On Sat, 9 Mar 2013, Alexandre Sieira wrote:
Thanks for the clarification, Luke.
That is really counter-intuitive behavior. I 100% agree with you that the "for"
documentation should state that assumption explicitly.
I would also like to suggest changing the "for" implementation to issue a warning if the
"seq" argument is a vector of a non-primitive type. That would have saved me a few hours
of debugging in the last few days, so I imagine it must be relevant for more people as well.
Issuing a warning unconditioally would result in far to many spurious
warnings. for() works perfecly fine now for most objects with a class
attribute. I don't see a reasonable way to detect the subset of cases
where this is not true (other than registering a list of classes for()
should warn about, which might be feasible, though might refult in a
significant performance hit.)
Best,
luke
--
Alexandre Sieira
CISA, CISSP, ISO 27001 Lead Auditor
"The truth is rarely pure and never simple."
Oscar Wilde, The Importance of Being Earnest, 1895, Act I
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On Saturday, 9 de March de 2013 at 13:10, luke-tier...@uiowa.edu wrote:
R's for loop is only designed to iterato over primitive types. The
help file says of the seq argument:
seq: An expression evaluating to a vector (including a list and an
expression) or to a pairlist or ¡NULL¢. A factor value will
be coerced to a character vector.
[This could be more emphatic by stating that any class attributes are
igonred or something of that nature.]
Having for() do anything else would require designing an iteration
protocol -- probably would be nice in principle but not easy to do.
Best,
luke
On Sat, 9 Mar 2013, Peter Ehlers wrote:
On 2013-03-09 11:14, R. Michael Weylandt wrote:
On Sat, Mar 9, 2013 at 6:50 PM, David Winsemius <dwinsem...@comcast.net
(mailto:dwinsem...@comcast.net)>
wrote:
I was unable to find the reason for the original coercion in the
help("for") page or the R
Language Definition entry regarding for-loops. On the hunch that coercion
via as.vector
might be occurring,
Behaviorally, it seems to, but the code for do_for in eval.c has
factors special-cased to call
asCharacterFactor so that might not be a robust detail to rely on. The
relevant behavior seems instead to be that there's a
switch on val_type which creates the loop index but doesn't copy all
attributes (like class)
Note that this means a user's as.vector wouldn't be called here:
as.vector.flub <- function(x, ...) letters[x]
foo <- 1:5
class(foo) <- "flub"
as.vector(foo)
for(j in foo) {print(j); print(class(j))}
as.vector.grub <- function(x, ...) match(x, letters)
bar <- letters[1:5]
class(bar) <- "grub"
as.vector(bar)
for(j in bar) {print(j); print(class(j))}
Cheers,
Michael
I think that Michael is right - the problem is with val_type
in the do_for code.
Here's a simplified version of Alexandre's example:
d <- as.Date("2013-03-10")
for(i in seq_along(d)) print(i)
#[1] 1
for(i in d) print(i)
#[1] 15774
where we might have expected to see "2013-03-10".
The essential line in the do_for code seems to me to be:
val_type = TYPEOF(val);
?typeof tells us that R does not have a 'date' type, so:
typeof(d)
#[1] "double"
And the for-loop results follow.
Peter Ehlers
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--
Luke Tierney
Chair, Statistics and Actuarial Science
Ralph E. Wareham Professor of Mathematical Sciences
University of Iowa Phone: 319-335-3386
Department of Statistics and Fax: 319-335-3017
Actuarial Science
241 Schaeffer Hall email: luke-tier...@uiowa.edu
Iowa City, IA 52242 WWW: http://www.stat.uiowa.edu
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