[R] adding logical vector to data frame

[Thomas Vogels]

Prof Brian D Ripley <ripley at stats.ox.ac.uk> writes:

> R version?
> 
> This works correctly in R-devel.  Yes, using $ on data frame subverts some
> of the conversions, but logical variables are full-status variables now.

Prof. Ripley,

thanks for the answer--both parts (confirmation of different treatment
of '$' and news from R-devel) are very helpful.


Emmanuel Paradis <paradis at isem.univ-montp2.fr> writes:

[snip]
> You'll need a more expert opinion than mine to explain the differences
> between [ and [[, but look at the following:
> 
> > x <- 1:9
> > names(x) <- LETTERS[x] # give names to the observations
> > x
> A B C D E F G H I 
> 1 2 3 4 5 6 7 8 9 
> > x["C"]
> C 
> 3 
> > x[["C"]]
> [1] 3
> > names(x["C"])
> [1] "C"
> > names(x[["C"]])
> NULL

Quite obviously, I'm not an expert, but in my simple-minded view, []
subsets, while [[]] extracts (one element, and one element only).
So x["C"] is the third row of x while x[["C"]] gives you the
third element (a scalar, no name).

> The same with numeric indexing, but not logical indexing:
> 
> > x[3]
> C 
> 3
> > x[[3]]
> [1] 3
> > x[x == 3]
> C 
> 3 
> > x[[x == 3]]
> Error: attempt to select more than one element

because 'x == 3' results in a vector of the same length as x.  Since
that length is > 1, an error occurs.  (You could argue that if the
index vector contains exactly one TRUE value, '[['() could proceed,
but that's probably more dangerous than helpful.)

  -tom

-- 
mailto:tov at ece.cmu.edu (Tom Vogels)   Tel: (412) 268-6638   FAX: -3204
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