date operations
On Wed, 2008-12-03 at 14:13 -0500, Gabor Grothendieck wrote:
<snip />
Why it works is not odd if you look at the help for ?`+.Date`, which shows that for this method (correct term?) we need 'date' + 'x', where 'date' is an object of class Date and 'x' is numeric.
In fact what it says is that x may be numeric OR difftime.
Yes, sorry, got distracted with dinner...
Also the above does not explain it in any case because:
"+.Date"(alpha1, alpha2)
[1] "2009-02-15"
I guess something like this is happening:
as.Date(as.numeric(alpha1) + as.numeric(alpha2), origin="1970-01-01")
[1] "2009-02-15" Although internally, `+.Date` deals with objects of class "difftime" for either date or x. So technically the help for ?`+.Date` is at odds with the code; although there is a degree of ambiguity in what a 'date object' is?
alpha1+alpha2
Time difference of 14290 days
Warning message:
Incompatible methods ("Ops.difftime", "+.Date") for "+"
But you are doing method dispatch yourself (by hand) in the first case and allowing `+` to deal with it in the latter. The warning is not coming from `+.Date`(). This looks like an infelicity in non-standard method dispatch that takes place within `+`, a primitive function. It would be nice if `+` didn't raise warnings about potential incompatible classes, but it can't know in advance what methods users might write for it.
The OP can avoid the warning by:
alpha2 + as.numeric(alpha1)
[1] "2009-02-15"
His question was how to avoid using as.numeric.
Well, I read it as not converting his "Date"s to numerics (I converted a "difftime"). But that is splitting hairs ;-) The OP can use `+.Date` directly as you suggest, or ignore the warnings as they are harmless, or coerce the difftime object to numeric as I suggested. It makes no odds in these cases. I guess the main point for the OP is to realise why the warning is there and determine if it is OK to ignore. G
If you put alpha1 first (class difftime) that method gets used and alpha2 is coerced to numeric to complete the operation:
alpha1+alpha2
Time difference of 14290 days
Warning message:
Incompatible methods ("Ops.difftime", "+.Date") for "+"
as.numeric(alpha2) + 56
[1] 14290 To answer the OP's question about conversion, no you don't need to convert Date objects to numerics to work with them. You just need to be aware of method dispatch for `+()` and check that adding objects of differing classes makes sense. Of course, this assumes that the OP wanted the answer as an object of class "Date"...? G
On Tue, Dec 2, 2008 at 4:10 PM, Christophe Dutang <dutangc at gmail.com> wrote:
Hi all,
I'm dealing with dates in R (2.7.2), but some basic operations raise a
warning.
Incompatible methods ("+.Date", "Ops.difftime") for "+"
I saw this topic in this mailing list, but I do not understand what to
do...
cf. https://stat.ethz.ch/pipermail/r-help/2008-June/165842.html
Do I have to convert Dates to numeric?
Thanks in advance
Christophe
PS : R script used
alpha2=as.Date("2008-12-21")
alpha1=as.Date("2008-12-21")-as.Date("2008-10-26")
alpha1
alpha2
alpha1+alpha2
[[alternative HTML version deleted]]
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