R help - May 2006 - sapply and Date objects

This is probably a dumb question, but I cannot figure it out. Why does
this happen?

 dt <- as.Date("1954-02-01")
> as.character(dt)
[1] "1954-02-01"
> sapply(c(dt), as.character)
[1] "-5813"

Thanks.

FS

>From ?sapply we see that sapply requires a list or
atomic vector.   If its not a list then looking at the first
few two lines of sapply we see it tries to convert
the first arg to a list using as.list.  Note that using
your dt:
> as.list(dt)
[[1]]
[1] -5813

so try this instead:

sapply(list(dt), as.character)



On 5/20/06, Fernando Saldanha <fsaldan1 at gmail.com>
wrote:
> This is probably a dumb question, but I cannot figure it out. Why does
> this happen?
>
>  dt <- as.Date("1954-02-01")
> > as.character(dt)
> [1] "1954-02-01"
> > sapply(c(dt), as.character)
> [1] "-5813"
>
> Thanks.
>
> FS
>
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>

On Sat, 2006-05-20 at 11:01 -0400, Fernando Saldanha
wrote:
> This is probably a dumb question, but I cannot figure it out. Why does
> this happen?
> 
>  dt <- as.Date("1954-02-01")
> > as.character(dt)
> [1] "1954-02-01"
> > sapply(c(dt), as.character)
> [1] "-5813"
> 
> Thanks.
> 
> FS
Does this help?
> dt <- as.Date("1954-02-01")
> dt
[1] "1954-02-01"

# Note the numeric value of dt is an offset from the default
# base date in R of 1970-01-01
> str(dt)
Class 'Date'  num -5813
> dt + 5813
[1] "1970-01-01"


When using sapply(), the 'X' argument is first coerced to a list which
is then passed to lapply(). So:
> as.list(dt)
[[1]]
[1] -5813

> str(as.list(dt))
List of 1
 $ : num -5813


Note that the result of the coercion to a list loses the Date class
attribute. Thus, you end up with just a numeric value, which you are
then coercing to a character vector.

So, in effect you are doing:
> unlist(lapply(as.list(dt), as.character))
[1] "-5813"


HTH,

Marc Schwartz

?sapply  says

        X: list or (atomic) vector to be used.

now dt is neither, and it has been coerced to a list, losing its class.

Reading the help page for the function often resolves such questions.


On Sat, 20 May 2006, Fernando Saldanha wrote:
> This is probably a dumb question, but I cannot figure it out. Why does
> this happen?
>
> dt <- as.Date("1954-02-01")
>> as.character(dt)
> [1] "1954-02-01"
>> sapply(c(dt), as.character)
> [1] "-5813"

-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
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