[R] Unexpected behavior looping through sequence of dates

[luke-tierney at uiowa.edu]

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
>
>
> Sent with Sparrow (http://www.sparrowmailapp.com/?sig)
>
>
> On Saturday, 9 de March de 2013 at 13:10, 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.
>>
>> 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 (mailto: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
>>>
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>>> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
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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-tierney at uiowa.edu (mailto:luke-tierney at uiowa.edu)
>> Iowa City, IA 52242 WWW: http://www.stat.uiowa.edu
>
>
>
>

-- 
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