(I enlarged this mail from r-core to r-devel since it's an interesting
question for general discussion.)
Douglas Bates wrote:>
> In a recent r-patched I notice that when df is a data.frame
> as(df, "list") does not propagate the names whereas as.list(df)
does.
> Is this intentional?
Well, it's explicit anyway. The as() function uses method dispatch for
coerce() to find methods.
R> selectMethod("coerce", c("data.frame",
"list"))
function (from, to, strict = TRUE)
{
value <- as.list(from)
if (strict)
attributes(value) <- NULL
value
}
(This is actually the method for c("ANY","list").)
The strict=FALSE option is analogous to what would happen in method
dispatch if "list" were a superclass of "data.frame" (which
it isn't
since "data.frame" is an S3 class.)
Two points about as.list:
1- Generally, it leaves all attributes alone.
2- There is an S3 method, as.list.data.frame, that deletes the "class"
and "row.names" attributes, but leaves the names.
So putting these all together: Given 1, I think the general behavior
needs to stay the same (in fact, one might argue that even with
strict=FALSE, the class attribute should be reset).
But it seems reasonable to promote the as.list.data.frame method to an
S4 method for coerce().
This prompts a general question: do we want to promote certain S3
methods? (It happens that as.list.data.frame is the only non-default
method for as.list, but there are 31 apparent non-default as.* methods.
Promoting them would encourage use of as() for consistent programming.)
John
PS: The comparative behavior of as() and as.list() is illustrated below:
R> attributes(as.list(women))
$names
[1] "height" "weight"
R> attributes(as(women, "list"))
NULL
R> attributes(as(women, "list", strict=F))
$names
[1] "height" "weight"
R> x <- list(a=1,b=2)
R> class(x) <- "something"
R> attr(x, "foo") <- pi
R> as.list(x)
$a
[1] 1
$b
[1] 2
attr(,"class")
[1] "something"
attr(,"foo")
[1] 3.141593
R> as(x, "list")
[[1]]
[1] 1
[[2]]
[1] 2
and as(x, "list", FALSE) is the same result as as.list(x).
>
> > data(women)
> > class(women)
> [1] "data.frame"
> > as.list(women)
> $height
> [1] 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72
>
> $weight
> [1] 115 117 120 123 126 129 132 135 139 142 146 150 154 159 164
>
> > as(women, "list")
> [[1]]
> [1] 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72
>
> [[2]]
> [1] 115 117 120 123 126 129 132 135 139 142 146 150 154 159 164
>
> _______________________________________________
--
John M. Chambers jmc@bell-labs.com
Bell Labs, Lucent Technologies office: (908)582-2681
700 Mountain Avenue, Room 2C-282 fax: (908)582-3340
Murray Hill, NJ 07974 web: http://www.cs.bell-labs.com/~jmc
