[R] vector of NA indices
Martin Maechler
maechler at stat.math.ethz.ch
Thu Jul 15 08:51:21 CEST 1999
>>>>> "BDR" == Prof Brian D Ripley <ripley at 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.
Martin Maechler <maechler at stat.math.ethz.ch> http://stat.ethz.ch/~maechler/
Seminar fuer Statistik, ETH-Zentrum SOL G1 Sonneggstr.33
ETH (Federal Inst. Technology) 8092 Zurich SWITZERLAND
phone: x-41-1-632-3408 fax: ...-1086 <><
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