Names of Greek letters stored as character strings; plotmath.

On Sat, May 19, 2012 at 11:07 AM, William Dunlap <wdunlap at tibco.com> wrote:

parse(text=paste(...)) works in simple cases but not in others. ?The
fortune about it is there because it is tempting to use but if you bury it
in a general purpose function it will cause problems when people
start using nonstandard names for variables. ?bquote(), substitute(),
call(), and relatives work in all cases. ?E.g.,

?> par(mfrow=c(2,1))
?> power <- "gamma" ; x <- "Waist" ; y <- "Weight" # valid R variable names
?> plot(0, main=bquote(.(as.name(x))^.(as.name(power))/.(as.name(y))))
?> plot(0, main=parse(text=paste0(x, "^", power, "/", y))) # same as previous
?>
?> power <- "gamma" ; x <- "Waist Size (cm)" ; y <- "Weight (kg)" # invalid R names
?> plot(0, main=bquote(.(as.name(x))^.(as.name(power))/.(as.name(y))))
?> plot(0, main=parse(text=paste0(x, "^", power, "/", y))) # whoops
?Error in parse(text = paste0(x, "^", power, "/", y)) :
? ?<text>:1:7: unexpected symbol
?1: Waist Size
? ? ? ? ? ^

Now you might say that serves me right for using weird variable names,
but some of us use R as a back end to a GUI system (one not designed
around R) and don't want to inflict on users R's rules for names when
we do not have to.

Bill Dunlap
Spotfire, TIBCO Software
wdunlap tibco.com

Bill's point is on the money here. We should aim to teach one way that
works always. But finding that one way is the hard part for me.

Lately, I'm bothered that R (or computers in general?) allows too many
ways to do the same thing that work SOME of the time. Without a very
deep understanding of the terminology, users are bewildered.

Go read ?plotmath. ?Do we see "as.name"? ? NO. ?Why does the idiom
expression(paste()) work as well? ? (Don't answer, that's a rhetorical
question).

There are too many ways to do the same thing. Or, well, too many of us
know one way that works sometime and don't find out it doesn't always
work until, as Bill says, it is buried in the bottom of some big
function that behaves erratically.

Gabor notes this works (sometimes):

plot(0, xlab = parse(text = xNm))