The example is confusing and debatable, but not an obvious bug. And
your presentation of it is the cause of much of the confusion
(unintentionally I'm sure).
First, slipping from the as() function to methods for the coerce()
function might surprise a less experienced user. And in fact, that is
the point here. If you look at the as() function, it jumps through
several hoops and in particular selects a method from coerce in such a
way as NOT to use inheritance on the from= argument. (I think this
makes sense in this case). So I would assert that your selectMethod()
output below came from a different session than the as(1:4, "numeric").
Starting from a clean session with R 2.10.1:
> class(as(1:4,"numeric"))
[1] "integer"
> selectMethod("coerce", c("integer","numeric"))
Method Definition:
function (from, to = "numeric", strict = TRUE)
if (strict) {
class(from) <- "numeric"
from
} else from
<environment: namespace:methods>
Signatures:
from to
target "integer" "numeric"
defined "integer" "numeric"
Note, no call to as.numeric(). In a session without a previous call to
as(), your selectMethod() call triggered a standard inherited method
selection. And if you had then gone on to as(), the result would have
been different.
In a different clean session:
> getMethod("coerce", c("integer", "numeric"))
Error in getMethod("coerce", c("integer", "numeric")) :
No method found for function "coerce" and signature integer, numeric
> selectMethod("coerce", c("integer", "numeric"))
Method Definition:
function (from, to, strict = TRUE)
{
value <- as.numeric(from)
if (strict)
attributes(value) <- NULL
value
}
<environment: namespace:methods>
Signatures:
from to
target "integer" "numeric"
defined "ANY" "numeric"
> class(as(1:4,"numeric"))
[1] "numeric"
No argument about this being confusing. Perhaps one should prohibit
standard selectMethod() on coerce() but that seems a bit arcane to
thwart folks like you!
Suggested improvements for the current implementation are welcome, so
long as they consider the best definition of as() in the general sense.
Regards,
John
On 3/31/10 3:52 PM, Hervé Pagès wrote:
Hi,
> class(as(1:4, "numeric"))
[1] "integer"
Surprising but an explanation could be that an integer
vector being a particular case of numeric vector, this
coercion has nothing to do because 1:4 is already numeric.
And indeed:
> is.numeric(1:4)
[1] TRUE
> is.numeric(as(1:4, "numeric"))
[1] TRUE
However, 'as(1:4, "numeric")' is inconsistent with
> class(as.numeric(1:4))
[1] "numeric"
And, even more confusing, if you look at the coerce,ANY,numeric
method:
> selectMethod("coerce", c("integer", "numeric"))
Method Definition:
function (from, to, strict = TRUE)
{
value <- as.numeric(from)
if (strict)
attributes(value) <- NULL
value
}
<environment: namespace:methods>
Signatures:
from to
target "integer" "numeric"
defined "ANY" "numeric"
it calls as.numeric()!
So how can 'as(1:4, "numeric")' not return the same thing as
'as.numeric(1:4)' looks like a mystery to me. Could it be
conceivable that I found a bug?
Cheers,
H.
> sessionInfo()
R version 2.11.0 Under development (unstable) (2010-03-15 r51282)
x86_64-unknown-linux-gnu
locale:
[1] LC_CTYPE=en_CA.UTF-8 LC_NUMERIC=C
[3] LC_TIME=en_CA.UTF-8 LC_COLLATE=en_CA.UTF-8
[5] LC_MONETARY=C LC_MESSAGES=en_CA.UTF-8
[7] LC_PAPER=en_CA.UTF-8 LC_NAME=C
[9] LC_ADDRESS=C LC_TELEPHONE=C
[11] LC_MEASUREMENT=en_CA.UTF-8 LC_IDENTIFICATION=C
attached base packages:
[1] stats graphics grDevices utils datasets methods base
______________________________________________
R-devel@r-project.org mailing list
https://stat.ethz.ch/mailman/listinfo/r-devel
