I had been thinking for years, without having ever checked (shame on me), that indexing a named vector by a factor 'f' produced the same results as indexing it by 'as.character(f)'. I was wrong, as the following example shows:
(m <- c(a=1,b=2))
(f <- factor(c(1,2),labels=c('b','a')))m[f] m[as.character(f)] m[as.numeric(f)]
When the labels of a factor are not sorted alphabetically, m[f] and m[as.character(f)] yield different results. m[f] is actually equivalent to m[as.numeric(f)].
I'm not the first to have been beaten by this (documented) feature, as I discovered in a recent thread ("[R] Indexing by factor misfeature", <http://tolstoy.newcastle.edu.au/R/help/04/04/0937.html> see http://tolstoy.newcastle.edu.au/R/help/04/04/0938.html).
If I am breaking the posting rules by writing again about a previous subject, it is only
to suggest to add a short notice about that behavior in the section of 'R-intro.pdf' about 'index vectors'. Currently, it does not mention the use of factors as indexes (It is true that factors are only defined later). At least, this manual could tell the reader to read the help about Extract, or type '?"["' (which did not occur to me before writing this message...).
Christophe Pallier
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