[R] Date Not Staying in Date Format

[Prof Brian Ripley]

tapply(df$Date, df$SomeFactor, max, simplify=FALSE)

works, i.e. it is the unlist() which is losing the class (and perhaps 
unlist or tapply should be a bit cleverer).

On Mon, 30 Jan 2006, David Randel wrote:

> I have a column in a data frame that has a class of "Date" and a mode of
> "numeric".  When I:
>
> max(df$Date)
>
> My output stays in Date format, i.e. "2006-01-03".
>
> However, when I run the following statment:
>
> tapply(df$Date, df$SomeFactor, max)
>
> my output looks like this:  9129   9493   9861  10226  10591  10956  11320
> 11687  12052  12417
>
> The returned object is of mode "numeric" and class "array".  Each array
> element is of mode "numeric" and class "numeric".  I believe that this is
> the integer representation of my date.  I can't seem to convert it back to a
> date.
>
> How do I get these to be intrepreted as a date instead of a number?
>
> Thanks,
> ~Dave R.
>
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-- 
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)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595