I have two lists. The first one has an empty element. I'd like to replace that empty element with the first vector of the third list element of another list.

```
l1 <- list(a=1:3,b=4:9,c="")
l2 <- list(aa=11:13,bb=14:19,cc=data.frame(matrix(100:103,ncol=2)))

l1[sapply(l1, `[[`, 1)==""] <- l2[[3]][[1]]

```

Using `sapply`, I can identify which elements are empty. However, when I try to assign a vector to this empty element: I get this error message:

> 
> Warning message: In l1[sapply(l1, `[[`, 1) == ""] <- l2[[3]][[1]] :  
> 
>  number of items to replace is not a multiple of replacement length
> 
> 
> 

This is only a warning, but the result I get is not the one I want. This is the l1 I get:

```
> l1
$a
[1] 1 2 3

$b
[1] 4 5 6 7 8 9

$c
[1] 100

```

This is what I need (two elements in $c):

```
> l1
$a
[1] 1 2 3

$b
[1] 4 5 6 7 8 9

$c
[1] 100 101

```