At first read, the logic of the following fragment in code of function 'factor'
was not clear to me.
if (missing(levels)) {
y <- unique(x, nmax = nmax)
ind <- sort.list(y) # or possibly order(x) which is more (too ?)
tolerant
y <- as.character(y)
levels <- unique(y[ind])
}
Code similar to the originally proposed in
https://stat.ethz.ch/pipermail/r-devel/2009-May/053316.html is more readable to
me.
I suggest using this.
if (missing(levels))
levels <- unique(as.character(
sort.int(unique(x, nmax = nmax), na.last = TRUE)# or possibly
sort(x) which is more (too ?) tolerant
))
I assume that as.character(y)[sort.list(y)] is equivalent to
as.character(sort.int(y, na.last = TRUE)). So, what I suggest above has the
same effect as code in current 'factor'. Function 'sort.int' instead of 'sort'
to be like 'sort.list' that fails for non-atomic input.
What I suggest is similar in form to default 'levels' in 'factor' in R before
version 2.10.0, which is
sort(unique.default(x), na.last = TRUE)
If this suggestion is used, the help page for 'factor' can be changed to say
"(by 'sort.int')" instead of "(by 'sort.list')".
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