Brian,
: -----Original Message-----
: From: Prof Brian Ripley [mailto:[EMAIL PROTECTED]
: Sent: Sunday, 1 May 2005 2:41 PM
: To: Venables, Bill (CMIS, Cleveland)
: Cc: r-devel@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 [EMAIL PROTECTED] 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@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:[EMAIL PROTECTED]
: > : http://www.cmis.csiro.au/bill.venables/
: > :
: > :
: >
: > ______________________________________________
: > R-devel@stat.math.ethz.ch mailing list
: > https://stat.ethz.ch/mailman/listinfo/r-devel
: >
: >
:
: --
: Brian D. Ripley, [EMAIL PROTECTED]
: 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
:
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