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List:       r-help
Subject:    Re: [R] Convert character date time to R date-time variable.
From:       Duncan Murdoch <murdoch.duncan () gmail ! com>
Date:       2023-12-08 21:56:10
Message-ID: ed18cc13-7836-416d-bd72-1a9bc03798ea () gmail ! com
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On 08/12/2023 4:30 p.m., David Winsemius wrote:
> 
> On 12/7/23 08:21, Sorkin, John wrote:
> > Colleagues,
> > 
> > I have a matrix of character data that represents date and time. The format of \
> > each element of the matrix is "2020-09-17_00:00:00"
> > How can I convert the elements into a valid R date-time constant?
> 
> You will not be able to store these datetime values in an R matrix, at
> least as class POSIXct. You could with class POSIXlt, but I've not seen
> it used before but it does appear possible since matrices can contain
> lists.
> 
> R matrices do no provide the capability to assign attributes, so only
> atomic types and lists can be elements. If you wanted to maintain the
> same structure, your first step might be to coerce to a data.frame and
> then proceed, or to first construct a vector and then use as the first
> argument to `matrix`.

The general sentiment is correct (it's hard to put POSIXct elements in 
matrices), but it's not impossible.  Almost all R objects can have 
attributes.

For example,

   m <- matrix(Sys.time(), 2,2)

converts the time to a numerical value, but you can view it as a POSIXlt 
object using

   class(m) <- c("POSIXct", class(m))

Subsetting works, e.g. m[1,1] is a time, etc.

Duncan Murdoch
> 
> 
> Best;
> 
> David
> 
> > 
> > Thank you,
> > John
> > 
> > 
> > 
> > John David Sorkin M.D., Ph.D.
> > Professor of Medicine, University of Maryland School of Medicine;
> > 
> > Associate Director for Biostatistics and Informatics, Baltimore VA Medical Center \
> > Geriatrics Research, Education, and Clinical Center; 
> > PI  Biostatistics and Informatics Core, University of Maryland School of Medicine \
> > Claude D. Pepper Older Americans Independence Center; 
> > Senior Statistician University of Maryland Center for Vascular Research;
> > 
> > Division of Gerontology and Paliative Care,
> > 10 North Greene Street
> > GRECC (BT/18/GR)
> > Baltimore, MD 21201-1524
> > Cell phone 443-418-5382
> > 
> > 
> > 
> > ______________________________________________
> > R-help@r-project.org mailing list -- To UNSUBSCRIBE and more, see
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> > and provide commented, minimal, self-contained, reproducible code.
> 
> ______________________________________________
> R-help@r-project.org mailing list -- To UNSUBSCRIBE and more, see
> https://stat.ethz.ch/mailman/listinfo/r-help
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> and provide commented, minimal, self-contained, reproducible code.

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