[R] Applying function to only numeric variable (plyr package?)

[Michael.Laviolette at dhhs.state.nh.us]

My data frame consists of character variables, factors, and proportions,
something like

c1 <- c("A", "B", "C", "C")
c2 <- factor(c(1, 1, 2, 2), labels = c("Y","N"))
x <- c(0.5234, 0.6919, 0.2307, 0.1160)
y <- c(0.9251, 0.7616, 0.3624, 0.4462)
df <- data.frame(c1, c2, x, y)
pct <- function(x) round(100*x, 1)

I want to apply the pct function to only the numeric variables so that the
proportions are computed to percentages, and retain all the columns:

  c1 c2   x1   x2
1  A  Y 52.3 92.5
2  B  Y 69.2 76.2
3  C  N 23.1 36.2
4  C  N 11.6 44.6

I've been approaching it with the ddply and colwise functions from the plyr
package, but in that case each I need each row to be its own group and
retain all columns. Am I on the right track? If not, what's the best way to
do this?

Thanks in advance,
M. L.