[R] Unexpected behavior looping through sequence of dates

[luke-tierney at uiowa.edu]

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> 
>> 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-tierney at uiowa.edu
Iowa City, IA 52242                 WWW:  http://www.stat.uiowa.edu