R help - Oct 2012 - What is behind class coercion of a factor into a character

Hello all,

Please review the following simple code:

# make a factor:
x <- factor(c("one", "two"))
       # what should be the output to the following expression?
c(x, "3")    # <===  ????
       # I expected it to be as the output of:
c(as.character(x), "3")
       # But in fact, the output is what would happen if we had ran the
next line:
c(as.character(as.numeric(x)), "3")
       # p.s: c(x, 3) would of course behave differently...

I imagine the above behavior is a "feature" (not a bug), but I am
curious
as to what is the rational behind it.  Is it because of computational
efficiency, or something that fixes some case study?

Thanks,
Tal

	[[alternative HTML version deleted]]

Tal:

There was a recent discussion on this list about this (Sam Steingold
was the OP IIRC).

The issue is ?c . In particular:

"c is sometimes used for its side effect of removing attributes except
names, for example to turn an array into a vector."

Hence, the factor attribute is removed and you get what you saw. As
regards it's "rationale," you may find Bill Dunlap's comments
on
"c()'s unfortunate history" relevant. The problem with factors is
"what should concatenation do, anyway?" If a <-
factor(c("x", "y"))
and b <- factor(c("y", "z")), what should c(a,b) be? --
There is no
reason to assume that the "y" in a is the same as the "y" in
b!

Cheers,
Bert

On Mon, Oct 22, 2012 at 6:46 AM, Tal Galili <tal.galili at gmail.com>
wrote:
> Hello all,
>
> Please review the following simple code:
>
> # make a factor:
> x <- factor(c("one", "two"))
>        # what should be the output to the following expression?
> c(x, "3")    # <===  ????
>        # I expected it to be as the output of:
> c(as.character(x), "3")
>        # But in fact, the output is what would happen if we had ran the
> next line:
> c(as.character(as.numeric(x)), "3")
>        # p.s: c(x, 3) would of course behave differently...
>
> I imagine the above behavior is a "feature" (not a bug), but I am
curious
> as to what is the rational behind it.  Is it because of computational
> efficiency, or something that fixes some case study?
>
> Thanks,
> Tal
>
>         [[alternative HTML version deleted]]
>
> ______________________________________________
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-- 

Bert Gunter
Genentech Nonclinical Biostatistics

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