[R] Unexpected behavior looping through sequence of dates

[David Winsemius]

On Mar 9, 2013, at 1:10 PM, <luke-tierney at 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.

except it works fine with lists:

for ( i in as.list(seq(Sys.Date(), length=4,by=1) ) ) print(class(i))
[1] "Date"
[1] "Date"
[1] "Date"
[1] "Date"


> 
> 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 <dwinsemius at 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
>> 
>> ______________________________________________
>> R-help at r-project.org mailing list
>> https://stat.ethz.ch/mailman/listinfo/r-help
>> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
>> and provide commented, minimal, self-contained, reproducible code.
>> 
> 
> -- 
> 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-tierney at uiowa.edu
> Iowa City, IA 52242                 WWW:  http://www.stat.uiowa.edu______________________________________________
> R-help at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.

David Winsemius
Alameda, CA, USA