[R] switch and factors

[Joshua Wiley]

Dear Baptiste,

On Tue, Mar 8, 2011 at 6:02 PM, baptiste auguie
<baptiste.auguie at googlemail.com> wrote:
> Dear list,
>
> Reading the help page for ?switch didn't give me more than a hint at
> what's going on here,
>
> x = 5
> y = 2
>
> foo <- function(a="x"){
>     switch(a, "x" = x,
>                     "y" = y)
> }
>
> foo(factor('x', levels=c('y', 'x')))
>
> # 2
>
> It seems that switch, when given a factor, uses the numeric codes
> rather than the string levels as I would have naively expected. Is
> this deliberate, should it be mentioned on the help page? I had an

This strikes me as similar to a discussion awhile back about whether
factors should behave primarily as their string levels or underlying
numeric representation:

> dput(factor('x', levels = c('y', 'x')))
structure(2L, .Label = c("y", "x"), class = "factor")

which of course also depends on whether methods are explicitly defined:

> dat <- factor('x', levels=c('y', 'x'))
> as.vector(dat) ## has methods
[1] "x"
> c(dat) ## does not
[1] 2

Because factors sort of live between worlds, "expecting" may be a bit
of a risky thing.

Cheers,

Josh

> input that had been invisibly converted to a factor by data.frame();
> using switch resulted in serious confusion.
>
> Thanks,
>
> baptiste
>
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-- 
Joshua Wiley
Ph.D. Student, Health Psychology
University of California, Los Angeles
http://www.joshuawiley.com/