which() does not handle NAs in named vectors. (PR#226)
ripley@stats.ox.ac.uk
ripley@stats.ox.ac.uk
Thu, 15 Jul 1999 09:14:46 +0200 (MET DST)
Version:
platform = sparc-sun-solaris2.6
arch = sparc
os = solaris2.6
system = sparc, solaris2.6
status =
status.rev = 0
major = 0
minor = 64.2
year = 1999
month = July
day = 3
language = R
-- It is unclear to me that the handling of NAs is desirable, and it
has problems with names:
> z <- c(T,T,NA,F,T)
> names(z) <- letters[1:5]
> which(z)
Error: names attribute must be the same length as the vector
(Why do the vector and its names have different subscripts? And while
you are correcting this,
Arguments:
x: a logical vector or array. `NA's are allowed an
omitted.
has a typo, and the logic can be simplified: see below.)
On Thu, 15 Jul 1999, Martin Maechler wrote:
> >>>>> "BDR" == Prof Brian D Ripley <ripley@stats.ox.ac.uk> writes:
>
> BDR> On Wed, 14 Jul 1999, Friedrich Leisch wrote:
> >> >>>>> On Wed, 14 Jul 1999 04:09:21, >>>>> Peter B Mandeville (PBM)
> >> wrote:
> >>
> PBM> I have a vector Pes with 600 elements some of which are NA's. How
> PBM> can I form a vector of the indices of the NA's.
> >>
> PBM> for(i in 1:600) if(is.na(Pes[i])) print(i)
> >>
> PBM> prints the indices of the NA's but I can't figure out how to put
> PBM> the results in a vector.
> >> try this:
> >>
> >> x <- (1:length(Pes))[is.na(Pes)]
>
> BDR> Tip: that sort of thing often fails for a length 0 vector. The
> BDR> `approved' spell is
>
> BDR> seq(along=Pes)[is.na(Pes)]
>
> BDR> In this case it does not matter as the subscript is of length 0,
> BDR> but it has floored enough library/package writers to be worth
> BDR> thinking about.
>
> Good teaching about seq() vs. 1:n
>
> However, the solution I gave
>
> which(is.na(Pes))
>
> is the one I stilly really recommend;
> it does deal with 0-length objects, and it keeps names when there are some,
> and it has an `arr.ind = FALSE' argument to return array indices instead of
> vector indices when so desired.
Yes, but
-- It is not in S (so causing difficulty in porting from R to S)
-- It looks a relatively expensive operation.
-- Internally which could be simplified by using seq(along=) as it
is a wrapper for this construct, but actually the separate
handling of n == 0 is unnecessary (as logic & !is.na(logic) will have
length zero.)
Brian
--
Brian D. Ripley, ripley@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 272860 (secr)
Oxford OX1 3TG, UK Fax: +44 1865 272595
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