rbind doing different things for single versus multiple arguments - r

Question

I am new to R and came over code that uses do.call("rbind", df.list) to combine a list of data frames.
The data frames have arrays as columns and rbind does remove the arrays, but only if there are at least two elements in the list to combine.
Quick example:
> class(rbind(data.frame(a=array(1,2)), data.frame(a=array(3,4)))$a)
[1] "numeric"
> class(rbind(data.frame(a=array(1,2)))$a)
[1] "array"
Is this a bug in rbind? It appears if it is called with one argument, it does just return that argument, while if called with multiple, it does remove arrays.
How can I "unarray" such a data frame if length(df.list) == 1?
Example of what I need:
> df.list1 <- list(data.frame(a=array(1,2), b=array("a")), data.frame(a=array(3,4), b=array("b")))
> df.list2 <- list(data.frame(a=array(1,2), b=array("a")))
> df.combined1 <- do.call("rbind", df.list1)
> df.combined2 <- do.call("rbind", df.list2)
> class(df.combined1$a)
[1] "numeric"
> class(df.combined2$a)
[1] "array"
The goal is to have a data frame df.combined not having array columns independent whether df.list had one or multiple elements. The type and number of the data frame columns are unknown in advance.

Answer 1

Lets start with:
class(rbind(data.frame(a=array(1,2)), data.frame(a=array(3,4))))
class(rbind(data.frame(a=array(1,2))))
Both these have class of data.frame.
Now, as you noticed:
> class(rbind(data.frame(a=array(1,2)), data.frame(a=array(3,4)))$a)
[1] "numeric"
> class(rbind(data.frame(a=array(1,2)))$a)
[1] "array"
The first one is expected, however, second one is unexpected due to the way rbind method for data.frame. As per the documentation for rbind:
... It [then] takes the classes of the columns from the first data frame,...
If you want to coerce the class in case of a single array to numeric, then you can use something like this:
ifelse(length(df.list) == 1,
class(rbind(data.frame(a=as.vector(array(1,2))))$a),
...)
Coercing it as.vector gets rid of the array class.
(EDIT: Depending on what you want, you might also benefit from the discussion in the comments below!)
Lastly, note that this is an issue only for a one-dimensional array. For higher dimensions, you get the appropriate class:
class(rbind(data.frame(a=array(as.numeric(1:10),c(2,5))))$a.1)
EDIT: Based on your update, I think here is what you want:
df.list1 <- list(data.frame(a=array(1,2), b=array("a")), data.frame(a=array(3,4), b=array("b")))
df.list2 <- list(data.frame(a=array(1,2), b=array("a")))
cobmineDFList <- function(df.list) {
temp <- do.call(rbind, df.list)
if(class(temp$a) == "array") temp$a <- as.numeric(temp$a)
temp
}
df.combined1 <- cobmineDFList(df.list1)
df.combined2 <- cobmineDFList(df.list2)
class(df.combined1$a)
class(df.combined2$a)
Hope this helps!