[R] some question about vector[-NULL]
PIKAL Petr
petr.pikal at precheza.cz
Thu Sep 11 10:22:38 CEST 2014
Hi
If you insist on this behaviour you need to redefine [ function to suit your needs. You can see there is already plenty of methods for this operator.
> methods("[")
[1] [.acf* [.arrow* [.AsIs [.bibentry*
[5] [.data.frame [.Date [.difftime [.Dlist
[9] [.factor [.formula* [.fractions* [.getAnywhere*
[13] [.gList* [.gpar* [.gtable* [.hexmode
[17] [.idf* [.indexed* [.listof [.noquote
[21] [.numeric_version [.octmode [.pdf_doc* [.person*
[25] [.POSIXct [.POSIXlt [.quoted* [.raster*
[29] [.roman* [.SavedPlots* [.shingle* [.simple.list
[33] [.split* [.terms* [.trellis* [.ts*
[37] [.tskernel* [.uneval* [.unit* [.unit.arithmetic*
[41] [.unit.list* [.vpPath* [.warnings
You can see how it is constructed eg. by
base:::"[.data.frame"
It can be done but if you do not persuade R core to do it for you, you need to do it yourself. As this operator is omnipresent and changing of its behaviour would change almost any R code I am afraid that you need to reprogram it yourself.
Regards
Petr
> -----Original Message-----
> From: PO SU [mailto:rhelpmaillist at 163.com]
> Sent: Thursday, September 11, 2014 9:27 AM
> To: PIKAL Petr
> Cc: R. Help
> Subject: Re:RE: [R] some question about vector[-NULL]
>
>
>
> It really suprise me that a[-c(0,1,2)] works as a[-c(1,2)].
> But, unfortunately, a[-0] still can't work . That 's mean:
> i want:
> a<-1:3
> a[-0]
> > 1 2 3
> or
> a[-NULL]
> > 1 2 3
>
>
>
> --
>
> PO SU
> mail: desolator88 at 163.com
> Majored in Statistics from SJTU
>
>
>
>
> At 2014-09-11 02:24:25, "PIKAL Petr" <petr.pikal at precheza.cz> wrote:
> >Hi
> >
> >You still do not disclose important info about details of your
> functions. However, when you want to perform indexing like you show,
> you maybe can get rid of NULL and use zero instead.
> >
> >> a<-1:5
> >> a[-c(1,3)]
> >[1] 2 4 5
> >> a[-c(0,1,3)]
> >[1] 2 4 5
> >> a[-c(1,0,3)]
> >[1] 2 4 5
> >> a[-c(0,1,0,3,0)]
> >[1] 2 4 5
> >
> >However I am almost sure that you are fishing in murky waters and what
> you do by cycle and fiddling with NULL elements can be achieved by more
> efficiently.
> >
> >Regards
> >Petr
> >
> >
> >> -----Original Message-----
> >> From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-
> >> project.org] On Behalf Of PO SU
> >> Sent: Thursday, September 11, 2014 3:54 AM
> >> To: Duncan Murdoch
> >> Cc: R. Help
> >> Subject: Re: [R] some question about vector[-NULL]
> >>
> >>
> >> Tks, i think using logical index is a way, but to do that, i have to
> >> keep a vector as long as the original vector. that's, to exclude
> >> position 1 and 3 from
> >> a<-1:5
> >> I have to let b<-c(F,T,F,T,T) and exec a[b], not a[-c(1,3)]. which
> >> c(1,3) is much shorter than b if a is a long vector. that's, b would
> >> be c(F,T,F,T,T,T,T,......,T) I thought a way ,
> >> let d<-c(a,1)
> >> that d<-c(1,2,3,4,5,1)
> >> and initialize the index vector iv to length(d). that is iv<-6.
> >> then, d[-iv] is always equal a[- i ] , whether i is NULL or not.
> >> Because if i is NULL ,then iv is 6, if i is 2.then iv is c(2,6) and
> so
> >> on.......
> >>
> >>
> >>
> >>
> >>
> >>
> >>
> >> --
> >>
> >> PO SU
> >> mail: desolator88 at 163.com
> >> Majored in Statistics from SJTU
> >>
> >>
> >>
> >>
> >> At 2014-09-11 01:58:46, "Duncan Murdoch" <murdoch.duncan at gmail.com>
> >> wrote:
> >> >On 10/09/2014 12:20 PM, William Dunlap wrote:
> >> >> Can you make your example a bit more concrete? E.g., is your
> 'index
> >> >> vector' A an integer vector? If so, integer(0), an integer
> vector
> >> >> with no elements, would be a more reasonable return value than
> NULL,
> >> >> an object of class NULL with length 0, for the 'not found' case
> and
> >> >> you could check for that case by asking if length(A)==0.
> >> >>
> >> >> Show us typical inputs and expected outputs for your function
> (i.e.,
> >> >> the problem you want to solve).
> >> >
> >> >I think the problem with integer(0) and NULL is the same: a[-i]
> >> doesn't
> >> >act as expected (leaving out all the elements of i, i.e. nothing)
> if i
> >> >is either of those. The solution is to use logical indexing, not
> >> >negative numerical indexing.
