[R] Transforming boolean subsetting into index subsetting
wdunlap at tibco.com
Sat Jun 29 03:00:09 CEST 2013
And there is no real magic in which(). You already know that
where 'I' is a logical vector the length of 'V' extracts the values
in 'V' corresponding to the TRUE's in 'I'. Replace 'V' with the vector
made by seq_along(I), (== 1, 2, ..., length(I)), and you have the
essentials of which(I).
which(I) does add an extra twist - it considers missing values in I to be the
same as FALSE's:
seq_along(I)[ !is.na(I) & I ]
Spotfire, TIBCO Software
> -----Original Message-----
> From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-project.org] On Behalf
> Of Bert Gunter
> Sent: Friday, June 28, 2013 5:01 PM
> To: Julio Sergio
> Cc: r-help at stat.math.ethz.ch
> Subject: Re: [R] Transforming boolean subsetting into index subsetting
> -- Bert
> On Fri, Jun 28, 2013 at 2:58 PM, Julio Sergio <juliosergio at gmail.com> wrote:
> > One of the techniques to subset, a vector for instance, is the following:
> > V <- c(18, 8, 5, 41, 8, 7)
> > V
> > ##  18 8 5 41 8 7
> > ( I <- abs(V - 9) <= 3 )
> > ##  FALSE TRUE FALSE FALSE TRUE TRUE
> > V[I]
> > ##  8 8 7
> > However, sometimes we are interested in the indexes of the elements where the
> > condition holds. Is there an easy way to transform the I vector into an
> > indexes vector similar to: I == c(2,5,6) ?
> > I know that I can traverse the I vector with a for() loop collecting the
> > indexes, I just wonder if such an operation can be avoided.
> > Thanks,
> > -Sergio.
> > ______________________________________________
> > 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.
> Bert Gunter
> Genentech Nonclinical Biostatistics
> Internal Contact Info:
> Phone: 467-7374
> R-help at r-project.org mailing list
> 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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