[R] extraction
Prof Brian Ripley
ripley at stats.ox.ac.uk
Thu Apr 19 16:31:29 CEST 2001
On Thu, 19 Apr 2001, Heberto Ghezzo wrote:
> Hello everybody
> I am trying to extract some observations from a data frame, ie the
> subjects that belong to a given group, and although t-test etc work
> if I try to obtain the number of subjects in the subgroup i get some
> funny numbers. All the subjects with NA for group are included in
> the subgroups 'group==1' etc.
> Is this a bug?
The description of BUGS in the FAQ will help you answer your own question.
> for example in Windoze
???
> R : Copyright 2001, The R Development Core Team
> Version 1.2.2 (2001-02-26)
>
>
> > z1 <- rnorm(100)
> > z2<- rep(1:5,20)
> > z3 <- z2
> > z3[z1>1] <- NA
> > z1[z2==1]
> [1] 1.16747148 0.58088013 1.00958356 1.35477371 -
> 1.19790468 0.07028495 -0.67707300
> [8] -0.10999675 1.78765730 0.52044707 0.47965798 -
> 2.05768565 1.16349068 -0.75485570
> [15] -0.70517830 0.86419410 -0.03514043 -0.84861275 -
> 0.50284191 0.64971596
> > z1[z3==1]
> [1] NA 0.58088013 NA NA NA NA
> NA
> [8] -1.19790468 NA 0.07028495 -0.67707300 -0.10999675
> NA NA
> [15] NA 0.52044707 0.47965798 -2.05768565 NA -
> 0.75485570 NA
> [22] -0.70517830 0.86419410 NA -0.03514043 -0.84861275
> NA -0.50284191
> [29] NA NA NA 0.64971596
> >
>
> the first with z2==1 gives me the 20 subjects OK but the second
> should have given me less than 20 but it returns 32! it adds all the
> NA's for z3.
> I would have thought that z3==1 excluded all the NA's
> Thanks for any explanation.
NA means unknown. So you don't know if those values are one, and all you
can get is a report that they might be. I suspect you intended
z1[z3 %in% 1]
--
Brian D. Ripley, ripley at 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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