapply instead of for loops. I have made up the following simple example:

a<-as.list(c(1,2))  
b<-as.list(c(6,7))  
results<-lapply(a, function(x) lapply(b, function(y) x+y))

> results  
[[1]]  
[[1]][[1]]  
[1] 7  

[[1]][[2]]  
[1] 8  


[[2]]  
[[2]][[1]]  
[1] 8

[[2]][[2]]  
[1] 9

how can I assign names to this list, I can assign names to the first level using names(result)<-a, but I do not know how to do it for the second level. And naming should be done in loop since in my main program length of b may change. I appreciate if any one can give me some hint.

Answer 1

You should note that lapply() itself is just a wrapper for a well constructed for() loop, so you're not gaining any efficiency, just perhaps readability. That aside, the easiest approach is to add names to the lists going into your nested lapply() calls:

a<-as.list(c(1,2))
b<-as.list(c(6,7)) 
names(a) <- c("a","b")
names(b) <- c("c", "d")
results<-lapply(a, function(x) lapply(b, function(y) x+y))

This approach yields:

> results
$a
$a$c
[1] 7

$a$d
[1] 8


$b
$b$c
[1] 8

$b$d
[1] 9

You can of course then rename nested components like this:

> names(results$a) <- c("e","f")
> results$a
$e
[1] 7

$f
[1] 8