[Rd] [R] which.na

[Duncan Murdoch]

Martin Maechler wrote:
>>>>>> "CAPE" == Charles Annis, P E <Charles.Annis at statisticalengineering.com>
>>>>>>     on Thu, 19 Mar 2009 22:46:10 -0400 writes:
>>>>>>             
>
>     >> ?is.na
>     >> x <- c(NA, 3, 4, 5, NA)
>     >> which(is.na(x))
>     CAPE> [1] 1 5
>
> well, of course.
>
> But note that  which.na(.) could be implemented to be 
> faster (because needing much less memory) than the above,
> notably when  x  is large and  has only few NAs
>
> But this now has *REALLY*  changed into a topic belonging to
> R-devel, not R-help
>
> --> hence I've diverted the thread to there.
>
> I have recently entertained similar thoughts, i.e. wished for R
> functions that compute  
> 	  which( function_returning_logical(..) )
> and also
> 	    any( function_returning_logical(..) )
>
> directly {on .Internal i.e. C-level} instead of going to
> construct the potentially huge logical vector.
>
> For what functions should this happen?
> I agree that  is.na() is one of them; but then, why not
>     is.nan() /  is.finite() 
> too?
>
> Instead of defining a slew of such functions  
> which.foo(), which.bar(), any.foo(), any.bar(), etc,
> it would be nice to have a generic interface such as
>
>    whichApply(x, is.na)
>    whichApply(x, is.nan)
>
>    anyApply(x, is.na) 
>
> where internally, for some functions {in a given internal
> table}, the fast shortcut would be used, and for others the
> interface would be equivalent to  which( thatFunction( x ) )
>   
A couple of different interfaces to the same idea:

  - which() could recognize a few thatFunction(x) calls before 
evaluating them, and do the fast internal version.  (This is hard 
because it needs to know
that the user hasn't redefined is.na, etc.  Probably not worth doing.)

  - which() could gain a new arg, so that

       which(x, test=is.na)

    would do as your whichApply() does.

Duncan Murdoch
> Martin Maechler, ETH Zurich (and R Core team)
>
>
>     CAPE> Charles Annis, P.E.
>
>     CAPE> Charles.Annis at StatisticalEngineering.com
>     CAPE> phone: 561-352-9699
>     CAPE> eFax:  614-455-3265
>     CAPE> http://www.StatisticalEngineering.com
>  
>
>     CAPE> -----Original Message-----
>     CAPE> From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-project.org] On
>     CAPE> Behalf Of Santosh
>     CAPE> Sent: Thursday, March 19, 2009 9:37 PM
>     CAPE> To: r-help at r-project.org
>     CAPE> Subject: [R] which.na
>
>     CAPE> Hi R- users
>
>     CAPE> I was wondering if there is any function equivalent to which.na used in S+?
>
>     CAPE> Thanks much in advance!
>
>     CAPE> Regards,
>     CAPE> Santosh
>
>     CAPE> [[alternative HTML version deleted]]
>
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