> >> >
> >> >Duncan Murdoch
> >> >>
> >> >> Bill Dunlap
> >> >> TIBCO Software
> >> >> wdunlap tibco.com
> >> >>
> >> >>
> >> >> On Wed, Sep 10, 2014 at 8:53 AM, PO SU <rhelpmaillist at 163.com>
> >> wrote:
> >> >> >
> >> >> > Tks for your
> >> >> >
> >> >> > a <- list(ress = 1, res = NULL)
> >> >> > And in my second question, let me explain it :
> >> >> > Actually i have two vectors in global enviroment, called A and
> B
> >> .A is initialized to NULL which used to record some index in B.
> >> >> > Then i would run a function F, and each time, i would get a
> index
> >> value or NULL. that's, D<-F(B). D would be NULL or some index
> >> position in B.
> >> >> > But in the function F, though input is B, i would exclude the
> >> index value from B recorded in A. That's :
> >> >> > F<-function( B ) {
> >> >> > B<-B[-A]
> >> >> > some processing...
> >> >> > res<-NULL or some new index not included in A
> >> >> > return(res)
> >> >> > }
> >> >> > so in a loop,
> >> >> > A<-NULL
> >> >> > for( i in 1:100000) {
> >> >> > D<-F(B)
> >> >> > A<-c(A,D)
> >> >> > }
> >> >> > I never know whether D is a NULL or a different index compared
> >> with indexes already recorded in A.
> >> >> > Actually, A<-c(A,D) work well, i never worry about whether D is
> >> NULL or a real index, but in the function F, B<-B[-A] won't work.
> >> >> > so i hope that, e.g.
> >> >> > a<-1:3
> >> >> > a[-NULL] wouldn't trigger an error but return a.
> >> >> > Because, if i wrote function like the following:
> >> >> >
> >> >> > F<-function( B ) {
> >> >> > if( is.null(A))
> >> >> > B<-B
> >> >> > else
> >> >> > B<-B[-A]
> >> >> > some processing...
> >> >> > res<-NULL or some new index not included in A
> >> >> > return(res)
> >> >> > }
> >> >> > May be after 5 or 10 loops, A would already not NULL, so the
> added
> >> if ..else statement would be repeated in left 9999 loops which i
> would
> >> not like to see.
> >> >> >
> >> >> >
> >> >> >
> >> >> >
> >> >> >
> >> >> > --
> >> >> >
> >> >> > PO SU
> >> >> > mail: desolator88 at 163.com
> >> >> > Majored in Statistics from SJTU
> >> >> >
> >> >> >
> >> >> >
> >> >> > At 2014-09-10 06:45:59, "Duncan Murdoch"
> >> <murdoch.duncan at gmail.com> wrote:
> >> >> >>On 10/09/2014, 3:21 AM, PO SU wrote:
> >> >> >>>
> >> >> >>> Dear expeRts,
> >> >> >>> I have some programming questions about NULL in R.There
> >> are listed as follows:
> >> >> >>> 1. I find i can't let a list have a element NULL:
> >> >> >>> a<-list()
> >> >> >>> a$ress<-1
> >> >> >>> a$res<-NULL
> >> >> >>> a
> >> >> >>> str(a)
> >> >> >>
> >> >> >>You can do it using
> >> >> >>
> >> >> >>a <- list(ress = 1, res = NULL)
> >> >> >>
> >> >> >>> How can i know i have a named element but it is NULL, not
> just
> >> get a$xxxx,a$iiii,a$oooo there all get NULL
> >> >> >>
> >> >> >>That's a little harder. There are a few ways:
> >> >> >>
> >> >> >>"res" %in% names(a) & is.null(a[["res"]])
> >> >> >>
> >> >> >>or
> >> >> >>
> >> >> >>identical(a["res"], list(res = NULL))
> >> >> >>
> >> >> >>or
> >> >> >>
> >> >> >>is.null(a[[2]])
> >> >> >>
> >> >> >>should all work.
> >> >> >>
> >> >> >>Generally because of the special handling needed, it's a bad
> idea
> >> to try
> >> >> >>to store NULL in a list.
> >> >> >>
> >> >> >>> 2.The most important thing:
> >> >> >>> a<-1:10
> >> >> >>> b<-NULL or 1
> >> >> >>> a<-c(a,b) will work so i don't need to know whether b is null
> or
> >> not,but:
> >> >> >>> a[-NULL] can't work!! i just need a[-NULL]==a , how can i
> reach
> >> this purpose?
> >> >> >>
> >> >> >>Using !, and a logical test, e.g.
> >> >> >>
> >> >> >>a[!nullentry(a)]
> >> >> >>
> >> >> >>where nullentry() is a function based on one of the tests
> above,
> >> but
> >> >> >>applied to all entries.
> >> >> >>
> >> >> >>Duncan Murdoch
> >> >> >>
> >> >> > ______________________________________________
> >> >> > R-help at r-project.org mailing list
> >> >> > https://stat.ethz.ch/mailman/listinfo/r-help
> >> >> > PLEASE do read the posting guide http://www.R-
> project.org/posting-
> >> guide.html
> >> >> > and provide commented, minimal, self-contained, reproducible
> code.
> >> >
> >> ______________________________________________
> >> R-help at r-project.org mailing list
> >> https://stat.ethz.ch/mailman/listinfo/r-help
> >> PLEASE do read the posting guide http://www.R-project.org/posting-
> >> guide.html
> >> and provide commented, minimal, self-contained, reproducible code.
> >
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