Unexpected behavior looping through sequence of dates

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