R help - Mar 2013 - Unexpected behavior looping through sequence of dates

I understand that the two following loops should produce the exact same output.
However, they do not. It appears that looping directly through the sequence of
Date objects somehow makes them be coerced to numeric:
> date1 = "20130301"
> date2 = "20130302"
> 
> d1 = as.Date(date1, format="%Y%m%d", tz="GMT")
> d2 = as.Date(date2, format="%Y%m%d", tz="GMT")
> 
> range = seq(from=d1, to=d2, by="day")
> for (i in 1:length(range)) print(class(range[i]))
[1] "Date"
[1] "Date"
> 
> for (i in seq(from=d1, to=d2, by="day")) print(class(i))
[1] "numeric"
[1] "numeric"

This is output from R version 2.15.2 (2012-10-26) on Mac OS X 10.8.2.

Is this expected behavior?

-- 
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 Mar 9, 2013, at 9:24 AM, Alexandre Sieira wrote:
> I understand that the two following loops should produce the exact same
output. However, they do not. It appears that looping directly through the
sequence of Date objects somehow makes them be coerced to numeric:
> 
>> date1 = "20130301"
>> date2 = "20130302"
>> 
>> d1 = as.Date(date1, format="%Y%m%d", tz="GMT")
>> d2 = as.Date(date2, format="%Y%m%d", tz="GMT")
>> 
>> range = seq(from=d1, to=d2, by="day")
>> for (i in 1:length(range)) print(class(range[i]))
> 
> [1] "Date"
> [1] "Date"
>> 
>> for (i in seq(from=d1, to=d2, by="day")) print(class(i))
> [1] "numeric"
> [1] "numeric"
> 
> This is output from R version 2.15.2 (2012-10-26) on Mac OS X 10.8.2.
> 
> Is this expected behavior?
It wasn't expected by me either, but it is what both my Mac R 2.15.3 and my
Windows(32bit) 2.15.1 display. If you put those dates in a list, you do not get
the coercion:
> for (i in list(d1,d2)) print(class(i))
[1] "Date"
[1] "Date"

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, I tried
with with a factor vector and did see that it was coerced to
"character" which is the behavior that would be expected if that were
happening behind the scenes:

 ff <- factor(letters[1:3])
 as.vector(ff)
#[1] "a" "b" "c"
 for (i in ff) print(class(i))
#[1] "character"
#[1] "character"
#[1] "character"

(I also tried searching for loop in the R Internals document and found nothing
that explained this to me.)

-- 
David.
> -- 
> 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)
> 
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David Winsemius
Alameda, CA, USA