[Rd] RE: as.numeric method for objects of class "difftime"

[Bill.Venables at csiro.au]

Brian,

: -----Original Message-----
: From: Prof Brian Ripley [mailto:ripley at stats.ox.ac.uk] 
: Sent: Sunday, 1 May 2005 2:41 PM
: To: Venables, Bill (CMIS, Cleveland)
: Cc: r-devel at stat.math.ethz.ch
: Subject: Re: [Rd] RE: as.numeric method for objects of class 
: "difftime"
: 
: 
: I am very surprised that people would expect as.numeric to do 
: something 
: meaningful on time differences (or times, come to that).  
: Perhaps the best 
: thing is to stop it altogether.

This would enforce one particular interpretation that may 
inhibit legitimate but unanticipated alternative uses.

: 
: The selection of "auto" was the best for the original 
: purpose: people in 
: general do not want a difference of 3 days expressed in seconds.  The 
: problems only arise if the result is unclassed, and I still do not 
: understand why that is a reasonable thing to do.

No, they may not want the answer in seconds, but if they have
a vector of time differences that includes zero they'll get
them all in seconds anyway.

Let me encapsulate my problem with an example.  I want to use
"elapsed time" as a predictor.  I don't particularly care what 
units the time lag is in, so long as they are all in the _same_ 
units.  What I don't like about the present default is this 
kind of possibility:

> d <- as.POSIXct(paste(2005, 5, 1:3, sep="-"))
> origin <- d[1]

> as.numeric(d - origin)
[1]      0  86400 172800

> as.numeric(d[-1] - origin)
[1] 1 2


Bill.

: On Sun, 1 May 2005 Bill.Venables at csiro.au wrote:
: 
: > I had a couple of private replies to the message below, all very
: > supportive of the idea.  I see that where I should have 
: looked first is
: > at the function difftime, the constructor (which will hardly ever be
: > used except by people who know about its separate existence from
: > Ops.POSIXt).
: >
: > Thus encouraged I formally propose that a method for as.numeric be
: > provided that will not break existing code, but will issue 
: a warning if
: > people convert from difftime to numeric without specifying 
: a time unit
: > to which the resulting numerical quantity will implicitly refer.
: >
: > This is the simplest acceptable solution I could think of.  
: The code I
: > propose is as follows:
: >
: > ###
: > ### S3 method for objects of class 'difftime'
: > ###
: > as.double.difftime <- function(x, units = attr(x, "units"), ...) {
: >  if(missing(units)) {
: >    warning('No time units specified for conversion to numeric.\n"',
: >             units, '" has been assumed.')
: >    return(as.vector(x))
: >  }
: >  cfToSecs <- cumprod(c(secs = 1, mins = 60, hours = 60, days = 24,
: > weeks = 7))
: >  if(!is.element(units, names(cfToSecs)))
: >    stop("Unknown time units. Acceptable time units are\n\t",
: >         paste('"', names(cfToSecs), '", ', sep = "", collapse = ""),
: >         "only.")
: >  as.vector(x) * (cfToSecs[attr(x, "units")]/cfToSecs[units])
: > }
: > ###
: > ### End S3 method
: > ###
: >
: > Note that if people want to continue business as usual and 
: the warning
: > irritates them, the simplest solution is to use as.vector instead of
: > as.numeric.  I think this is reasonable: to me as.vector 
: implies simply
: > a change of structure to the object, whereas as.numeric implies a
: > conversion to a reasonably intuitive numerical object 
: corresponding to
: > the original.
: >
: > In retrospect the best thing to have done would have been 
: not to make
: > "auto" the default units for difftime, but "secs" instead, 
: but I think
: > that particular horse has bolted by now.
: >
: > I would welcome further discussion.
: >
: > Bill.
: >
: > : -----Original Message-----
: > : From: Venables, Bill (CMIS, Cleveland)
: > : Sent: Saturday, 30 April 2005 4:03 PM
: > : To: 'R-Devel (r-devel at stat.math.ethz.ch)'
: > : Subject: as.numeric method for objects of class "difftime"
: > :
: > :
: > : I have just become painfully aware that objects of class
: > : "difftime", generated by the difference of two POSIXct
: > : objects, carry a "units" attribute, which flashes by when the
: > : object is printed, for example.
: > :
: > : The pain was occasioned when I tried to turn these objects
: > : into numberic objects for use elsewhere as a covariate.
: > :
: > : as.numeric(difftime object)
: > :
: > : simply turns off the units attribute and provides a numeric
: > : object which may represent a number of seconds or a number of
: > : days, with no warning as to which.
: > :
: > : I think this is an unfortunate situation, but I can't see how
: > : to rectify it without breaking code that may rely on this
: > : quirky feature.  My inclination is to suggest a method for
: > : as.numeric (ie for as.double) that settles on a single unit,
: > : which for consistency with as.numeric(POSIXct object) should
: > : probably be seconds:
: > :
: > : as.double.difftime <- function(x, ...)
: > :   if(attr(x, "units") == "days") as.vector(x) * 86400 else
: > : as.vector(x)
: > :
: > : but there must now be lots of code out there that has
: > : blythely assumed that the difference will always be a number
: > : of days and others assume it is always seconds.
: > :
: > : At the very least I think the help information should carry a
: > : big red warning about this rather unusual feature.  (It may,
: > : I suppose, but I couldn't find it.)
: > :
: > : Comments?
: > :
: > : Bill Venables,
: > : CMIS, CSIRO Laboratories,
: > : PO Box 120, Cleveland, Qld. 4163
: > : AUSTRALIA
: > : Phone:  +61 7 3826 7251
: > : Fax:    +61 7 3826 7304
: > : Mobile: +61 4 1963 4642
: > : Home:   +61 7 3286 7700
: > : mailto:Bill.Venables at csiro.au
: > : http://www.cmis.csiro.au/bill.venables/
: > :
: > :
: >
: > ______________________________________________
: > R-devel at stat.math.ethz.ch mailing list
: > https://stat.ethz.ch/mailman/listinfo/r-devel
: >
: >
: 
: -- 
: 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
: